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diff --git a/doc/src/sgml/html/glossary.html b/doc/src/sgml/html/glossary.html new file mode 100644 index 0000000..67c1e3d --- /dev/null +++ b/doc/src/sgml/html/glossary.html @@ -0,0 +1,998 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 V1.79.1" /><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 xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 13.4 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></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 process 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. + </p><p> + For more information, see + <a class="xref" href="routine-vacuuming.html#AUTOVACUUM" title="24.1.6. The Autovacuum Daemon">Section 24.1.6</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 47. Background Worker Processes">Chapter 47</a>. + </p></dd><dt id="GLOSSARY-BACKGROUND-WRITER"><span class="glossterm">Background writer (process)</span></dt><dd class="glossdef"><p> + A process 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="19.4.5. Background Writer">Section 19.4.5</a>. + </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="22.1. Overview">Section 22.1</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="29.4. WAL Configuration">Section 29.4</a>. + </p></dd><dt id="GLOSSARY-CHECKPOINTER"><span class="glossterm">Checkpointer (process)</span></dt><dd class="glossdef"><p> + A specialized process responsible for executing checkpoints. + </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="19.3. Connections and Authentication">Section 19.3</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 + + <em class="firstterm">domain</em>. + </p><p> + For more information, see + <a class="xref" href="ddl-constraints.html" title="5.4. Constraints">Section 5.4</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="22.1. Overview">Section 22.1</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="69.1. Database File Layout">Section 69.1</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-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="37.17. Packaging Related Objects into an Extension">Section 37.17</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="69.1. Database File Layout">Section 69.1</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="69.3. Free Space Map">Section 69.3</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 backend and auxiliary processes 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="24.3. Log File Maintenance">Section 24.3</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> + If activated, the process + 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="19.8. Error Reporting and Logging">Section 19.8</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 other auxiliary processes 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="18.3. Starting the Database Server">Section 18.3</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="19.4.1. Memory">Section 19.4.1</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 belong to 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 belong to 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="22.1. Overview">Section 22.1</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-STATS-COLLECTOR"><span class="glossterm">Stats collector (process)</span></dt><dd class="glossdef"><p> + This process collects 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="27.2. The Statistics Collector">Section 27.2</a>. + </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 36. The Information Schema">Chapter 36</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="22.6. Tablespaces">Section 22.6</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="69.2. TOAST">Section 69.2</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="24.1. Routine Vacuuming">Section 24.1</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> + A process that 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="25.3. Continuous Archiving and Point-in-Time Recovery (PITR)">Section 25.3</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="29.5. WAL Internals">Section 29.5</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="29.5. WAL Internals">Section 29.5</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-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="19.5. Write Ahead Log">Section 19.5</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 xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="acronyms.html" title="Appendix L. 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