diff options
Diffstat (limited to 'doc/src/sgml/glossary.sgml')
| -rw-r--r-- | doc/src/sgml/glossary.sgml | 2213 |
1 files changed, 2213 insertions, 0 deletions
diff --git a/doc/src/sgml/glossary.sgml b/doc/src/sgml/glossary.sgml new file mode 100644 index 0000000..5815fa4 --- /dev/null +++ b/doc/src/sgml/glossary.sgml @@ -0,0 +1,2213 @@ +<appendix id="glossary"> + <title>Glossary</title> + <para> + This is a list of terms and their meaning in the context of + <productname>PostgreSQL</productname> and relational database + systems in general. + </para> + + <glosslist> + <glossentry id="glossary-acid"> + <glossterm>ACID</glossterm> + <glossdef> + <para> + <glossterm linkend="glossary-atomicity">Atomicity</glossterm>, + <glossterm linkend="glossary-consistency">Consistency</glossterm>, + <glossterm linkend="glossary-isolation">Isolation</glossterm>, and + <glossterm linkend="glossary-durability">Durability</glossterm>. + This set of properties of database transactions is intended to + guarantee validity in concurrent operation and even in event of + errors, power failures, etc. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-aggregate"> + <glossterm>Aggregate function (routine)</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-function">function</glossterm> that + combines (<firstterm>aggregates</firstterm>) multiple input values, + for example by counting, averaging or adding, + yielding a single output value. + </para> + <para> + For more information, see + <xref linkend="functions-aggregate"/>. + </para> + <glossseealso otherterm="glossary-window-function" /> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Analytic function</glossterm> + <glosssee otherterm="glossary-window-function" /> + </glossentry> + + <glossentry id="glossary-analyze"> + <glossterm>Analyze (operation)</glossterm> + <glossdef> + <para> + The act of collecting statistics from data in + <glossterm linkend="glossary-table">tables</glossterm> + and other <glossterm linkend="glossary-relation">relations</glossterm> + to help the <glossterm linkend="glossary-planner">query planner</glossterm> + to make decisions about how to execute + <glossterm linkend="glossary-query">queries</glossterm>. + </para> + <para> + (Don't confuse this term with the <literal>ANALYZE</literal> option + to the <xref linkend="sql-explain"/> command.) + </para> + <para> + For more information, see + <xref linkend="sql-analyze"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-atomic"> + <glossterm>Atomic</glossterm> + <glossdef> + <para> + In reference to a <glossterm linkend="glossary-datum">datum</glossterm>: + the fact that its value cannot be broken down into smaller + components. + </para> + </glossdef> + <glossdef> + <para> + In reference to a + <glossterm linkend="glossary-transaction">database transaction</glossterm>: + see <glossterm linkend="glossary-atomicity">atomicity</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-atomicity"> + <glossterm>Atomicity</glossterm> + <glossdef> + <para> + The property of a <glossterm linkend="glossary-transaction">transaction</glossterm> + 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>ACID</acronym> properties. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-attribute"> + <glossterm>Attribute</glossterm> + <glossdef> + <para> + An element with a certain name and data type found within a + <glossterm linkend="glossary-tuple">tuple</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-autovacuum"> + <glossterm>Autovacuum (process)</glossterm> + <glossdef> + <para> + A set of background processes that routinely perform + <glossterm linkend="glossary-vacuum">vacuum</glossterm> + and <glossterm linkend="glossary-analyze">analyze</glossterm> operations. + The <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + that coordinates the work and is always present (unless autovacuum + is disabled) is known as the <firstterm>autovacuum launcher</firstterm>, + and the processes that carry out the tasks are known as the + <firstterm>autovacuum workers</firstterm>. + </para> + <para> + For more information, see + <xref linkend="autovacuum"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-auxiliary-proc"> + <glossterm>Auxiliary process</glossterm> + <glossdef> + <para> + A process within an <glossterm linkend="glossary-instance">instance</glossterm> + that is in charge of some specific background task for the instance. + The auxiliary processes consist of <!-- in alphabetical order --> + <!-- NB: In the code, the autovac launcher doesn't use the auxiliary + process scaffolding; however it does behave as one so we list it + here anyway. In addition, logger isn't connected to shared memory so + most code outside postmaster.c doesn't even consider it a "proc" in + the first place. + --> + the <glossterm linkend="glossary-autovacuum">autovacuum launcher</glossterm> + (but not the autovacuum workers), + the <glossterm linkend="glossary-background-writer">background writer</glossterm>, + the <glossterm linkend="glossary-checkpointer">checkpointer</glossterm>, + the <glossterm linkend="glossary-logger">logger</glossterm>, + the <glossterm linkend="glossary-startup-process">startup process</glossterm>, + the <glossterm linkend="glossary-wal-archiver">WAL archiver</glossterm>, + the <glossterm linkend="glossary-wal-receiver">WAL receiver</glossterm> + (but not the <glossterm linkend="glossary-wal-sender">WAL senders</glossterm>), + and the <glossterm linkend="glossary-wal-writer">WAL writer</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-backend"> + <glossterm>Backend (process)</glossterm> + <glossdef> + <para> + Process of an <glossterm linkend="glossary-instance">instance</glossterm> + which acts on behalf of a <glossterm linkend="glossary-session">client session</glossterm> + and handles its requests. + </para> + <para> + (Don't confuse this term with the similar terms + <glossterm linkend="glossary-background-worker">Background Worker</glossterm> or + <glossterm linkend="glossary-background-writer">Background Writer</glossterm>). + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-background-worker"> + <glossterm>Background worker (process)</glossterm> + <glossdef> + <para> + Process within an <glossterm linkend="glossary-instance">instance</glossterm>, + which runs system- or user-supplied code. + Serves as infrastructure for several features in + <productname>PostgreSQL</productname>, such as + <glossterm linkend="glossary-replication">logical replication</glossterm> + and <glossterm linkend="glossary-parallel-query">parallel queries</glossterm>. + In addition, <glossterm linkend="glossary-extension">Extensions</glossterm> can add + custom background worker processes. + </para> + <para> + For more information, see + <xref linkend="bgworker"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-background-writer"> + <glossterm>Background writer (process)</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + that writes dirty + <glossterm linkend="glossary-data-page">data pages</glossterm> from + <glossterm linkend="glossary-shared-memory">shared memory</glossterm> to + the file system. It wakes up periodically, but works only for a short + period in order to distribute its expensive <acronym>I/O</acronym> + activity over time to avoid generating larger + <acronym>I/O</acronym> peaks which could block other processes. + </para> + <para> + For more information, see + <xref linkend="runtime-config-resource-background-writer"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-basebackup"> + <glossterm>Base Backup</glossterm> + <glossdef> + <para> + A binary copy of all + <glossterm linkend="glossary-db-cluster">database cluster</glossterm> + files. It is generated by the tool <xref linkend="app-pgbasebackup"/>. + In combination with WAL files it can be used as the starting point + for recovery, log shipping, or streaming replication. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-bloat"> + <glossterm>Bloat</glossterm> + <glossdef> + <para> + Space in data pages which does not contain current row versions, + such as unused (free) space or outdated row versions. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-bootstrap-superuser"> + <glossterm>Bootstrap superuser</glossterm> + <glossdef> + <para> + The first <glossterm linkend="glossary-user">user</glossterm> initialized in a + <glossterm linkend="glossary-db-cluster">database cluster</glossterm>. + </para> + <para> + This user owns all system catalog tables in each database. It is also the role + from which all granted permissions originate. Because of these things, this + role may not be dropped. + </para> + <para> + This role also behaves as a normal + <glossterm linkend="glossary-database-superuser">database superuser</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-buffer-access-strategy"> + <glossterm>Buffer Access Strategy</glossterm> + <glossdef> + <para> + Some operations will access a large number of + <glossterm linkend="glossary-data-page">pages</glossterm>. A + <firstterm>Buffer Access Strategy</firstterm> helps to prevent these + operations from evicting too many pages from + <glossterm linkend="glossary-shared-memory">shared buffers</glossterm>. + </para> + <para> + A Buffer Access Strategy sets up references to a limited number of + <glossterm linkend="glossary-shared-memory">shared buffers</glossterm> and + reuses them circularly. When the operation requires a new page, a victim + buffer is chosen from the buffers in the strategy ring, which may require + flushing the page's dirty data and possibly also unflushed + <glossterm linkend="glossary-wal">WAL</glossterm> to permanent storage. + </para> + <para> + Buffer Access Strategies are used for various operations such as + sequential scans of large tables, <command>VACUUM</command>, + <command>COPY</command>, <command>CREATE TABLE AS SELECT</command>, + <command>ALTER TABLE</command>, <command>CREATE DATABASE</command>, + <command>CREATE INDEX</command>, and <command>CLUSTER</command>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-cast"> + <glossterm>Cast</glossterm> + <glossdef> + <para> + A conversion of a <glossterm linkend="glossary-datum">datum</glossterm> + from its current data type to another data type. + </para> + <para> + For more information, see + <xref linkend="sql-createcast"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-catalog"> + <glossterm>Catalog</glossterm> + <glossdef> + <para> + The <acronym>SQL</acronym> standard uses this term to + indicate what is called a + <glossterm linkend="glossary-database">database</glossterm> in + <productname>PostgreSQL</productname>'s terminology. + </para> + <para> + (Don't confuse this term with + <glossterm linkend="glossary-system-catalog">system catalog</glossterm>). + </para> + <para> + For more information, see + <xref linkend="manage-ag-overview"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-check-constraint"> + <glossterm>Check constraint</glossterm> + <glossdef> + <para> + A type of <glossterm linkend="glossary-constraint">constraint</glossterm> + defined on a <glossterm linkend="glossary-relation">relation</glossterm> + which restricts the values allowed in one or more + <glossterm linkend="glossary-attribute">attributes</glossterm>. 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. + </para> + <para> + For more information, see + <xref linkend="ddl-constraints"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-checkpoint"> + <glossterm>Checkpoint</glossterm> + <glossdef> + <para> + A point in the <glossterm linkend="glossary-wal">WAL</glossterm> sequence + at which it is guaranteed that the heap and index data files have been + updated with all information from + <glossterm linkend="glossary-shared-memory">shared memory</glossterm> + modified before that checkpoint; + a <firstterm>checkpoint record</firstterm> is written and flushed to WAL + to mark that point. + </para> + <para> + 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 <command>CHECKPOINT</command>. + </para> + <para> + For more information, see + <xref linkend="wal-configuration"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-checkpointer"> + <glossterm>Checkpointer (process)</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + that is responsible for executing + <glossterm linkend="glossary-checkpoint">checkpoints</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Class (archaic)</glossterm> + <glosssee otherterm="glossary-relation" /> + </glossentry> + + <glossentry id="glossary-client"> + <glossterm>Client (process)</glossterm> + <glossdef> + <para> + Any process, possibly remote, that establishes a + <glossterm linkend="glossary-session">session</glossterm> + by <glossterm linkend="glossary-connection">connecting</glossterm> to an + <glossterm linkend="glossary-instance">instance</glossterm> + to interact with a <glossterm linkend="glossary-database">database</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-cluster-owner"> + <glossterm>Cluster owner</glossterm> + <glossdef> + <para> + The operating system user that owns the + <glossterm linkend="glossary-data-directory">data directory</glossterm> + and under which the <literal>postgres</literal> process is run. + It is required that this user exist prior to creating a new + <glossterm linkend="glossary-db-cluster">database cluster</glossterm>. + </para> + <para> + On operating systems with a <literal>root</literal> user, + said user is not allowed to be the cluster owner. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-column"> + <glossterm>Column</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-attribute">attribute</glossterm> found in + a <glossterm linkend="glossary-table">table</glossterm> or + <glossterm linkend="glossary-view">view</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-commit"> + <glossterm>Commit</glossterm> + <glossdef> + <para> + The act of finalizing a + <glossterm linkend="glossary-transaction">transaction</glossterm> within + the <glossterm linkend="glossary-database">database</glossterm>, which + makes it visible to other transactions and assures its + <glossterm linkend="glossary-durability">durability</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-commit"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-concurrency"> + <glossterm>Concurrency</glossterm> + <glossdef> + <para> + The concept that multiple independent operations happen within the + <glossterm linkend="glossary-database">database</glossterm> at the same time. + In <productname>PostgreSQL</productname>, concurrency is controlled by + the <glossterm linkend="glossary-mvcc">multiversion concurrency control</glossterm> + mechanism. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-connection"> + <glossterm>Connection</glossterm> + <glossdef> + <para> + An established line of communication between a client process and a + <glossterm linkend="glossary-backend">backend</glossterm> process, + usually over a network, supporting a + <glossterm linkend="glossary-session">session</glossterm>. This term is + sometimes used as a synonym for session. + </para> + <para> + For more information, see + <xref linkend="runtime-config-connection"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-consistency"> + <glossterm>Consistency</glossterm> + <glossdef> + <para> + The property that the data in the + <glossterm linkend="glossary-database">database</glossterm> + is always in compliance with + <glossterm linkend="glossary-constraint">integrity constraints</glossterm>. + 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 + <glossterm linkend="glossary-rollback">rolled back</glossterm>. + This is one of the <acronym>ACID</acronym> properties. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-constraint"> + <glossterm>Constraint</glossterm> + <glossdef> + <para> + A restriction on the values of data allowed within a + <glossterm linkend="glossary-table">table</glossterm>, + or in attributes of a + <glossterm linkend="glossary-domain">domain</glossterm>. + </para> + <para> + For more information, see + <xref linkend="ddl-constraints"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-cumulative-statistics"> + <glossterm>Cumulative Statistics System</glossterm> + <glossdef> + <para> + A system which, if enabled, accumulates statistical information + about the <glossterm linkend="glossary-instance">instance</glossterm>'s + activities. + </para> + <para> + For more information, see + <xref linkend="monitoring-stats"/>. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Data area</glossterm> + <glosssee otherterm="glossary-data-directory" /> + </glossentry> + + <glossentry id="glossary-database"> + <glossterm>Database</glossterm> + <glossdef> + <para> + A named collection of + <glossterm linkend="glossary-sql-object">local SQL objects</glossterm>. + </para> + <para> + For more information, see + <xref linkend="manage-ag-overview"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-db-cluster"> + <glossterm>Database cluster</glossterm> + <glossdef> + <para> + A collection of databases and global SQL objects, + and their common static and dynamic metadata. + Sometimes referred to as a + <firstterm>cluster</firstterm>. + A database cluster is created using the + <xref linkend="app-initdb" /> program. + </para> + <para> + In <productname>PostgreSQL</productname>, the term + <firstterm>cluster</firstterm> is also sometimes used to refer to an instance. + (Don't confuse this term with the SQL command <command>CLUSTER</command>.) + </para> + <para> + See also <glossterm linkend="glossary-cluster-owner">cluster owner</glossterm>, + the operating-system owner of a cluster, + and <glossterm linkend="glossary-bootstrap-superuser">bootstrap superuser</glossterm>, + the <productname>PostgreSQL</productname> owner of a cluster. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Database server</glossterm> + <glosssee otherterm="glossary-instance" /> + </glossentry> + + <glossentry id="glossary-database-superuser"> + <glossterm>Database superuser</glossterm> + <glossdef> + <para> + A role having <firstterm>superuser status</firstterm> + (see <xref linkend="role-attributes"/>). + </para> + <para> + Frequently referred to as <firstterm>superuser</firstterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-data-directory"> + <glossterm>Data directory</glossterm> + <glossdef> + <para> + The base directory on the file system of a + <glossterm linkend="glossary-server">server</glossterm> that contains all + data files and subdirectories associated with a + <glossterm linkend="glossary-db-cluster">database cluster</glossterm> + (with the exception of + <glossterm linkend="glossary-tablespace">tablespaces</glossterm>, + and optionally <glossterm linkend="glossary-wal">WAL</glossterm>). + The environment variable <literal>PGDATA</literal> is commonly used to + refer to the data directory. + </para> + <para> + A <glossterm linkend="glossary-db-cluster">cluster</glossterm>'s storage + space comprises the data directory plus any additional tablespaces. + </para> + <para> + For more information, see + <xref linkend="storage-file-layout"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-data-page"> + <glossterm>Data page</glossterm> + <glossdef> + <para> + 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 <glossterm linkend="glossary-shared-memory">shared buffers</glossterm> + where they can be modified, becoming + <firstterm>dirty</firstterm>. They become clean when written + to disk. New pages, which initially exist in memory only, are also + dirty until written. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-datum"> + <glossterm>Datum</glossterm> + <glossdef> + <para> + The internal representation of one value of an <acronym>SQL</acronym> + data type. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-delete"> + <glossterm>Delete</glossterm> + <glossdef> + <para> + An <acronym>SQL</acronym> command which removes + <glossterm linkend="glossary-tuple">rows</glossterm> from a given + <glossterm linkend="glossary-table">table</glossterm> + or <glossterm linkend="glossary-relation">relation</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-delete"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-domain"> + <glossterm>Domain</glossterm> + <glossdef> + <para> + A user-defined data type that is based on another underlying data type. + It acts the same as the underlying type except for possibly restricting + the set of allowed values. + </para> + <para> + For more information, see <xref linkend="domains"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-durability"> + <glossterm>Durability</glossterm> + <glossdef> + <para> + The assurance that once a + <glossterm linkend="glossary-transaction">transaction</glossterm> has + been <glossterm linkend="glossary-commit">committed</glossterm>, the + changes remain even after a system failure or crash. + This is one of the <acronym>ACID</acronym> properties. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Epoch</glossterm> + <glosssee otherterm="glossary-xid" /> + </glossentry> + + <glossentry id="glossary-extension"> + <glossterm>Extension</glossterm> + <glossdef> + <para> + A software add-on package that can be installed on an + <glossterm linkend="glossary-instance">instance</glossterm> to + get extra features. + </para> + <para> + For more information, see + <xref linkend="extend-extensions" />. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-file-segment"> + <glossterm>File segment</glossterm> + <glossdef> + <para> + A physical file which stores data for a given + <glossterm linkend="glossary-relation">relation</glossterm>. + 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. + </para> + <para> + For more information, see + <xref linkend="storage-file-layout"/>. + </para> + <para> + (Don't confuse this term with the similar term + <glossterm linkend="glossary-wal-file">WAL segment</glossterm>). + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-foreign-data-wrapper"> + <glossterm>Foreign data wrapper</glossterm> + <glossdef> + <para> + A means of representing data that is not contained in the local + <glossterm linkend="glossary-database">database</glossterm> so that it appears as if were in local + <glossterm linkend="glossary-table">table(s)</glossterm>. With a foreign data wrapper it is + possible to define a <glossterm linkend="glossary-foreign-server">foreign server</glossterm> and + <glossterm linkend="glossary-foreign-table">foreign tables</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-createforeigndatawrapper"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-foreign-key"> + <glossterm>Foreign key</glossterm> + <glossdef> + <para> + A type of <glossterm linkend="glossary-constraint">constraint</glossterm> + defined on one or more <glossterm linkend="glossary-column">columns</glossterm> + in a <glossterm linkend="glossary-table">table</glossterm> which + requires the value(s) in those <glossterm linkend="glossary-column">columns</glossterm> to + identify zero or one <glossterm linkend="glossary-tuple">row</glossterm> + in another (or, infrequently, the same) + <glossterm linkend="glossary-table">table</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-foreign-server"> + <glossterm>Foreign server</glossterm> + <glossdef> + <para> + A named collection of + <glossterm linkend="glossary-foreign-table">foreign tables</glossterm> which + all use the same + <glossterm linkend="glossary-foreign-data-wrapper">foreign data wrapper</glossterm> + and have other configuration values in common. + </para> + <para> + For more information, see + <xref linkend="sql-createserver"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-foreign-table"> + <glossterm>Foreign table (relation)</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-relation">relation</glossterm> which appears to have + <glossterm linkend="glossary-tuple">rows</glossterm> and + <glossterm linkend="glossary-column">columns</glossterm> similar to a + regular <glossterm linkend="glossary-table">table</glossterm>, but will forward + requests for data through its + <glossterm linkend="glossary-foreign-data-wrapper">foreign data wrapper</glossterm>, + which will return <glossterm linkend="glossary-result-set">result sets</glossterm> + structured according to the definition of the + <glossterm linkend="glossary-foreign-table">foreign table</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-createforeigntable"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-fork"> + <glossterm>Fork</glossterm> + <glossdef> + <para> + Each of the separate segmented file sets in which a relation is stored. + The <firstterm>main fork</firstterm> is where the actual data resides. + There also exist two secondary forks for metadata: + the <glossterm linkend="glossary-fsm">free space map</glossterm> + and the <glossterm linkend="glossary-vm">visibility map</glossterm>. + <glossterm linkend="glossary-unlogged">Unlogged relations</glossterm> + also have an <firstterm>init fork</firstterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-fsm"> + <glossterm>Free space map (fork)</glossterm> + <glossdef> + <para> + 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. + </para> + <para> + For more information, see + <xref linkend="storage-fsm"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-function"> + <glossterm>Function (routine)</glossterm> + <glossdef> + <para> + 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 + <command>SELECT</command>. + Certain functions can return + <glossterm linkend="glossary-result-set">sets</glossterm>; those are + called <firstterm>set-returning functions</firstterm>. + </para> + <para> + Functions can also be used for + <glossterm linkend="glossary-trigger">triggers</glossterm> to invoke. + </para> + <para> + For more information, see + <xref linkend="sql-createfunction"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-grant"> + <glossterm>Grant</glossterm> + <glossdef> + <para> + An <acronym>SQL</acronym> command that is used to allow a + <glossterm linkend="glossary-user">user</glossterm> or + <glossterm linkend="glossary-role">role</glossterm> to access + specific objects within the <glossterm linkend="glossary-database">database</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-grant"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-heap"> + <glossterm>Heap</glossterm> + <glossdef> + <para> + Contains the values of <glossterm linkend="glossary-tuple">row</glossterm> + attributes (i.e., the data) for a + <glossterm linkend="glossary-relation">relation</glossterm>. + The heap is realized within one or more + <glossterm linkend="glossary-file-segment">file segments</glossterm> + in the relation's <glossterm linkend="glossary-fork">main fork</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-host"> + <glossterm>Host</glossterm> + <glossdef> + <para> + A computer that communicates with other computers over a network. + This is sometimes used as a synonym for + <glossterm linkend="glossary-server">server</glossterm>. + It is also used to refer to a computer where + <glossterm linkend="glossary-client">client processes</glossterm> run. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-index"> + <glossterm>Index (relation)</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-relation">relation</glossterm> that contains + data derived from a <glossterm linkend="glossary-table">table</glossterm> + or <glossterm linkend="glossary-materialized-view">materialized view</glossterm>. + Its internal structure supports fast retrieval of and access to the original + data. + </para> + <para> + For more information, see + <xref linkend="sql-createindex"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-insert"> + <glossterm>Insert</glossterm> + <glossdef> + <para> + An <acronym>SQL</acronym> command used to add new data into a + <glossterm linkend="glossary-table">table</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-insert"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-instance"> + <glossterm>Instance</glossterm> + <glossdef> + <para> + A group of <glossterm linkend="glossary-backend">backend</glossterm> and + <glossterm linkend="glossary-auxiliary-proc">auxiliary processes</glossterm> + that communicate using a common shared memory area. One + <glossterm linkend="glossary-postmaster">postmaster process</glossterm> + manages the instance; one instance manages exactly one + <glossterm linkend="glossary-db-cluster">database cluster</glossterm> + with all its databases. Many instances can run on the same + <glossterm linkend="glossary-server">server</glossterm> + as long as their <acronym>TCP</acronym> ports do not conflict. + </para> + <para> + The instance handles all key features of a <acronym>DBMS</acronym>: + read and write access to files and shared memory, + assurance of the <acronym>ACID</acronym> properties, + <glossterm linkend="glossary-connection">connections</glossterm> to + <glossterm linkend="glossary-client">client processes</glossterm>, + privilege verification, crash recovery, replication, etc. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-isolation"> + <glossterm>Isolation</glossterm> + <glossdef> + <para> + The property that the effects of a transaction are not visible to + <glossterm linkend="glossary-concurrency">concurrent transactions</glossterm> + before it commits. + This is one of the <acronym>ACID</acronym> properties. + </para> + <para> + For more information, see <xref linkend="transaction-iso" />. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-join"> + <glossterm>Join</glossterm> + <glossdef> + <para> + An operation and <acronym>SQL</acronym> keyword used in + <glossterm linkend="glossary-query">queries</glossterm> + for combining data from multiple + <glossterm linkend="glossary-relation">relations</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-key"> + <glossterm>Key</glossterm> + <glossdef> + <para> + A means of identifying a <glossterm linkend="glossary-tuple">row</glossterm> within a + <glossterm linkend="glossary-table">table</glossterm> or + other <glossterm linkend="glossary-relation">relation</glossterm> by + values contained within one or more + <glossterm linkend="glossary-attribute">attributes</glossterm> + in that relation. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-lock"> + <glossterm>Lock</glossterm> + <glossdef> + <para> + A mechanism that allows a process to limit or prevent simultaneous + access to a resource. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-log-file"> + <glossterm>Log file</glossterm> + <glossdef> + <para> + Log files contain human-readable text lines about events. + Examples include login failures, long-running queries, etc. + </para> + <para> + For more information, see + <xref linkend="logfile-maintenance"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-logged"> + <glossterm>Logged</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-table">table</glossterm> is considered + <glossterm linkend="glossary-logged">logged</glossterm> if changes to it are sent to the + <glossterm linkend="glossary-wal">WAL</glossterm>. By default, all regular + tables are logged. A table can be specified as + <glossterm linkend="glossary-unlogged">unlogged</glossterm> either at + creation time or via the <command>ALTER TABLE</command> command. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-logger"> + <glossterm>Logger (process)</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + which, if enabled, writes information about database events into the current + <glossterm linkend="glossary-log-file">log file</glossterm>. + When reaching certain time- or + volume-dependent criteria, a new log file is created. + Also called <firstterm>syslogger</firstterm>. + </para> + <para> + For more information, see + <xref linkend="runtime-config-logging"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-log-record"> + <glossterm>Log record</glossterm> + <glossdef> + <para> + Archaic term for a <glossterm linkend="glossary-wal-record">WAL record</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-log-sequence-number"> + <glossterm>Log sequence number</glossterm> + <acronym>LSN</acronym> + <glossdef> + <para> + Byte offset into the <glossterm linkend="glossary-wal">WAL</glossterm>, + increasing monotonically with each new <glossterm + linkend="glossary-wal-record">WAL record</glossterm>. + </para> + <para> + For more information, see <link + linkend="datatype-pg-lsn"><type>pg_lsn</type></link> and <xref + linkend="wal-internals"/>. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>LSN</glossterm> + <glosssee otherterm="glossary-log-sequence-number"/> + </glossentry> + + <glossentry> + <glossterm>Master (server)</glossterm> + <glosssee otherterm="glossary-primary-server" /> + </glossentry> + + <glossentry id="glossary-materialized"> + <glossterm>Materialized</glossterm> + <glossdef> + <para> + The property that some information has been pre-computed and stored + for later use, rather than computing it on-the-fly. + </para> + <para> + This term is used in + <glossterm linkend="glossary-materialized-view">materialized view</glossterm>, + to mean that the data derived from the view's query is stored on + disk separately from the sources of that data. + </para> + <para> + 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. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-materialized-view"> + <glossterm>Materialized view (relation)</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-relation">relation</glossterm> that is + defined by a <command>SELECT</command> statement + (just like a <glossterm linkend="glossary-view">view</glossterm>), + but stores data in the same way that a + <glossterm linkend="glossary-table">table</glossterm> does. It cannot be + modified via <command>INSERT</command>, <command>UPDATE</command>, + <command>DELETE</command>, or <command>MERGE</command> operations. + </para> + <para> + For more information, see + <xref linkend="sql-creatematerializedview"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-merge"> + <glossterm>Merge</glossterm> + <glossdef> + <para> + An <acronym>SQL</acronym> command used to conditionally add, modify, + or remove <glossterm linkend="glossary-tuple">rows</glossterm> + in a given <glossterm linkend="glossary-table">table</glossterm>, + using data from a source + <glossterm linkend="glossary-relation">relation</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-merge"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-mvcc"> + <glossterm>Multi-version concurrency control (MVCC)</glossterm> + <glossdef> + <para> + A mechanism designed to allow several + <glossterm linkend="glossary-transaction">transactions</glossterm> to be + reading and writing the same rows without one process causing other + processes to stall. + In <productname>PostgreSQL</productname>, MVCC is implemented by + creating copies (<firstterm>versions</firstterm>) of + <glossterm linkend="glossary-tuple">tuples</glossterm> as they are + modified; after transactions that can see the old versions terminate, + those old versions need to be removed. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-null"> + <glossterm>Null</glossterm> + <glossdef> + <para> + A concept of non-existence that is a central tenet of relational + database theory. It represents the absence of a definite value. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Optimizer</glossterm> + <glosssee otherterm="glossary-planner" /> + </glossentry> + + <glossentry id="glossary-parallel-query"> + <glossterm>Parallel query</glossterm> + <glossdef> + <para> + The ability to handle parts of executing a + <glossterm linkend="glossary-query">query</glossterm> to take advantage + of parallel processes on servers with multiple <acronym>CPU</acronym>s. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-partition"> + <glossterm>Partition</glossterm> + <glossdef> + <para> + One of several disjoint (not overlapping) subsets of a larger set. + </para> + </glossdef> + <glossdef> + <para> + In reference to a + <glossterm linkend="glossary-partitioned-table">partitioned table</glossterm>: + One of the tables that each contain part of the data of the partitioned table, + which is said to be the <firstterm>parent</firstterm>. + 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. + </para> + </glossdef> + <glossdef> + <para> + In reference to a <glossterm linkend="glossary-window-function">window function</glossterm> + in a <glossterm linkend="glossary-query">query</glossterm>, + a partition is a user-defined criterion that identifies which neighboring + <glossterm linkend="glossary-tuple">rows</glossterm> + of the <glossterm linkend="glossary-result-set">query's result set</glossterm> + can be considered by the function. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-partitioned-table"> + <glossterm>Partitioned table (relation)</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-relation">relation</glossterm> that is + in semantic terms the same as a <glossterm linkend="glossary-table">table</glossterm>, + but whose storage is distributed across several + <glossterm linkend="glossary-partition">partitions</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-postmaster"> + <glossterm>Postmaster (process)</glossterm> + <glossdef> + <para> + The very first process of an <glossterm linkend="glossary-instance">instance</glossterm>. + It starts and manages the + <glossterm linkend="glossary-auxiliary-proc">auxiliary processes</glossterm> + and creates <glossterm linkend="glossary-backend">backend processes</glossterm> + on demand. + </para> + <para> + For more information, see + <xref linkend="server-start"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-primary-key"> + <glossterm>Primary key</glossterm> + <glossdef> + <para> + A special case of a + <glossterm linkend="glossary-unique-constraint">unique constraint</glossterm> + defined on a + <glossterm linkend="glossary-table">table</glossterm> or other + <glossterm linkend="glossary-relation">relation</glossterm> that also + guarantees that all of the + <glossterm linkend="glossary-attribute">attributes</glossterm> + within the <glossterm linkend="glossary-primary-key">primary key</glossterm> + do not have <glossterm linkend="glossary-null">null</glossterm> 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. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-primary-server"> + <glossterm>Primary (server)</glossterm> + <glossdef> + <para> + When two or more <glossterm linkend="glossary-database">databases</glossterm> + are linked via <glossterm linkend="glossary-replication">replication</glossterm>, + the <glossterm linkend="glossary-server">server</glossterm> + that is considered the authoritative source of information is called + the <firstterm>primary</firstterm>, + also known as a <firstterm>master</firstterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-procedure"> + <glossterm>Procedure (routine)</glossterm> + <glossdef> + <para> + 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 <command>COMMIT</command> and <command>ROLLBACK</command>. + They are invoked via the <command>CALL</command> command. + </para> + <para> + For more information, see + <xref linkend="sql-createprocedure"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-query"> + <glossterm>Query</glossterm> + <glossdef> + <para> + A request sent by a client to a <glossterm linkend="glossary-backend">backend</glossterm>, + usually to return results or to modify data on the database. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-planner"> + <glossterm>Query planner</glossterm> + <glossdef> + <para> + The part of <productname>PostgreSQL</productname> that is devoted to + determining (<firstterm>planning</firstterm>) the most efficient way to + execute <glossterm linkend="glossary-query">queries</glossterm>. + Also known as <firstterm>query optimizer</firstterm>, + <firstterm>optimizer</firstterm>, or simply <firstterm>planner</firstterm>. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Record</glossterm> + <glosssee otherterm="glossary-tuple" /> + </glossentry> + + <glossentry> + <glossterm>Recycling</glossterm> + <glosssee otherterm="glossary-wal-file" /> + </glossentry> + + <glossentry id="glossary-referential-integrity"> + <glossterm>Referential integrity</glossterm> + <glossdef> + <para> + A means of restricting data in one <glossterm linkend="glossary-relation">relation</glossterm> + by a <glossterm linkend="glossary-foreign-key">foreign key</glossterm> + so that it must have matching data in another + <glossterm linkend="glossary-relation">relation</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-relation"> + <glossterm>Relation</glossterm> + <glossdef> + <para> + The generic term for all objects in a + <glossterm linkend="glossary-database">database</glossterm> + that have a name and a list of + <glossterm linkend="glossary-attribute">attributes</glossterm> + defined in a specific order. + <glossterm linkend="glossary-table">Tables</glossterm>, + <glossterm linkend="glossary-sequence">sequences</glossterm>, + <glossterm linkend="glossary-view">views</glossterm>, + <glossterm linkend="glossary-foreign-table">foreign tables</glossterm>, + <glossterm linkend="glossary-materialized-view">materialized views</glossterm>, + composite types, and + <glossterm linkend="glossary-index">indexes</glossterm> are all relations. + </para> + <para> + More generically, a relation is a set of tuples; for example, + the result of a query is also a relation. + </para> + <para> + In <productname>PostgreSQL</productname>, + <firstterm>Class</firstterm> is an archaic synonym for + <firstterm>relation</firstterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-replica"> + <glossterm>Replica (server)</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-database">database</glossterm> that is paired + with a <glossterm linkend="glossary-primary-server">primary</glossterm> + 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 <glossterm linkend="glossary-primary-server">primary</glossterm> + becomes unavailable. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-replication"> + <glossterm>Replication</glossterm> + <glossdef> + <para> + The act of reproducing data on one + <glossterm linkend="glossary-server">server</glossterm> onto another + server called a <glossterm linkend="glossary-replica">replica</glossterm>. + This can take the form of <firstterm>physical replication</firstterm>, + where all file changes from one server are copied verbatim, + or <firstterm>logical replication</firstterm> where a defined subset + of data changes are conveyed using a higher-level representation. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-restartpoint"> + <glossterm>Restartpoint</glossterm> + <glossdef> + <para> + A variant of a <glossterm + linkend="glossary-checkpoint">checkpoint</glossterm> performed on a + <glossterm linkend="glossary-replica">replica</glossterm>. + </para> + <para> + For more information, see <xref linkend="wal-configuration"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-result-set"> + <glossterm>Result set</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-relation">relation</glossterm> transmitted + from a <glossterm linkend="glossary-backend">backend process</glossterm> + to a <glossterm linkend="glossary-client">client</glossterm> upon the + completion of an <acronym>SQL</acronym> command, usually a + <command>SELECT</command> but it can be an + <command>INSERT</command>, <command>UPDATE</command>, or + <command>DELETE</command> command if the <literal>RETURNING</literal> + clause is specified. + </para> + <para> + The fact that a result set is a relation means that a query can be used + in the definition of another query, becoming a + <firstterm>subquery</firstterm>. + </para> + </glossdef> + <glossdef> + <para> + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-revoke"> + <glossterm>Revoke</glossterm> + <glossdef> + <para> + A command to prevent access to a named set of + <glossterm linkend="glossary-database">database</glossterm> objects for a + named list of <glossterm linkend="glossary-role">roles</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-revoke"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-role"> + <glossterm>Role</glossterm> + <glossdef> + <para> + A collection of access privileges to the + <glossterm linkend="glossary-database">instance</glossterm>. + Roles are themselves a privilege that can be granted to other roles. + This is often done for convenience or to ensure completeness + when multiple <glossterm linkend="glossary-user">users</glossterm> need + the same privileges. + </para> + <para> + For more information, see + <xref linkend="sql-createrole"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-rollback"> + <glossterm>Rollback</glossterm> + <glossdef> + <para> + A command to undo all of the operations performed since the beginning + of a <glossterm linkend="glossary-transaction">transaction</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-rollback"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-routine"> + <glossterm>Routine</glossterm> + <glossdef> + <para> + 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 + <glossterm linkend="glossary-function">functions</glossterm> + (including set-returning functions and + <glossterm linkend="glossary-trigger">trigger functions</glossterm>), + <glossterm linkend="glossary-aggregate">aggregate functions</glossterm>, + and <glossterm linkend="glossary-procedure">procedures</glossterm>. + </para> + <para> + Many routines are already defined within <productname>PostgreSQL</productname> + itself, but user-defined ones can also be added. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Row</glossterm> + <glosssee otherterm="glossary-tuple" /> + </glossentry> + + <glossentry id="glossary-savepoint"> + <glossterm>Savepoint</glossterm> + <glossdef> + <para> + A special mark in the sequence of steps in a + <glossterm linkend="glossary-transaction">transaction</glossterm>. + Data modifications after this point in time may be reverted + to the time of the savepoint. + </para> + <para> + For more information, see + <xref linkend="sql-savepoint"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-schema"> + <glossterm>Schema</glossterm> + <glossdef> + <para> + A schema is a namespace for + <glossterm linkend="glossary-sql-object">SQL objects</glossterm>, + which all reside in the same + <glossterm linkend="glossary-database">database</glossterm>. + Each SQL object must reside in exactly one schema. + </para> + <para> + All system-defined SQL objects reside in schema <literal>pg_catalog</literal>. + </para> + </glossdef> + <glossdef> + <para> + More generically, the term <firstterm>schema</firstterm> is used to mean + all data descriptions (<glossterm linkend="glossary-table">table</glossterm> definitions, + <glossterm linkend="glossary-constraint">constraints</glossterm>, comments, etc.) + for a given <glossterm linkend="glossary-database">database</glossterm> or + subset thereof. + </para> + <para> + For more information, see + <xref linkend="ddl-schemas"/>. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Segment</glossterm> + <glosssee otherterm="glossary-file-segment" /> + </glossentry> + + <glossentry id="glossary-select"> + <glossterm>Select</glossterm> + <glossdef> + <para> + The <acronym>SQL</acronym> command used to request data from a + <glossterm linkend="glossary-database">database</glossterm>. + Normally, <command>SELECT</command> commands are not expected to modify the + <glossterm linkend="glossary-database">database</glossterm> in any way, + but it is possible that + <glossterm linkend="glossary-function">functions</glossterm> invoked within + the query could have side effects that do modify data. + </para> + <para> + For more information, see + <xref linkend="sql-select"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-sequence"> + <glossterm>Sequence (relation)</glossterm> + <glossdef> + <para> + 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 + <glossterm linkend="glossary-primary-key">primary key</glossterm> + values. + </para> + </glossdef> + </glossentry> + +<!-- XXX should define all other isolation levels (and improve this definition) + <glossentry id="glossary-serializable"> + <glossterm>Serializable (isolation level)</glossterm> + <glossdef> + <para> + Transactions defined as <literal>SERIALIZABLE</literal> are unable to + see changes made within other transactions. In effect, for the + initializing session the entire <glossterm linkend="glossary-database">database</glossterm> + appears to be frozen for the duration of the + <glossterm linkend="glossary-transaction">Transaction</glossterm>. + </para> + </glossdef> + </glossentry> +--> + + <glossentry id="glossary-server"> + <glossterm>Server</glossterm> + <glossdef> + <para> + A computer on which <productname>PostgreSQL</productname> + <glossterm linkend="glossary-instance">instances</glossterm> run. + The term <firstterm>server</firstterm> denotes real hardware, a + container, or a <firstterm>virtual machine</firstterm>. + </para> + <para> + This term is sometimes used to refer to an instance or to a host. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-session"> + <glossterm>Session</glossterm> + <glossdef> + <para> + A state that allows a client and a backend to interact, + communicating over a <glossterm linkend="glossary-connection">connection</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-shared-memory"> + <glossterm>Shared memory</glossterm> + <glossdef> + <para> + <acronym>RAM</acronym> which is used by the processes common to an + <glossterm linkend="glossary-instance">instance</glossterm>. + It mirrors parts of <glossterm linkend="glossary-database">database</glossterm> + files, provides a transient area for + <glossterm linkend="glossary-wal-record">WAL records</glossterm>, + and stores additional common information. + Note that shared memory belongs to the complete instance, not to a single + database. + </para> + <para> + The largest part of shared memory is known as <firstterm>shared buffers</firstterm> + 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. + </para> + <para> + For more information, see + <xref linkend="runtime-config-resource-memory"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-sql-object"> + <glossterm>SQL object</glossterm> + <glossdef> + <para> + Any object that can be created with a <command>CREATE</command> + command. Most objects are specific to one database, and are commonly + known as <firstterm>local objects</firstterm>. + </para> + <para> + Most local objects reside in a specific + <glossterm linkend="glossary-schema">schema</glossterm> in their + containing database, such as + <glossterm linkend="glossary-relation">relations</glossterm> (all types), + <glossterm linkend="glossary-function">routines</glossterm> (all types), + data types, etc. + The names of such objects of the same type in the same schema + are enforced to be unique. + </para> + <para> + There also exist local objects that do not reside in schemas; some examples are + <glossterm linkend="glossary-extension">extensions</glossterm>, + <glossterm linkend="glossary-cast">data type casts</glossterm>, and + <glossterm linkend="glossary-foreign-data-wrapper">foreign data wrappers</glossterm>. + The names of such objects of the same type are enforced to be unique + within the database. + </para> + <para> + Other object types, such as + <glossterm linkend="glossary-role">roles</glossterm>, + <glossterm linkend="glossary-tablespace">tablespaces</glossterm>, + 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 <firstterm>global objects</firstterm>. + The names of such objects are enforced to be unique within the whole + database cluster. + </para> + <para> + For more information, see + <xref linkend="manage-ag-overview"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-sql-standard"> + <glossterm>SQL standard</glossterm> + <glossdef> + <para> + A series of documents that define the <acronym>SQL</acronym> language. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>Standby (server)</glossterm> + <glosssee otherterm="glossary-replica" /> + </glossentry> + + <glossentry id="glossary-startup-process"> + <glossterm>Startup process</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + that replays WAL during crash recovery and in a + <glossterm linkend="glossary-replication">physical replica</glossterm>. + </para> + <para> + (The name is historical: the startup process was named before + replication was implemented; the name refers to its task as it + relates to the server startup following a crash.) + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-superuser"> + <glossterm>Superuser</glossterm> + <glossdef> + <para> + As used in this documentation, it is a synonym for + <glossterm linkend="glossary-database-superuser">database superuser</glossterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-system-catalog"> + <glossterm>System catalog</glossterm> + <glossdef> + <para> + A collection of <glossterm linkend="glossary-table">tables</glossterm> + which describe the structure of all + <glossterm linkend="glossary-sql-object">SQL objects</glossterm> + of the instance. + The system catalog resides in the schema <literal>pg_catalog</literal>. + These tables contain data in internal representation and are + not typically considered useful for user examination; + a number of user-friendlier <glossterm linkend="glossary-view">views</glossterm>, + also in schema <literal>pg_catalog</literal>, offer more convenient access to + some of that information, while additional tables and views + exist in schema <literal>information_schema</literal> + (see <xref linkend="information-schema" />) that expose some + of the same and additional information as mandated by the + <glossterm linkend="glossary-sql-standard">SQL standard</glossterm>. + </para> + <para> + For more information, see + <xref linkend="ddl-schemas"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-table"> + <glossterm>Table</glossterm> + <glossdef> + <para> + A collection of <glossterm linkend="glossary-tuple">tuples</glossterm> having + a common data structure (the same number of + <glossterm linkend="glossary-attribute">attributes</glossterm>, in the same + order, having the same name and type per position). + A table is the most common form of + <glossterm linkend="glossary-relation">relation</glossterm> in + <productname>PostgreSQL</productname>. + </para> + <para> + For more information, see + <xref linkend="sql-createtable"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-tablespace"> + <glossterm>Tablespace</glossterm> + <glossdef> + <para> + A named location on the server file system. + All <glossterm linkend="glossary-sql-object">SQL objects</glossterm> + which require storage beyond their definition in the + <glossterm linkend="glossary-system-catalog">system catalog</glossterm> + 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 <literal>pg_default</literal>. + </para> + <para> + For more information, see + <xref linkend="manage-ag-tablespaces"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-temporary-table"> + <glossterm>Temporary table</glossterm> + <glossdef> + <para> + <glossterm linkend="glossary-table">Tables</glossterm> that exist either + for the lifetime of a + <glossterm linkend="glossary-session">session</glossterm> or a + <glossterm linkend="glossary-transaction">transaction</glossterm>, as + specified at the time of creation. + The data in them is not visible to other sessions, and is not + <glossterm linkend="glossary-logged">logged</glossterm>. + Temporary tables are often used to store intermediate data for a + multi-step operation. + </para> + <para> + For more information, see + <xref linkend="sql-createtable"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-toast"> + <glossterm>TOAST</glossterm> + <glossdef> + <para> + A mechanism by which large attributes of table rows are split and + stored in a secondary table, called the <firstterm>TOAST table</firstterm>. + Each relation with large attributes has its own TOAST table. + </para> + <para> + For more information, see + <xref linkend="storage-toast" />. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-transaction"> + <glossterm>Transaction</glossterm> + <glossdef> + <para> + A combination of commands that must act as a single + <glossterm linkend="glossary-atomic">atomic</glossterm> command: they all + succeed or all fail as a single unit, and their effects are not visible to + other <glossterm linkend="glossary-session">sessions</glossterm> until + the transaction is complete, and possibly even later, depending on the + isolation level. + </para> + <para> + For more information, see + <xref linkend="transaction-iso"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-xid"> + <glossterm>Transaction ID</glossterm> + <glossdef> + <para> + The numerical, unique, sequentially-assigned identifier that each + transaction receives when it first causes a database modification. + Frequently abbreviated as <firstterm>xid</firstterm>. + 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, + <firstterm>epochs</firstterm> are used, also 32 bits wide. + When the counter reaches the maximum xid value, it starts over at + <literal>3</literal> (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; see <xref + linkend="transaction-id"/> for more details. + </para> + <para> + For more information, see + <xref linkend="datatype-oid"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-tps"> + <glossterm>Transactions per second (TPS)</glossterm> + <glossdef> + <para> + 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. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-trigger"> + <glossterm>Trigger</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-function">function</glossterm> which can + be defined to execute whenever a certain operation (<command>INSERT</command>, + <command>UPDATE</command>, <command>DELETE</command>, + <command>TRUNCATE</command>) is applied to a + <glossterm linkend="glossary-relation">relation</glossterm>. + A trigger executes within the same + <glossterm linkend="glossary-transaction">transaction</glossterm> as the + statement which invoked it, and if the function fails, then the invoking + statement also fails. + </para> + <para> + For more information, see + <xref linkend="sql-createtrigger"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-tuple"> + <glossterm>Tuple</glossterm> + <glossdef> + <para> + A collection of <glossterm linkend="glossary-attribute">attributes</glossterm> + in a fixed order. + That order may be defined by the <glossterm linkend="glossary-table">table</glossterm> + (or other <glossterm linkend="glossary-relation">relation</glossterm>) + where the tuple is contained, in which case the tuple is often called a + <firstterm>row</firstterm>. It may also be defined by the structure of a + result set, in which case it is sometimes called a <firstterm>record</firstterm>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-unique-constraint"> + <glossterm>Unique constraint</glossterm> + <glossdef> + <para> + A type of <glossterm linkend="glossary-constraint">constraint</glossterm> + defined on a <glossterm linkend="glossary-relation">relation</glossterm> + 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. + </para> + <para> + Because <glossterm linkend="glossary-null">null values</glossterm> are + not considered equal to each other, multiple rows with null values are + allowed to exist without violating the unique constraint. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-unlogged"> + <glossterm>Unlogged</glossterm> + <glossdef> + <para> + The property of certain <glossterm linkend="glossary-relation">relations</glossterm> + that the changes to them are not reflected in the + <glossterm linkend="glossary-wal">WAL</glossterm>. + This disables replication and crash recovery for these relations. + </para> + <para> + The primary use of unlogged tables is for storing + transient work data that must be shared across processes. + </para> + <para> + <glossterm linkend="glossary-temporary-table">Temporary tables</glossterm> + are always unlogged. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-update"> + <glossterm>Update</glossterm> + <glossdef> + <para> + An <acronym>SQL</acronym> command used to modify + <glossterm linkend="glossary-tuple">rows</glossterm> + that may already exist in a specified <glossterm linkend="glossary-table">table</glossterm>. + It cannot create or remove rows. + </para> + <para> + For more information, see + <xref linkend="sql-update"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-user"> + <glossterm>User</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-role">role</glossterm> that has the + <firstterm>login privilege</firstterm> + (see <xref linkend="role-attributes"/>). + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-user-mapping"> + <glossterm>User mapping</glossterm> + <glossdef> + <para> + The translation of login credentials in the local + <glossterm linkend="glossary-database">database</glossterm> to credentials + in a remote data system defined by a + <glossterm linkend="glossary-foreign-data-wrapper">foreign data wrapper</glossterm>. + </para> + <para> + For more information, see + <xref linkend="sql-createusermapping"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-vacuum"> + <glossterm>Vacuum</glossterm> + <glossdef> + <para> + The process of removing outdated + <glossterm linkend="glossary-tuple">tuple versions</glossterm> + from tables or materialized views, and other closely related + processing required by <productname>PostgreSQL</productname>'s + implementation of <glossterm linkend="glossary-mvcc">MVCC</glossterm>. + This can be initiated through the use of + the <command>VACUUM</command> command, but can also be handled automatically + via <glossterm linkend="glossary-autovacuum">autovacuum</glossterm> processes. + </para> + <para> + For more information, see + <xref linkend="routine-vacuuming"/> . + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-view"> + <glossterm>View</glossterm> + <glossdef> + <para> + A <glossterm linkend="glossary-relation">relation</glossterm> that is defined by a + <command>SELECT</command> 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. + </para> + <para> + For more information, see + <xref linkend="sql-createview"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-vm"> + <glossterm>Visibility map (fork)</glossterm> + <glossdef> + <para> + 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 + (<literal>all-visible</literal>) indicates that all tuples + in the page are visible to all transactions. The second one + (<literal>all-frozen</literal>) indicates that all tuples + in the page are marked frozen. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>WAL</glossterm> + <glosssee otherterm="glossary-wal" /> + </glossentry> + + <glossentry id="glossary-wal-archiver"> + <glossterm>WAL archiver (process)</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + which, if enabled, saves copies of + <glossterm linkend="glossary-wal-file">WAL files</glossterm> + for the purpose of creating backups or keeping + <glossterm linkend="glossary-replica">replicas</glossterm> current. + </para> + <para> + For more information, see + <xref linkend="continuous-archiving"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-wal-file"> + <glossterm>WAL file</glossterm> + <glossdef> + <para> + Also known as <firstterm>WAL segment</firstterm> or + <firstterm>WAL segment file</firstterm>. + Each of the sequentially-numbered files that provide storage space for + <glossterm linkend="glossary-wal">WAL</glossterm>. + 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. + </para> + <para> + Each WAL file can be released after a + <glossterm linkend="glossary-checkpoint">checkpoint</glossterm> + 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 + <firstterm>recycling</firstterm>. + </para> + <para> + For more information, see + <xref linkend="wal-internals"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-wal-record"> + <glossterm>WAL record</glossterm> + <glossdef> + <para> + A low-level description of an individual data change. + It contains sufficient information for the data change to be + re-executed (<firstterm>replayed</firstterm>) in case a system failure + causes the change to be lost. + WAL records use a non-printable binary format. + </para> + <para> + For more information, see + <xref linkend="wal-internals"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-wal-receiver"> + <glossterm>WAL receiver (process)</glossterm> + <glossdef> + <para> + An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm> + that runs on a <glossterm linkend="glossary-replica">replica</glossterm> + to receive WAL from the + <glossterm linkend="glossary-primary-server">primary server</glossterm> + for replay by the + <glossterm linkend="glossary-startup-process">startup process</glossterm>. + </para> + + <para> + For more information, see + <xref linkend="warm-standby"/>. + </para> + </glossdef> + </glossentry> + + <glossentry> + <glossterm>WAL segment</glossterm> + <glosssee otherterm="glossary-wal-file" /> + </glossentry> + + <glossentry id="glossary-wal-sender"> + <glossterm>WAL sender (process)</glossterm> + <glossdef> + <para> + A special <glossterm linkend="glossary-backend">backend process</glossterm> + that streams WAL over a network. The receiving end can be a + <glossterm linkend="glossary-wal-receiver">WAL receiver</glossterm> + in a <glossterm linkend="glossary-replica">replica</glossterm>, + <xref linkend="app-pgreceivewal"/>, or any other client program + that speaks the replication protocol. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-wal-writer"> + <glossterm>WAL writer (process)</glossterm> + <glossdef> + <para> + A process that writes <glossterm linkend="glossary-wal-record">WAL records</glossterm> + from <glossterm linkend="glossary-shared-memory">shared memory</glossterm> to + <glossterm linkend="glossary-wal-file">WAL files</glossterm>. + </para> + <para> + For more information, see + <xref linkend="runtime-config-wal"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-window-function"> + <glossterm>Window function (routine)</glossterm> + <glossdef> + <para> + A type of <glossterm linkend="glossary-function">function</glossterm> + used in a <glossterm linkend="glossary-query">query</glossterm> + that applies to a <glossterm linkend="glossary-partition">partition</glossterm> + of the query's <glossterm linkend="glossary-result-set">result set</glossterm>; + the function's result is based on values found in + <glossterm linkend="glossary-tuple">rows</glossterm> of the same partition or frame. + </para> + <para> + All <glossterm linkend="glossary-aggregate">aggregate functions</glossterm> + 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 <firstterm>analytic functions</firstterm>. + </para> + <para> + For more information, see + <xref linkend="tutorial-window"/>. + </para> + </glossdef> + </glossentry> + + <glossentry id="glossary-wal"> + <glossterm>Write-ahead log</glossterm> + <glossdef> + <para> + The journal that keeps track of the changes in the + <glossterm linkend="glossary-db-cluster">database cluster</glossterm> + as user- and system-invoked operations take place. + It comprises many individual + <glossterm linkend="glossary-wal-record">WAL records</glossterm> written + sequentially to <glossterm linkend="glossary-wal-file">WAL files</glossterm>. + </para> + </glossdef> + </glossentry> + </glosslist> +</appendix> |