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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Appendix M. Glossary</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="acronyms.html" title="Appendix L. Acronyms" /><link rel="next" href="color.html" title="Appendix N. Color Support" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Appendix M. Glossary</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="acronyms.html" title="Appendix L. Acronyms">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="appendixes.html" title="Part VIII. Appendixes">Up</a></td><th width="60%" align="center">Part VIII. Appendixes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="color.html" title="Appendix N. Color Support">Next</a></td></tr></table><hr /></div><div class="appendix" id="GLOSSARY"><div class="titlepage"><div><div><h2 class="title">Appendix M. Glossary</h2></div></div></div><p>
+ This is a list of terms and their meaning in the context of
+ <span class="productname">PostgreSQL</span> and relational database
+ systems in general.
+ </p><div class="glosslist"><dl><dt id="GLOSSARY-ACID"><span class="glossterm">ACID</span></dt><dd class="glossdef"><p>
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY" title="Atomicity">Atomicity</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-CONSISTENCY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSISTENCY" title="Consistency">Consistency</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-ISOLATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ISOLATION" title="Isolation">Isolation</a></em></a>, and
+ <a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY" title="Durability">Durability</a></em></a>.
+ This set of properties of database transactions is intended to
+ guarantee validity in concurrent operation and even in event of
+ errors, power failures, etc.
+ </p></dd><dt id="GLOSSARY-AGGREGATE"><span class="glossterm">Aggregate function (routine)</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">function</a></em></a> that
+ combines (<em class="firstterm">aggregates</em>) multiple input values,
+ for example by counting, averaging or adding,
+ yielding a single output value.
+ </p><p>
+ For more information, see
+ <a class="xref" href="functions-aggregate.html" title="9.21. Aggregate Functions">Section 9.21</a>.
+ </p><p>See Also <a class="glossseealso" href="glossary.html#GLOSSARY-WINDOW-FUNCTION">Window function (routine)</a>.</p></dd><dt><span class="glossterm">Analytic function</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WINDOW-FUNCTION">Window function (routine)</a>.</p></dd><dt id="GLOSSARY-ANALYZE"><span class="glossterm">Analyze (operation)</span></dt><dd class="glossdef"><p>
+ The act of collecting statistics from data in
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">tables</a></em></a>
+ and other <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a>
+ to help the <a class="glossterm" href="glossary.html#GLOSSARY-PLANNER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PLANNER" title="Query planner">query planner</a></em></a>
+ to make decisions about how to execute
+ <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">queries</a></em></a>.
+ </p><p>
+ (Don't confuse this term with the <code class="literal">ANALYZE</code> option
+ to the <a class="xref" href="sql-explain.html" title="EXPLAIN"><span class="refentrytitle">EXPLAIN</span></a> command.)
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a>.
+ </p></dd><dt id="GLOSSARY-ATOMIC"><span class="glossterm">Atomic</span></dt><dd class="glossdef"><p>
+ In reference to a <a class="glossterm" href="glossary.html#GLOSSARY-DATUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATUM" title="Datum">datum</a></em></a>:
+ the fact that its value cannot be broken down into smaller
+ components.
+ </p></dd><dd class="glossdef"><p>
+ In reference to a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">database transaction</a></em></a>:
+ see <a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY" title="Atomicity">atomicity</a></em></a>.
+ </p></dd><dt id="GLOSSARY-ATOMICITY"><span class="glossterm">Atomicity</span></dt><dd class="glossdef"><p>
+ The property of a <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>
+ that either all its operations complete as a single unit or none do.
+ In addition, if a system failure occurs during the execution of a
+ transaction, no partial results are visible after recovery.
+ This is one of the <acronym class="acronym">ACID</acronym> properties.
+ </p></dd><dt id="GLOSSARY-ATTRIBUTE"><span class="glossterm">Attribute</span></dt><dd class="glossdef"><p>
+ An element with a certain name and data type found within a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuple</a></em></a>.
+ </p></dd><dt id="GLOSSARY-AUTOVACUUM"><span class="glossterm">Autovacuum (process)</span></dt><dd class="glossdef"><p>
+ A set of background processes that routinely perform
+ <a class="glossterm" href="glossary.html#GLOSSARY-VACUUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VACUUM" title="Vacuum">vacuum</a></em></a>
+ and <a class="glossterm" href="glossary.html#GLOSSARY-ANALYZE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ANALYZE" title="Analyze (operation)">analyze</a></em></a> operations.
+ The <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ that coordinates the work and is always present (unless autovacuum
+ is disabled) is known as the <em class="firstterm">autovacuum launcher</em>,
+ and the processes that carry out the tasks are known as the
+ <em class="firstterm">autovacuum workers</em>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="routine-vacuuming.html#AUTOVACUUM" title="25.1.6. The Autovacuum Daemon">Section 25.1.6</a>.
+ </p></dd><dt id="GLOSSARY-AUXILIARY-PROC"><span class="glossterm">Auxiliary process</span></dt><dd class="glossdef"><p>
+ A process within an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>
+ that is in charge of some specific background task for the instance.
+ The auxiliary processes consist of
+
+ the <a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM" title="Autovacuum (process)">autovacuum launcher</a></em></a>
+ (but not the autovacuum workers),
+ the <a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER" title="Background writer (process)">background writer</a></em></a>,
+ the <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINTER" title="Checkpointer (process)">checkpointer</a></em></a>,
+ the <a class="glossterm" href="glossary.html#GLOSSARY-LOGGER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOGGER" title="Logger (process)">logger</a></em></a>,
+ the <a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS" title="Startup process">startup process</a></em></a>,
+ the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-ARCHIVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-ARCHIVER" title="WAL archiver (process)">WAL archiver</a></em></a>,
+ the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER" title="WAL receiver (process)">WAL receiver</a></em></a>
+ (but not the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-SENDER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-SENDER" title="WAL sender (process)">WAL senders</a></em></a>),
+ and the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-WRITER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-WRITER" title="WAL writer (process)">WAL writer</a></em></a>.
+ </p></dd><dt id="GLOSSARY-BACKEND"><span class="glossterm">Backend (process)</span></dt><dd class="glossdef"><p>
+ Process of an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>
+ which acts on behalf of a <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">client session</a></em></a>
+ and handles its requests.
+ </p><p>
+ (Don't confuse this term with the similar terms
+ <a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WORKER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WORKER" title="Background worker (process)">Background Worker</a></em></a> or
+ <a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER" title="Background writer (process)">Background Writer</a></em></a>).
+ </p></dd><dt id="GLOSSARY-BACKGROUND-WORKER"><span class="glossterm">Background worker (process)</span></dt><dd class="glossdef"><p>
+ Process within an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>,
+ which runs system- or user-supplied code.
+ Serves as infrastructure for several features in
+ <span class="productname">PostgreSQL</span>, such as
+ <a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION" title="Replication">logical replication</a></em></a>
+ and <a class="glossterm" href="glossary.html#GLOSSARY-PARALLEL-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARALLEL-QUERY" title="Parallel query">parallel queries</a></em></a>.
+ In addition, <a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION" title="Extension">Extensions</a></em></a> can add
+ custom background worker processes.
+ </p><p>
+ For more information, see
+ <a class="xref" href="bgworker.html" title="Chapter 48. Background Worker Processes">Chapter 48</a>.
+ </p></dd><dt id="GLOSSARY-BACKGROUND-WRITER"><span class="glossterm">Background writer (process)</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ that writes dirty
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATA-PAGE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATA-PAGE" title="Data page">data pages</a></em></a> from
+ <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared memory</a></em></a> to
+ the file system. It wakes up periodically, but works only for a short
+ period in order to distribute its expensive <acronym class="acronym">I/O</acronym>
+ activity over time to avoid generating larger
+ <acronym class="acronym">I/O</acronym> peaks which could block other processes.
+ </p><p>
+ For more information, see
+ <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER" title="20.4.5. Background Writer">Section 20.4.5</a>.
+ </p></dd><dt id="GLOSSARY-BASEBACKUP"><span class="glossterm">Base Backup</span></dt><dd class="glossdef"><p>
+ A binary copy of all
+ <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>
+ files. It is generated by the tool <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a>.
+ In combination with WAL files it can be used as the starting point
+ for recovery, log shipping, or streaming replication.
+ </p></dd><dt id="GLOSSARY-BLOAT"><span class="glossterm">Bloat</span></dt><dd class="glossdef"><p>
+ Space in data pages which does not contain current row versions,
+ such as unused (free) space or outdated row versions.
+ </p></dd><dt id="GLOSSARY-CAST"><span class="glossterm">Cast</span></dt><dd class="glossdef"><p>
+ A conversion of a <a class="glossterm" href="glossary.html#GLOSSARY-DATUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATUM" title="Datum">datum</a></em></a>
+ from its current data type to another data type.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createcast.html" title="CREATE CAST"><span class="refentrytitle">CREATE CAST</span></a>.
+ </p></dd><dt id="GLOSSARY-CATALOG"><span class="glossterm">Catalog</span></dt><dd class="glossdef"><p>
+ The <acronym class="acronym">SQL</acronym> standard uses this term to
+ indicate what is called a
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> in
+ <span class="productname">PostgreSQL</span>'s terminology.
+ </p><p>
+ (Don't confuse this term with
+ <a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG" title="System catalog">system catalog</a></em></a>).
+ </p><p>
+ For more information, see
+ <a class="xref" href="manage-ag-overview.html" title="23.1. Overview">Section 23.1</a>.
+ </p></dd><dt id="GLOSSARY-CHECK-CONSTRAINT"><span class="glossterm">Check constraint</span></dt><dd class="glossdef"><p>
+ A type of <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraint</a></em></a>
+ defined on a <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>
+ which restricts the values allowed in one or more
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>. The
+ check constraint can make reference to any attribute of the same row in
+ the relation, but cannot reference other rows of the same relation or
+ other relations.
+ </p><p>
+ For more information, see
+ <a class="xref" href="ddl-constraints.html" title="5.4. Constraints">Section 5.4</a>.
+ </p></dd><dt id="GLOSSARY-CHECKPOINT"><span class="glossterm">Checkpoint</span></dt><dd class="glossdef"><p>
+ A point in the <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a> sequence
+ at which it is guaranteed that the heap and index data files have been
+ updated with all information from
+ <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared memory</a></em></a>
+ modified before that checkpoint;
+ a <em class="firstterm">checkpoint record</em> is written and flushed to WAL
+ to mark that point.
+ </p><p>
+ A checkpoint is also the act of carrying out all the actions that
+ are necessary to reach a checkpoint as defined above.
+ This process is initiated when predefined conditions are met,
+ such as a specified amount of time has passed, or a certain volume
+ of records has been written; or it can be invoked by the user
+ with the command <code class="command">CHECKPOINT</code>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="wal-configuration.html" title="30.5. WAL Configuration">Section 30.5</a>.
+ </p></dd><dt id="GLOSSARY-CHECKPOINTER"><span class="glossterm">Checkpointer (process)</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ that is responsible for executing
+ <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT" title="Checkpoint">checkpoints</a></em></a>.
+ </p></dd><dt><span class="glossterm">Class (archaic)</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-RELATION">Relation</a>.</p></dd><dt id="GLOSSARY-CLIENT"><span class="glossterm">Client (process)</span></dt><dd class="glossdef"><p>
+ Any process, possibly remote, that establishes a
+ <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">session</a></em></a>
+ by <a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION" title="Connection">connecting</a></em></a> to an
+ <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>
+ to interact with a <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.
+ </p></dd><dt id="GLOSSARY-COLUMN"><span class="glossterm">Column</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attribute</a></em></a> found in
+ a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> or
+ <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">view</a></em></a>.
+ </p></dd><dt id="GLOSSARY-COMMIT"><span class="glossterm">Commit</span></dt><dd class="glossdef"><p>
+ The act of finalizing a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a> within
+ the <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>, which
+ makes it visible to other transactions and assures its
+ <a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY" title="Durability">durability</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-commit.html" title="COMMIT"><span class="refentrytitle">COMMIT</span></a>.
+ </p></dd><dt id="GLOSSARY-CONCURRENCY"><span class="glossterm">Concurrency</span></dt><dd class="glossdef"><p>
+ The concept that multiple independent operations happen within the
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> at the same time.
+ In <span class="productname">PostgreSQL</span>, concurrency is controlled by
+ the <a class="glossterm" href="glossary.html#GLOSSARY-MVCC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MVCC" title="Multi-version concurrency control (MVCC)">multiversion concurrency control</a></em></a>
+ mechanism.
+ </p></dd><dt id="GLOSSARY-CONNECTION"><span class="glossterm">Connection</span></dt><dd class="glossdef"><p>
+ An established line of communication between a client process and a
+ <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend</a></em></a> process,
+ usually over a network, supporting a
+ <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">session</a></em></a>. This term is
+ sometimes used as a synonym for session.
+ </p><p>
+ For more information, see
+ <a class="xref" href="runtime-config-connection.html" title="20.3. Connections and Authentication">Section 20.3</a>.
+ </p></dd><dt id="GLOSSARY-CONSISTENCY"><span class="glossterm">Consistency</span></dt><dd class="glossdef"><p>
+ The property that the data in the
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>
+ is always in compliance with
+ <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">integrity constraints</a></em></a>.
+ Transactions may be allowed to violate some of the constraints
+ transiently before it commits, but if such violations are not resolved
+ by the time it commits, such a transaction is automatically
+ <a class="glossterm" href="glossary.html#GLOSSARY-ROLLBACK"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLLBACK" title="Rollback">rolled back</a></em></a>.
+ This is one of the <acronym class="acronym">ACID</acronym> properties.
+ </p></dd><dt id="GLOSSARY-CONSTRAINT"><span class="glossterm">Constraint</span></dt><dd class="glossdef"><p>
+ A restriction on the values of data allowed within a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>,
+ or in attributes of a
+ <a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN" title="Domain">domain</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="ddl-constraints.html" title="5.4. Constraints">Section 5.4</a>.
+ </p></dd><dt id="GLOSSARY-CUMULATIVE-STATISTICS"><span class="glossterm">Cumulative Statistics System</span></dt><dd class="glossdef"><p>
+ A system which, if enabled, accumulates statistical information
+ about the <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>'s
+ activities.
+ </p><p>
+ For more information, see
+ <a class="xref" href="monitoring-stats.html" title="28.2. The Cumulative Statistics System">Section 28.2</a>.
+ </p></dd><dt><span class="glossterm">Data area</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-DATA-DIRECTORY">Data directory</a>.</p></dd><dt id="GLOSSARY-DATABASE"><span class="glossterm">Database</span></dt><dd class="glossdef"><p>
+ A named collection of
+ <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">local SQL objects</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="manage-ag-overview.html" title="23.1. Overview">Section 23.1</a>.
+ </p></dd><dt id="GLOSSARY-DB-CLUSTER"><span class="glossterm">Database cluster</span></dt><dd class="glossdef"><p>
+ A collection of databases and global SQL objects,
+ and their common static and dynamic metadata.
+ Sometimes referred to as a
+ <em class="firstterm">cluster</em>.
+ </p><p>
+ In <span class="productname">PostgreSQL</span>, the term
+ <em class="firstterm">cluster</em> is also sometimes used to refer to an instance.
+ (Don't confuse this term with the SQL command <code class="command">CLUSTER</code>.)
+ </p></dd><dt><span class="glossterm">Database server</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-INSTANCE">Instance</a>.</p></dd><dt id="GLOSSARY-DATA-DIRECTORY"><span class="glossterm">Data directory</span></dt><dd class="glossdef"><p>
+ The base directory on the file system of a
+ <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a> that contains all
+ data files and subdirectories associated with a
+ <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>
+ (with the exception of
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE" title="Tablespace">tablespaces</a></em></a>,
+ and optionally <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>).
+ The environment variable <code class="literal">PGDATA</code> is commonly used to
+ refer to the data directory.
+ </p><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">cluster</a></em></a>'s storage
+ space comprises the data directory plus any additional tablespaces.
+ </p><p>
+ For more information, see
+ <a class="xref" href="storage-file-layout.html" title="73.1. Database File Layout">Section 73.1</a>.
+ </p></dd><dt id="GLOSSARY-DATA-PAGE"><span class="glossterm">Data page</span></dt><dd class="glossdef"><p>
+ The basic structure used to store relation data.
+ All pages are of the same size.
+ Data pages are typically stored on disk, each in a specific file,
+ and can be read to <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared buffers</a></em></a>
+ where they can be modified, becoming
+ <em class="firstterm">dirty</em>. They become clean when written
+ to disk. New pages, which initially exist in memory only, are also
+ dirty until written.
+ </p></dd><dt id="GLOSSARY-DATUM"><span class="glossterm">Datum</span></dt><dd class="glossdef"><p>
+ The internal representation of one value of an <acronym class="acronym">SQL</acronym>
+ data type.
+ </p></dd><dt id="GLOSSARY-DELETE"><span class="glossterm">Delete</span></dt><dd class="glossdef"><p>
+ An <acronym class="acronym">SQL</acronym> command which removes
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a> from a given
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>
+ or <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-delete.html" title="DELETE"><span class="refentrytitle">DELETE</span></a>.
+ </p></dd><dt id="GLOSSARY-DOMAIN"><span class="glossterm">Domain</span></dt><dd class="glossdef"><p>
+ A user-defined data type that is based on another underlying data type.
+ It acts the same as the underlying type except for possibly restricting
+ the set of allowed values.
+ </p><p>
+ For more information, see <a class="xref" href="domains.html" title="8.18. Domain Types">Section 8.18</a>.
+ </p></dd><dt id="GLOSSARY-DURABILITY"><span class="glossterm">Durability</span></dt><dd class="glossdef"><p>
+ The assurance that once a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a> has
+ been <a class="glossterm" href="glossary.html#GLOSSARY-COMMIT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COMMIT" title="Commit">committed</a></em></a>, the
+ changes remain even after a system failure or crash.
+ This is one of the <acronym class="acronym">ACID</acronym> properties.
+ </p></dd><dt><span class="glossterm">Epoch</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-XID">Transaction ID</a>.</p></dd><dt id="GLOSSARY-EXTENSION"><span class="glossterm">Extension</span></dt><dd class="glossdef"><p>
+ A software add-on package that can be installed on an
+ <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a> to
+ get extra features.
+ </p><p>
+ For more information, see
+ <a class="xref" href="extend-extensions.html" title="38.17. Packaging Related Objects into an Extension">Section 38.17</a>.
+ </p></dd><dt id="GLOSSARY-FILE-SEGMENT"><span class="glossterm">File segment</span></dt><dd class="glossdef"><p>
+ A physical file which stores data for a given
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.
+ File segments are limited in size by a configuration value
+ (typically 1 gigabyte),
+ so if a relation exceeds that size, it is split into multiple segments.
+ </p><p>
+ For more information, see
+ <a class="xref" href="storage-file-layout.html" title="73.1. Database File Layout">Section 73.1</a>.
+ </p><p>
+ (Don't confuse this term with the similar term
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL segment</a></em></a>).
+ </p></dd><dt id="GLOSSARY-FOREIGN-DATA-WRAPPER"><span class="glossterm">Foreign data wrapper</span></dt><dd class="glossdef"><p>
+ A means of representing data that is not contained in the local
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> so that it appears as if were in local
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table(s)</a></em></a>. With a foreign data wrapper it is
+ possible to define a <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-SERVER" title="Foreign server">foreign server</a></em></a> and
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign tables</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createforeigndatawrapper.html" title="CREATE FOREIGN DATA WRAPPER"><span class="refentrytitle">CREATE FOREIGN DATA WRAPPER</span></a>.
+ </p></dd><dt id="GLOSSARY-FOREIGN-KEY"><span class="glossterm">Foreign key</span></dt><dd class="glossdef"><p>
+ A type of <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraint</a></em></a>
+ defined on one or more <a class="glossterm" href="glossary.html#GLOSSARY-COLUMN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COLUMN" title="Column">columns</a></em></a>
+ in a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> which
+ requires the value(s) in those <a class="glossterm" href="glossary.html#GLOSSARY-COLUMN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COLUMN" title="Column">columns</a></em></a> to
+ identify zero or one <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">row</a></em></a>
+ in another (or, infrequently, the same)
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>.
+ </p></dd><dt id="GLOSSARY-FOREIGN-SERVER"><span class="glossterm">Foreign server</span></dt><dd class="glossdef"><p>
+ A named collection of
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign tables</a></em></a> which
+ all use the same
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrapper</a></em></a>
+ and have other configuration values in common.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createserver.html" title="CREATE SERVER"><span class="refentrytitle">CREATE SERVER</span></a>.
+ </p></dd><dt id="GLOSSARY-FOREIGN-TABLE"><span class="glossterm">Foreign table (relation)</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> which appears to have
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a> and
+ <a class="glossterm" href="glossary.html#GLOSSARY-COLUMN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COLUMN" title="Column">columns</a></em></a> similar to a
+ regular <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>, but will forward
+ requests for data through its
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrapper</a></em></a>,
+ which will return <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">result sets</a></em></a>
+ structured according to the definition of the
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign table</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a>.
+ </p></dd><dt id="GLOSSARY-FORK"><span class="glossterm">Fork</span></dt><dd class="glossdef"><p>
+ Each of the separate segmented file sets in which a relation is stored.
+ The <em class="firstterm">main fork</em> is where the actual data resides.
+ There also exist two secondary forks for metadata:
+ the <a class="glossterm" href="glossary.html#GLOSSARY-FSM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FSM" title="Free space map (fork)">free space map</a></em></a>
+ and the <a class="glossterm" href="glossary.html#GLOSSARY-VM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VM" title="Visibility map (fork)">visibility map</a></em></a>.
+ <a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED" title="Unlogged">Unlogged relations</a></em></a>
+ also have an <em class="firstterm">init fork</em>.
+ </p></dd><dt id="GLOSSARY-FSM"><span class="glossterm">Free space map (fork)</span></dt><dd class="glossdef"><p>
+ A storage structure that keeps metadata about each data page of a table's
+ main fork. The free space map entry for each page stores the
+ amount of free space that's available for future tuples, and is structured
+ to be efficiently searched for available space for a new tuple of a given
+ size.
+ </p><p>
+ For more information, see
+ <a class="xref" href="storage-fsm.html" title="73.3. Free Space Map">Section 73.3</a>.
+ </p></dd><dt id="GLOSSARY-FUNCTION"><span class="glossterm">Function (routine)</span></dt><dd class="glossdef"><p>
+ A type of routine that receives zero or more arguments, returns zero or more
+ output values, and is constrained to run within one transaction.
+ Functions are invoked as part of a query, for example via
+ <code class="command">SELECT</code>.
+ Certain functions can return
+ <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">sets</a></em></a>; those are
+ called <em class="firstterm">set-returning functions</em>.
+ </p><p>
+ Functions can also be used for
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER" title="Trigger">triggers</a></em></a> to invoke.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>.
+ </p></dd><dt id="GLOSSARY-GRANT"><span class="glossterm">Grant</span></dt><dd class="glossdef"><p>
+ An <acronym class="acronym">SQL</acronym> command that is used to allow a
+ <a class="glossterm" href="glossary.html#GLOSSARY-USER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-USER" title="User">user</a></em></a> or
+ <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">role</a></em></a> to access
+ specific objects within the <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-grant.html" title="GRANT"><span class="refentrytitle">GRANT</span></a>.
+ </p></dd><dt id="GLOSSARY-HEAP"><span class="glossterm">Heap</span></dt><dd class="glossdef"><p>
+ Contains the values of <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">row</a></em></a>
+ attributes (i.e., the data) for a
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.
+ The heap is realized within one or more
+ <a class="glossterm" href="glossary.html#GLOSSARY-FILE-SEGMENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FILE-SEGMENT" title="File segment">file segments</a></em></a>
+ in the relation's <a class="glossterm" href="glossary.html#GLOSSARY-FORK"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FORK" title="Fork">main fork</a></em></a>.
+ </p></dd><dt id="GLOSSARY-HOST"><span class="glossterm">Host</span></dt><dd class="glossdef"><p>
+ A computer that communicates with other computers over a network.
+ This is sometimes used as a synonym for
+ <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a>.
+ It is also used to refer to a computer where
+ <a class="glossterm" href="glossary.html#GLOSSARY-CLIENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLIENT" title="Client (process)">client processes</a></em></a> run.
+ </p></dd><dt id="GLOSSARY-INDEX"><span class="glossterm">Index (relation)</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that contains
+ data derived from a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>
+ or <a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW" title="Materialized view (relation)">materialized view</a></em></a>.
+ Its internal structure supports fast retrieval of and access to the original
+ data.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createindex.html" title="CREATE INDEX"><span class="refentrytitle">CREATE INDEX</span></a>.
+ </p></dd><dt id="GLOSSARY-INSERT"><span class="glossterm">Insert</span></dt><dd class="glossdef"><p>
+ An <acronym class="acronym">SQL</acronym> command used to add new data into a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-insert.html" title="INSERT"><span class="refentrytitle">INSERT</span></a>.
+ </p></dd><dt id="GLOSSARY-INSTANCE"><span class="glossterm">Instance</span></dt><dd class="glossdef"><p>
+ A group of <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend</a></em></a> and
+ <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary processes</a></em></a>
+ that communicate using a common shared memory area. One
+ <a class="glossterm" href="glossary.html#GLOSSARY-POSTMASTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-POSTMASTER" title="Postmaster (process)">postmaster process</a></em></a>
+ manages the instance; one instance manages exactly one
+ <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>
+ with all its databases. Many instances can run on the same
+ <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a>
+ as long as their <acronym class="acronym">TCP</acronym> ports do not conflict.
+ </p><p>
+ The instance handles all key features of a <acronym class="acronym">DBMS</acronym>:
+ read and write access to files and shared memory,
+ assurance of the <acronym class="acronym">ACID</acronym> properties,
+ <a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION" title="Connection">connections</a></em></a> to
+ <a class="glossterm" href="glossary.html#GLOSSARY-CLIENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLIENT" title="Client (process)">client processes</a></em></a>,
+ privilege verification, crash recovery, replication, etc.
+ </p></dd><dt id="GLOSSARY-ISOLATION"><span class="glossterm">Isolation</span></dt><dd class="glossdef"><p>
+ The property that the effects of a transaction are not visible to
+ <a class="glossterm" href="glossary.html#GLOSSARY-CONCURRENCY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONCURRENCY" title="Concurrency">concurrent transactions</a></em></a>
+ before it commits.
+ This is one of the <acronym class="acronym">ACID</acronym> properties.
+ </p><p>
+ For more information, see <a class="xref" href="transaction-iso.html" title="13.2. Transaction Isolation">Section 13.2</a>.
+ </p></dd><dt id="GLOSSARY-JOIN"><span class="glossterm">Join</span></dt><dd class="glossdef"><p>
+ An operation and <acronym class="acronym">SQL</acronym> keyword used in
+ <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">queries</a></em></a>
+ for combining data from multiple
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a>.
+ </p></dd><dt id="GLOSSARY-KEY"><span class="glossterm">Key</span></dt><dd class="glossdef"><p>
+ A means of identifying a <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">row</a></em></a> within a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> or
+ other <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> by
+ values contained within one or more
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>
+ in that relation.
+ </p></dd><dt id="GLOSSARY-LOCK"><span class="glossterm">Lock</span></dt><dd class="glossdef"><p>
+ A mechanism that allows a process to limit or prevent simultaneous
+ access to a resource.
+ </p></dd><dt id="GLOSSARY-LOG-FILE"><span class="glossterm">Log file</span></dt><dd class="glossdef"><p>
+ Log files contain human-readable text lines about events.
+ Examples include login failures, long-running queries, etc.
+ </p><p>
+ For more information, see
+ <a class="xref" href="logfile-maintenance.html" title="25.3. Log File Maintenance">Section 25.3</a>.
+ </p></dd><dt id="GLOSSARY-LOGGED"><span class="glossterm">Logged</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> is considered
+ <a class="glossterm" href="glossary.html#GLOSSARY-LOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOGGED" title="Logged">logged</a></em></a> if changes to it are sent to the
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>. By default, all regular
+ tables are logged. A table can be specified as
+ <a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED" title="Unlogged">unlogged</a></em></a> either at
+ creation time or via the <code class="command">ALTER TABLE</code> command.
+ </p></dd><dt id="GLOSSARY-LOGGER"><span class="glossterm">Logger (process)</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ which, if enabled, writes information about database events into the current
+ <a class="glossterm" href="glossary.html#GLOSSARY-LOG-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOG-FILE" title="Log file">log file</a></em></a>.
+ When reaching certain time- or
+ volume-dependent criteria, a new log file is created.
+ Also called <em class="firstterm">syslogger</em>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="runtime-config-logging.html" title="20.8. Error Reporting and Logging">Section 20.8</a>.
+ </p></dd><dt id="GLOSSARY-LOG-RECORD"><span class="glossterm">Log record</span></dt><dd class="glossdef"><p>
+ Archaic term for a <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL record</a></em></a>.
+ </p></dd><dt><span class="glossterm">Master (server)</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-PRIMARY-SERVER">Primary (server)</a>.</p></dd><dt id="GLOSSARY-MATERIALIZED"><span class="glossterm">Materialized</span></dt><dd class="glossdef"><p>
+ The property that some information has been pre-computed and stored
+ for later use, rather than computing it on-the-fly.
+ </p><p>
+ This term is used in
+ <a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW" title="Materialized view (relation)">materialized view</a></em></a>,
+ to mean that the data derived from the view's query is stored on
+ disk separately from the sources of that data.
+ </p><p>
+ This term is also used to refer to some multi-step queries to mean that
+ the data resulting from executing a given step is stored in memory
+ (with the possibility of spilling to disk), so that it can be read multiple
+ times by another step.
+ </p></dd><dt id="GLOSSARY-MATERIALIZED-VIEW"><span class="glossterm">Materialized view (relation)</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that is
+ defined by a <code class="command">SELECT</code> statement
+ (just like a <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">view</a></em></a>),
+ but stores data in the same way that a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> does. It cannot be
+ modified via <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or
+ <code class="command">DELETE</code> operations.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-creatematerializedview.html" title="CREATE MATERIALIZED VIEW"><span class="refentrytitle">CREATE MATERIALIZED VIEW</span></a>.
+ </p></dd><dt id="GLOSSARY-MVCC"><span class="glossterm">Multi-version concurrency control (MVCC)</span></dt><dd class="glossdef"><p>
+ A mechanism designed to allow several
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transactions</a></em></a> to be
+ reading and writing the same rows without one process causing other
+ processes to stall.
+ In <span class="productname">PostgreSQL</span>, MVCC is implemented by
+ creating copies (<em class="firstterm">versions</em>) of
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuples</a></em></a> as they are
+ modified; after transactions that can see the old versions terminate,
+ those old versions need to be removed.
+ </p></dd><dt id="GLOSSARY-NULL"><span class="glossterm">Null</span></dt><dd class="glossdef"><p>
+ A concept of non-existence that is a central tenet of relational
+ database theory. It represents the absence of a definite value.
+ </p></dd><dt><span class="glossterm">Optimizer</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-PLANNER">Query planner</a>.</p></dd><dt id="GLOSSARY-PARALLEL-QUERY"><span class="glossterm">Parallel query</span></dt><dd class="glossdef"><p>
+ The ability to handle parts of executing a
+ <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">query</a></em></a> to take advantage
+ of parallel processes on servers with multiple <acronym class="acronym">CPU</acronym>s.
+ </p></dd><dt id="GLOSSARY-PARTITION"><span class="glossterm">Partition</span></dt><dd class="glossdef"><p>
+ One of several disjoint (not overlapping) subsets of a larger set.
+ </p></dd><dd class="glossdef"><p>
+ In reference to a
+ <a class="glossterm" href="glossary.html#GLOSSARY-PARTITIONED-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARTITIONED-TABLE" title="Partitioned table (relation)">partitioned table</a></em></a>:
+ One of the tables that each contain part of the data of the partitioned table,
+ which is said to be the <em class="firstterm">parent</em>.
+ The partition is itself a table, so it can also be queried directly;
+ at the same time, a partition can sometimes be a partitioned table,
+ allowing hierarchies to be created.
+ </p></dd><dd class="glossdef"><p>
+ In reference to a <a class="glossterm" href="glossary.html#GLOSSARY-WINDOW-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WINDOW-FUNCTION" title="Window function (routine)">window function</a></em></a>
+ in a <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">query</a></em></a>,
+ a partition is a user-defined criterion that identifies which neighboring
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a>
+ of the <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">query's result set</a></em></a>
+ can be considered by the function.
+ </p></dd><dt id="GLOSSARY-PARTITIONED-TABLE"><span class="glossterm">Partitioned table (relation)</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that is
+ in semantic terms the same as a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>,
+ but whose storage is distributed across several
+ <a class="glossterm" href="glossary.html#GLOSSARY-PARTITION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARTITION" title="Partition">partitions</a></em></a>.
+ </p></dd><dt id="GLOSSARY-POSTMASTER"><span class="glossterm">Postmaster (process)</span></dt><dd class="glossdef"><p>
+ The very first process of an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>.
+ It starts and manages the
+ <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary processes</a></em></a>
+ and creates <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend processes</a></em></a>
+ on demand.
+ </p><p>
+ For more information, see
+ <a class="xref" href="server-start.html" title="19.3. Starting the Database Server">Section 19.3</a>.
+ </p></dd><dt id="GLOSSARY-PRIMARY-KEY"><span class="glossterm">Primary key</span></dt><dd class="glossdef"><p>
+ A special case of a
+ <a class="glossterm" href="glossary.html#GLOSSARY-UNIQUE-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-UNIQUE-CONSTRAINT" title="Unique constraint">unique constraint</a></em></a>
+ defined on a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> or other
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that also
+ guarantees that all of the
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>
+ within the <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY" title="Primary key">primary key</a></em></a>
+ do not have <a class="glossterm" href="glossary.html#GLOSSARY-NULL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-NULL" title="Null">null</a></em></a> values.
+ As the name implies, there can be only one
+ primary key per table, though it is possible to have multiple unique
+ constraints that also have no null-capable attributes.
+ </p></dd><dt id="GLOSSARY-PRIMARY-SERVER"><span class="glossterm">Primary (server)</span></dt><dd class="glossdef"><p>
+ When two or more <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">databases</a></em></a>
+ are linked via <a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION" title="Replication">replication</a></em></a>,
+ the <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a>
+ that is considered the authoritative source of information is called
+ the <em class="firstterm">primary</em>,
+ also known as a <em class="firstterm">master</em>.
+ </p></dd><dt id="GLOSSARY-PROCEDURE"><span class="glossterm">Procedure (routine)</span></dt><dd class="glossdef"><p>
+ A type of routine.
+ Their distinctive qualities are that they do not return values,
+ and that they are allowed to make transactional statements such
+ as <code class="command">COMMIT</code> and <code class="command">ROLLBACK</code>.
+ They are invoked via the <code class="command">CALL</code> command.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createprocedure.html" title="CREATE PROCEDURE"><span class="refentrytitle">CREATE PROCEDURE</span></a>.
+ </p></dd><dt id="GLOSSARY-QUERY"><span class="glossterm">Query</span></dt><dd class="glossdef"><p>
+ A request sent by a client to a <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend</a></em></a>,
+ usually to return results or to modify data on the database.
+ </p></dd><dt id="GLOSSARY-PLANNER"><span class="glossterm">Query planner</span></dt><dd class="glossdef"><p>
+ The part of <span class="productname">PostgreSQL</span> that is devoted to
+ determining (<em class="firstterm">planning</em>) the most efficient way to
+ execute <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">queries</a></em></a>.
+ Also known as <em class="firstterm">query optimizer</em>,
+ <em class="firstterm">optimizer</em>, or simply <em class="firstterm">planner</em>.
+ </p></dd><dt><span class="glossterm">Record</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-TUPLE">Tuple</a>.</p></dd><dt><span class="glossterm">Recycling</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WAL-FILE">WAL file</a>.</p></dd><dt id="GLOSSARY-REFERENTIAL-INTEGRITY"><span class="glossterm">Referential integrity</span></dt><dd class="glossdef"><p>
+ A means of restricting data in one <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>
+ by a <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-KEY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-KEY" title="Foreign key">foreign key</a></em></a>
+ so that it must have matching data in another
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.
+ </p></dd><dt id="GLOSSARY-RELATION"><span class="glossterm">Relation</span></dt><dd class="glossdef"><p>
+ The generic term for all objects in a
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>
+ that have a name and a list of
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>
+ defined in a specific order.
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">Tables</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-SEQUENCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SEQUENCE" title="Sequence (relation)">sequences</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">views</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign tables</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW" title="Materialized view (relation)">materialized views</a></em></a>,
+ composite types, and
+ <a class="glossterm" href="glossary.html#GLOSSARY-INDEX"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INDEX" title="Index (relation)">indexes</a></em></a> are all relations.
+ </p><p>
+ More generically, a relation is a set of tuples; for example,
+ the result of a query is also a relation.
+ </p><p>
+ In <span class="productname">PostgreSQL</span>,
+ <em class="firstterm">Class</em> is an archaic synonym for
+ <em class="firstterm">relation</em>.
+ </p></dd><dt id="GLOSSARY-REPLICA"><span class="glossterm">Replica (server)</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> that is paired
+ with a <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER" title="Primary (server)">primary</a></em></a>
+ database and is maintaining a copy of some or all of the primary database's
+ data. The foremost reasons for doing this are to allow for greater access
+ to that data, and to maintain availability of the data in the event that
+ the <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER" title="Primary (server)">primary</a></em></a>
+ becomes unavailable.
+ </p></dd><dt id="GLOSSARY-REPLICATION"><span class="glossterm">Replication</span></dt><dd class="glossdef"><p>
+ The act of reproducing data on one
+ <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a> onto another
+ server called a <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>.
+ This can take the form of <em class="firstterm">physical replication</em>,
+ where all file changes from one server are copied verbatim,
+ or <em class="firstterm">logical replication</em> where a defined subset
+ of data changes are conveyed using a higher-level representation.
+ </p></dd><dt id="GLOSSARY-RESULT-SET"><span class="glossterm">Result set</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> transmitted
+ from a <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend process</a></em></a>
+ to a <a class="glossterm" href="glossary.html#GLOSSARY-CLIENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLIENT" title="Client (process)">client</a></em></a> upon the
+ completion of an <acronym class="acronym">SQL</acronym> command, usually a
+ <code class="command">SELECT</code> but it can be an
+ <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or
+ <code class="command">DELETE</code> command if the <code class="literal">RETURNING</code>
+ clause is specified.
+ </p><p>
+ The fact that a result set is a relation means that a query can be used
+ in the definition of another query, becoming a
+ <em class="firstterm">subquery</em>.
+ </p></dd><dd class="glossdef"><p>
+ </p></dd><dt id="GLOSSARY-REVOKE"><span class="glossterm">Revoke</span></dt><dd class="glossdef"><p>
+ A command to prevent access to a named set of
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> objects for a
+ named list of <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">roles</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-revoke.html" title="REVOKE"><span class="refentrytitle">REVOKE</span></a>.
+ </p></dd><dt id="GLOSSARY-ROLE"><span class="glossterm">Role</span></dt><dd class="glossdef"><p>
+ A collection of access privileges to the
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">instance</a></em></a>.
+ Roles are themselves a privilege that can be granted to other roles.
+ This is often done for convenience or to ensure completeness
+ when multiple <a class="glossterm" href="glossary.html#GLOSSARY-USER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-USER" title="User">users</a></em></a> need
+ the same privileges.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createrole.html" title="CREATE ROLE"><span class="refentrytitle">CREATE ROLE</span></a>.
+ </p></dd><dt id="GLOSSARY-ROLLBACK"><span class="glossterm">Rollback</span></dt><dd class="glossdef"><p>
+ A command to undo all of the operations performed since the beginning
+ of a <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-rollback.html" title="ROLLBACK"><span class="refentrytitle">ROLLBACK</span></a>.
+ </p></dd><dt id="GLOSSARY-ROUTINE"><span class="glossterm">Routine</span></dt><dd class="glossdef"><p>
+ A defined set of instructions stored in the database system
+ that can be invoked for execution.
+ A routine can be written in a variety of programming
+ languages. Routines can be
+ <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">functions</a></em></a>
+ (including set-returning functions and
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER" title="Trigger">trigger functions</a></em></a>),
+ <a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE" title="Aggregate function (routine)">aggregate functions</a></em></a>,
+ and <a class="glossterm" href="glossary.html#GLOSSARY-PROCEDURE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PROCEDURE" title="Procedure (routine)">procedures</a></em></a>.
+ </p><p>
+ Many routines are already defined within <span class="productname">PostgreSQL</span>
+ itself, but user-defined ones can also be added.
+ </p></dd><dt><span class="glossterm">Row</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-TUPLE">Tuple</a>.</p></dd><dt id="GLOSSARY-SAVEPOINT"><span class="glossterm">Savepoint</span></dt><dd class="glossdef"><p>
+ A special mark in the sequence of steps in a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>.
+ Data modifications after this point in time may be reverted
+ to the time of the savepoint.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-savepoint.html" title="SAVEPOINT"><span class="refentrytitle">SAVEPOINT</span></a>.
+ </p></dd><dt id="GLOSSARY-SCHEMA"><span class="glossterm">Schema</span></dt><dd class="glossdef"><p>
+ A schema is a namespace for
+ <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">SQL objects</a></em></a>,
+ which all reside in the same
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.
+ Each SQL object must reside in exactly one schema.
+ </p><p>
+ All system-defined SQL objects reside in schema <code class="literal">pg_catalog</code>.
+ </p></dd><dd class="glossdef"><p>
+ More generically, the term <em class="firstterm">schema</em> is used to mean
+ all data descriptions (<a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> definitions,
+ <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraints</a></em></a>, comments, etc.)
+ for a given <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> or
+ subset thereof.
+ </p><p>
+ For more information, see
+ <a class="xref" href="ddl-schemas.html" title="5.9. Schemas">Section 5.9</a>.
+ </p></dd><dt><span class="glossterm">Segment</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-FILE-SEGMENT">File segment</a>.</p></dd><dt id="GLOSSARY-SELECT"><span class="glossterm">Select</span></dt><dd class="glossdef"><p>
+ The <acronym class="acronym">SQL</acronym> command used to request data from a
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.
+ Normally, <code class="command">SELECT</code> commands are not expected to modify the
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> in any way,
+ but it is possible that
+ <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">functions</a></em></a> invoked within
+ the query could have side effects that do modify data.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-select.html" title="SELECT"><span class="refentrytitle">SELECT</span></a>.
+ </p></dd><dt id="GLOSSARY-SEQUENCE"><span class="glossterm">Sequence (relation)</span></dt><dd class="glossdef"><p>
+ A type of relation that is used to generate values.
+ Typically the generated values are sequential non-repeating numbers.
+ They are commonly used to generate surrogate
+ <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY" title="Primary key">primary key</a></em></a>
+ values.
+ </p></dd><dt id="GLOSSARY-SERVER"><span class="glossterm">Server</span></dt><dd class="glossdef"><p>
+ A computer on which <span class="productname">PostgreSQL</span>
+ <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instances</a></em></a> run.
+ The term <em class="firstterm">server</em> denotes real hardware, a
+ container, or a <em class="firstterm">virtual machine</em>.
+ </p><p>
+ This term is sometimes used to refer to an instance or to a host.
+ </p></dd><dt id="GLOSSARY-SESSION"><span class="glossterm">Session</span></dt><dd class="glossdef"><p>
+ A state that allows a client and a backend to interact,
+ communicating over a <a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION" title="Connection">connection</a></em></a>.
+ </p></dd><dt id="GLOSSARY-SHARED-MEMORY"><span class="glossterm">Shared memory</span></dt><dd class="glossdef"><p>
+ <acronym class="acronym">RAM</acronym> which is used by the processes common to an
+ <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>.
+ It mirrors parts of <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>
+ files, provides a transient area for
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL records</a></em></a>,
+ and stores additional common information.
+ Note that shared memory belongs to the complete instance, not to a single
+ database.
+ </p><p>
+ The largest part of shared memory is known as <em class="firstterm">shared buffers</em>
+ and is used to mirror part of data files, organized into pages.
+ When a page is modified, it is called a dirty page until it is
+ written back to the file system.
+ </p><p>
+ For more information, see
+ <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-MEMORY" title="20.4.1. Memory">Section 20.4.1</a>.
+ </p></dd><dt id="GLOSSARY-SQL-OBJECT"><span class="glossterm">SQL object</span></dt><dd class="glossdef"><p>
+ Any object that can be created with a <code class="command">CREATE</code>
+ command. Most objects are specific to one database, and are commonly
+ known as <em class="firstterm">local objects</em>.
+ </p><p>
+ Most local objects reside in a specific
+ <a class="glossterm" href="glossary.html#GLOSSARY-SCHEMA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SCHEMA" title="Schema">schema</a></em></a> in their
+ containing database, such as
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a> (all types),
+ <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">routines</a></em></a> (all types),
+ data types, etc.
+ The names of such objects of the same type in the same schema
+ are enforced to be unique.
+ </p><p>
+ There also exist local objects that do not reside in schemas; some examples are
+ <a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION" title="Extension">extensions</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-CAST"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CAST" title="Cast">data type casts</a></em></a>, and
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrappers</a></em></a>.
+ The names of such objects of the same type are enforced to be unique
+ within the database.
+ </p><p>
+ Other object types, such as
+ <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">roles</a></em></a>,
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE" title="Tablespace">tablespaces</a></em></a>,
+ replication origins, subscriptions for logical replication, and
+ databases themselves are not local SQL objects since they exist
+ entirely outside of any specific database;
+ they are called <em class="firstterm">global objects</em>.
+ The names of such objects are enforced to be unique within the whole
+ database cluster.
+ </p><p>
+ For more information, see
+ <a class="xref" href="manage-ag-overview.html" title="23.1. Overview">Section 23.1</a>.
+ </p></dd><dt id="GLOSSARY-SQL-STANDARD"><span class="glossterm">SQL standard</span></dt><dd class="glossdef"><p>
+ A series of documents that define the <acronym class="acronym">SQL</acronym> language.
+ </p></dd><dt><span class="glossterm">Standby (server)</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-REPLICA">Replica (server)</a>.</p></dd><dt id="GLOSSARY-STARTUP-PROCESS"><span class="glossterm">Startup process</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ that replays WAL during crash recovery and in a
+ <a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION" title="Replication">physical replica</a></em></a>.
+ </p><p>
+ (The name is historical: the startup process was named before
+ replication was implemented; the name refers to its task as it
+ relates to the server startup following a crash.)
+ </p></dd><dt id="GLOSSARY-SYSTEM-CATALOG"><span class="glossterm">System catalog</span></dt><dd class="glossdef"><p>
+ A collection of <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">tables</a></em></a>
+ which describe the structure of all
+ <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">SQL objects</a></em></a>
+ of the instance.
+ The system catalog resides in the schema <code class="literal">pg_catalog</code>.
+ These tables contain data in internal representation and are
+ not typically considered useful for user examination;
+ a number of user-friendlier <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">views</a></em></a>,
+ also in schema <code class="literal">pg_catalog</code>, offer more convenient access to
+ some of that information, while additional tables and views
+ exist in schema <code class="literal">information_schema</code>
+ (see <a class="xref" href="information-schema.html" title="Chapter 37. The Information Schema">Chapter 37</a>) that expose some
+ of the same and additional information as mandated by the
+ <a class="glossterm" href="glossary.html#GLOSSARY-SQL-STANDARD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-STANDARD" title="SQL standard">SQL standard</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="ddl-schemas.html" title="5.9. Schemas">Section 5.9</a>.
+ </p></dd><dt id="GLOSSARY-TABLE"><span class="glossterm">Table</span></dt><dd class="glossdef"><p>
+ A collection of <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuples</a></em></a> having
+ a common data structure (the same number of
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>, in the same
+ order, having the same name and type per position).
+ A table is the most common form of
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> in
+ <span class="productname">PostgreSQL</span>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>.
+ </p></dd><dt id="GLOSSARY-TABLESPACE"><span class="glossterm">Tablespace</span></dt><dd class="glossdef"><p>
+ A named location on the server file system.
+ All <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">SQL objects</a></em></a>
+ which require storage beyond their definition in the
+ <a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG" title="System catalog">system catalog</a></em></a>
+ must belong to a single tablespace.
+ Initially, a database cluster contains a single usable tablespace which is
+ used as the default for all SQL objects, called <code class="literal">pg_default</code>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="manage-ag-tablespaces.html" title="23.6. Tablespaces">Section 23.6</a>.
+ </p></dd><dt id="GLOSSARY-TEMPORARY-TABLE"><span class="glossterm">Temporary table</span></dt><dd class="glossdef"><p>
+ <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">Tables</a></em></a> that exist either
+ for the lifetime of a
+ <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">session</a></em></a> or a
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>, as
+ specified at the time of creation.
+ The data in them is not visible to other sessions, and is not
+ <a class="glossterm" href="glossary.html#GLOSSARY-LOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOGGED" title="Logged">logged</a></em></a>.
+ Temporary tables are often used to store intermediate data for a
+ multi-step operation.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>.
+ </p></dd><dt id="GLOSSARY-TOAST"><span class="glossterm">TOAST</span></dt><dd class="glossdef"><p>
+ A mechanism by which large attributes of table rows are split and
+ stored in a secondary table, called the <em class="firstterm">TOAST table</em>.
+ Each relation with large attributes has its own TOAST table.
+ </p><p>
+ For more information, see
+ <a class="xref" href="storage-toast.html" title="73.2. TOAST">Section 73.2</a>.
+ </p></dd><dt id="GLOSSARY-TRANSACTION"><span class="glossterm">Transaction</span></dt><dd class="glossdef"><p>
+ A combination of commands that must act as a single
+ <a class="glossterm" href="glossary.html#GLOSSARY-ATOMIC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATOMIC" title="Atomic">atomic</a></em></a> command: they all
+ succeed or all fail as a single unit, and their effects are not visible to
+ other <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">sessions</a></em></a> until
+ the transaction is complete, and possibly even later, depending on the
+ isolation level.
+ </p><p>
+ For more information, see
+ <a class="xref" href="transaction-iso.html" title="13.2. Transaction Isolation">Section 13.2</a>.
+ </p></dd><dt id="GLOSSARY-XID"><span class="glossterm">Transaction ID</span></dt><dd class="glossdef"><p>
+ The numerical, unique, sequentially-assigned identifier that each
+ transaction receives when it first causes a database modification.
+ Frequently abbreviated as <em class="firstterm">xid</em>.
+ When stored on disk, xids are only 32-bits wide, so only
+ approximately four billion write transaction IDs can be generated;
+ to permit the system to run for longer than that,
+ <em class="firstterm">epochs</em> are used, also 32 bits wide.
+ When the counter reaches the maximum xid value, it starts over at
+ <code class="literal">3</code> (values under that are reserved) and the
+ epoch value is incremented by one.
+ In some contexts, the epoch and xid values are
+ considered together as a single 64-bit value.
+ </p><p>
+ For more information, see
+ <a class="xref" href="datatype-oid.html" title="8.19. Object Identifier Types">Section 8.19</a>.
+ </p></dd><dt id="GLOSSARY-TPS"><span class="glossterm">Transactions per second (TPS)</span></dt><dd class="glossdef"><p>
+ Average number of transactions that are executed per second,
+ totaled across all sessions active for a measured run.
+ This is used as a measure of the performance characteristics of
+ an instance.
+ </p></dd><dt id="GLOSSARY-TRIGGER"><span class="glossterm">Trigger</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">function</a></em></a> which can
+ be defined to execute whenever a certain operation (<code class="command">INSERT</code>,
+ <code class="command">UPDATE</code>, <code class="command">DELETE</code>,
+ <code class="command">TRUNCATE</code>) is applied to a
+ <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.
+ A trigger executes within the same
+ <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a> as the
+ statement which invoked it, and if the function fails, then the invoking
+ statement also fails.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createtrigger.html" title="CREATE TRIGGER"><span class="refentrytitle">CREATE TRIGGER</span></a>.
+ </p></dd><dt id="GLOSSARY-TUPLE"><span class="glossterm">Tuple</span></dt><dd class="glossdef"><p>
+ A collection of <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>
+ in a fixed order.
+ That order may be defined by the <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>
+ (or other <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>)
+ where the tuple is contained, in which case the tuple is often called a
+ <em class="firstterm">row</em>. It may also be defined by the structure of a
+ result set, in which case it is sometimes called a <em class="firstterm">record</em>.
+ </p></dd><dt id="GLOSSARY-UNIQUE-CONSTRAINT"><span class="glossterm">Unique constraint</span></dt><dd class="glossdef"><p>
+ A type of <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraint</a></em></a>
+ defined on a <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>
+ which restricts the values allowed in one or a combination of columns
+ so that each value or combination of values can only appear once in the
+ relation — that is, no other row in the relation contains values
+ that are equal to those.
+ </p><p>
+ Because <a class="glossterm" href="glossary.html#GLOSSARY-NULL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-NULL" title="Null">null values</a></em></a> are
+ not considered equal to each other, multiple rows with null values are
+ allowed to exist without violating the unique constraint.
+ </p></dd><dt id="GLOSSARY-UNLOGGED"><span class="glossterm">Unlogged</span></dt><dd class="glossdef"><p>
+ The property of certain <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a>
+ that the changes to them are not reflected in the
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>.
+ This disables replication and crash recovery for these relations.
+ </p><p>
+ The primary use of unlogged tables is for storing
+ transient work data that must be shared across processes.
+ </p><p>
+ <a class="glossterm" href="glossary.html#GLOSSARY-TEMPORARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TEMPORARY-TABLE" title="Temporary table">Temporary tables</a></em></a>
+ are always unlogged.
+ </p></dd><dt id="GLOSSARY-UPDATE"><span class="glossterm">Update</span></dt><dd class="glossdef"><p>
+ An <acronym class="acronym">SQL</acronym> command used to modify
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a>
+ that may already exist in a specified <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>.
+ It cannot create or remove rows.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-update.html" title="UPDATE"><span class="refentrytitle">UPDATE</span></a>.
+ </p></dd><dt id="GLOSSARY-USER"><span class="glossterm">User</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">role</a></em></a> that has the
+ <code class="literal">LOGIN</code> privilege.
+ </p></dd><dt id="GLOSSARY-USER-MAPPING"><span class="glossterm">User mapping</span></dt><dd class="glossdef"><p>
+ The translation of login credentials in the local
+ <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> to credentials
+ in a remote data system defined by a
+ <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrapper</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createusermapping.html" title="CREATE USER MAPPING"><span class="refentrytitle">CREATE USER MAPPING</span></a>.
+ </p></dd><dt id="GLOSSARY-VACUUM"><span class="glossterm">Vacuum</span></dt><dd class="glossdef"><p>
+ The process of removing outdated
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuple versions</a></em></a>
+ from tables or materialized views, and other closely related
+ processing required by <span class="productname">PostgreSQL</span>'s
+ implementation of <a class="glossterm" href="glossary.html#GLOSSARY-MVCC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MVCC" title="Multi-version concurrency control (MVCC)">MVCC</a></em></a>.
+ This can be initiated through the use of
+ the <code class="command">VACUUM</code> command, but can also be handled automatically
+ via <a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM" title="Autovacuum (process)">autovacuum</a></em></a> processes.
+ </p><p>
+ For more information, see
+ <a class="xref" href="routine-vacuuming.html" title="25.1. Routine Vacuuming">Section 25.1</a> .
+ </p></dd><dt id="GLOSSARY-VIEW"><span class="glossterm">View</span></dt><dd class="glossdef"><p>
+ A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that is defined by a
+ <code class="command">SELECT</code> statement, but has no storage of its own.
+ Any time a query references a view, the definition of the view is
+ substituted into the query as if the user had typed it as a subquery
+ instead of the name of the view.
+ </p><p>
+ For more information, see
+ <a class="xref" href="sql-createview.html" title="CREATE VIEW"><span class="refentrytitle">CREATE VIEW</span></a>.
+ </p></dd><dt id="GLOSSARY-VM"><span class="glossterm">Visibility map (fork)</span></dt><dd class="glossdef"><p>
+ A storage structure that keeps metadata about each data page
+ of a table's main fork. The visibility map entry for
+ each page stores two bits: the first one
+ (<code class="literal">all-visible</code>) indicates that all tuples
+ in the page are visible to all transactions. The second one
+ (<code class="literal">all-frozen</code>) indicates that all tuples
+ in the page are marked frozen.
+ </p></dd><dt><span class="glossterm">WAL</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WAL">Write-ahead log</a>.</p></dd><dt id="GLOSSARY-WAL-ARCHIVER"><span class="glossterm">WAL archiver (process)</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ which, if enabled, saves copies of
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL files</a></em></a>
+ for the purpose of creating backups or keeping
+ <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replicas</a></em></a> current.
+ </p><p>
+ For more information, see
+ <a class="xref" href="continuous-archiving.html" title="26.3. Continuous Archiving and Point-in-Time Recovery (PITR)">Section 26.3</a>.
+ </p></dd><dt id="GLOSSARY-WAL-FILE"><span class="glossterm">WAL file</span></dt><dd class="glossdef"><p>
+ Also known as <em class="firstterm">WAL segment</em> or
+ <em class="firstterm">WAL segment file</em>.
+ Each of the sequentially-numbered files that provide storage space for
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>.
+ The files are all of the same predefined size
+ and are written in sequential order, interspersing changes
+ as they occur in multiple simultaneous sessions.
+ If the system crashes, the files are read in order, and each of the
+ changes is replayed to restore the system to the state it was in
+ before the crash.
+ </p><p>
+ Each WAL file can be released after a
+ <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT" title="Checkpoint">checkpoint</a></em></a>
+ writes all the changes in it to the corresponding data files.
+ Releasing the file can be done either by deleting it, or by changing its
+ name so that it will be used in the future, which is called
+ <em class="firstterm">recycling</em>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="wal-internals.html" title="30.6. WAL Internals">Section 30.6</a>.
+ </p></dd><dt id="GLOSSARY-WAL-RECORD"><span class="glossterm">WAL record</span></dt><dd class="glossdef"><p>
+ A low-level description of an individual data change.
+ It contains sufficient information for the data change to be
+ re-executed (<em class="firstterm">replayed</em>) in case a system failure
+ causes the change to be lost.
+ WAL records use a non-printable binary format.
+ </p><p>
+ For more information, see
+ <a class="xref" href="wal-internals.html" title="30.6. WAL Internals">Section 30.6</a>.
+ </p></dd><dt id="GLOSSARY-WAL-RECEIVER"><span class="glossterm">WAL receiver (process)</span></dt><dd class="glossdef"><p>
+ An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>
+ that runs on a <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>
+ to receive WAL from the
+ <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER" title="Primary (server)">primary server</a></em></a>
+ for replay by the
+ <a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS" title="Startup process">startup process</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="warm-standby.html" title="27.2. Log-Shipping Standby Servers">Section 27.2</a>.
+ </p></dd><dt><span class="glossterm">WAL segment</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WAL-FILE">WAL file</a>.</p></dd><dt id="GLOSSARY-WAL-SENDER"><span class="glossterm">WAL sender (process)</span></dt><dd class="glossdef"><p>
+ A special <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend process</a></em></a>
+ that streams WAL over a network. The receiving end can be a
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER" title="WAL receiver (process)">WAL receiver</a></em></a>
+ in a <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>,
+ <a class="xref" href="app-pgreceivewal.html" title="pg_receivewal"><span class="refentrytitle"><span class="application">pg_receivewal</span></span></a>, or any other client program
+ that speaks the replication protocol.
+ </p></dd><dt id="GLOSSARY-WAL-WRITER"><span class="glossterm">WAL writer (process)</span></dt><dd class="glossdef"><p>
+ A process that writes <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL records</a></em></a>
+ from <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared memory</a></em></a> to
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL files</a></em></a>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="runtime-config-wal.html" title="20.5. Write Ahead Log">Section 20.5</a>.
+ </p></dd><dt id="GLOSSARY-WINDOW-FUNCTION"><span class="glossterm">Window function (routine)</span></dt><dd class="glossdef"><p>
+ A type of <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">function</a></em></a>
+ used in a <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">query</a></em></a>
+ that applies to a <a class="glossterm" href="glossary.html#GLOSSARY-PARTITION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARTITION" title="Partition">partition</a></em></a>
+ of the query's <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">result set</a></em></a>;
+ the function's result is based on values found in
+ <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a> of the same partition or frame.
+ </p><p>
+ All <a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE" title="Aggregate function (routine)">aggregate functions</a></em></a>
+ can be used as window functions, but window functions can also be
+ used to, for example, give ranks to each of the rows in the partition.
+ Also known as <em class="firstterm">analytic functions</em>.
+ </p><p>
+ For more information, see
+ <a class="xref" href="tutorial-window.html" title="3.5. Window Functions">Section 3.5</a>.
+ </p></dd><dt id="GLOSSARY-WAL"><span class="glossterm">Write-ahead log</span></dt><dd class="glossdef"><p>
+ The journal that keeps track of the changes in the
+ <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>
+ as user- and system-invoked operations take place.
+ It comprises many individual
+ <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL records</a></em></a> written
+ sequentially to <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL files</a></em></a>.
+ </p></dd></dl></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="acronyms.html" title="Appendix L. Acronyms">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendixes.html" title="Part VIII. Appendixes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="color.html" title="Appendix N. Color Support">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Appendix L. Acronyms </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Appendix N. Color Support</td></tr></table></div></body></html> \ No newline at end of file