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doc/src/sgml/glossary.sgml
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Daniel Gustafsson
doc: Fix glossary entry for data checksums workers
01 авг 2026, 22:35
01 авг 2026, 22:35c5ad377
<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 id="glossary-am">
   <glossterm>Access Method</glossterm>
   <glossdef>
    <para>
     Interfaces which <productname>PostgreSQL</productname> use in order to
     access data in tables and indexes.  This abstraction allows for adding
     support for new types of data storage.
    </para>
    <para>
     For more information, see <xref linkend="tableam" /> and
     <xref linkend="indexam" />.
    </para>
   </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-application-time">
   <glossterm>Application time</glossterm>
   <glossdef>
    <para>
     In a <glossterm linkend="glossary-temporal-table">temporal table</glossterm>,
     the dimension of time that represents when the entity described by the table
     changed (as opposed to the table itself).
    </para>
    <para>
     For more information, see
     <xref linkend="ddl-temporal-tables"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-aio">
   <glossterm>Asynchronous <acronym>I/O</acronym></glossterm>
   <acronym>AIO</acronym>
   <indexterm>
    <primary>Asynchronous <acronym>I/O</acronym></primary>
   </indexterm>
   <glossdef>
    <para>
     Asynchronous <acronym>I/O</acronym> (<acronym>AIO</acronym>) describes
     performing <acronym>I/O</acronym> in a non-blocking way (asynchronously),
     in contrast to synchronous <acronym>I/O</acronym>, which blocks for the
     entire duration of the <acronym>I/O</acronym>.
    </para>
    <para>
     With <acronym>AIO</acronym>, starting an <acronym>I/O</acronym> operation
     is separated from waiting for the result of the operation, allowing
     multiple <acronym>I/O</acronym> operations to be initiated concurrently,
     as well as performing <acronym>CPU</acronym> heavy operations
     concurrently with <acronym>I/O</acronym>. The price for that increased
     concurrency is increased complexity.
    </para>
    <glossseealso otherterm="glossary-io" />
   </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>),
     the <glossterm linkend="glossary-wal-summarizer">WAL summarizer</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>,
     <glossterm linkend="glossary-parallel-query">parallel queries</glossterm> and
     <glossterm linkend="glossary-data-checksums-process">data checksums processing</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>,
     and its superuser status cannot be removed.
    </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-conflict-schema">
   <glossterm>Conflict schema</glossterm>
   <glossdef>
    <para>
     The <literal>pg_conflict</literal> schema that contains system-managed
     conflict log tables for logical replication. These tables are created
     and maintained automatically by the system and are not intended for
     direct user manipulation. See <xref linkend="ddl-schemas-conflict"/>.
    </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-data-checksums-process">
   <glossterm>Data Checksums (process)</glossterm>
   <glossdef>
    <para>
     A set of
     <glossterm linkend="glossary-background-worker">background worker</glossterm>
     processes which can enable or disable data checksums in a running cluster.
     The process which coordinates the work is known as the
     <firstterm>data checksums launcher</firstterm> and the process which
     operates on the individual databases is known as the
     <firstterm>data checksums worker</firstterm>.
    </para>
   </glossdef>
  </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>
   <glossterm>GMT</glossterm>
   <glosssee otherterm="glossary-utc" />
  </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-guc">
   <glossterm>GUC</glossterm>
   <glossdef>
    <para>
     Based on a short-hand for Grand Unified Configuration, the subsystem
     which controls server configuration, it is now a commonly used term
     for the individual configuration parameters themselves. Typically meant
     to refer to user settable parameters, it can also refer to internal or
     build-time parameters.
    </para>
    <para>
     For information on working with configuration parameters, see
     <xref linkend="config-setting"/>.
    </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-incremental-backup">
   <glossterm>Incremental backup</glossterm>
   <glossdef>
    <para>
     A special <glossterm linkend="glossary-basebackup">base backup</glossterm>
     that for some files may contain only those pages that were modified since
     a previous backup, as opposed to the full contents of every file. Like
     base backups, it is generated by the tool <xref linkend="app-pgbasebackup"/>.
    </para>
    <para>
     To restore incremental backups the tool <xref linkend="app-pgcombinebackup"/>
     is used, which combines incremental backups with a base backup.
     Afterwards, recovery can use
     <glossterm linkend="glossary-wal">WAL</glossterm> to bring the
     <glossterm linkend="glossary-db-cluster">database cluster</glossterm> to
     a consistent state.
    </para>
    <para>
     For more information, see <xref linkend="backup-incremental-backup"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-io">
   <glossterm>Input/Output</glossterm>
   <acronym>I/O</acronym>
   <glossdef>
    <para>
     Input/Output (<acronym>I/O</acronym>) describes the communication between
     a program and peripheral devices. In the context of database systems,
     <acronym>I/O</acronym> commonly, but not exclusively, refers to
     interaction with storage devices or the network.
    </para>
    <glossseealso otherterm="glossary-aio" />
   </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-logical-replication-cluster">
   <glossterm>Logical replication cluster</glossterm>
   <glossdef>
    <para>
     A set of publisher and subscriber instances with the publisher instance
     replicating changes to the subscriber instance.
    </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>
     Mathematically, a <firstterm>relation</firstterm> is a set of
     <glossterm linkend="glossary-tuple">tuples</glossterm>;
     this is the sense meant in the term "relational database".
    </para>

    <para>
     In <productname>PostgreSQL</productname>, "relation" is commonly used to
     mean an <glossterm linkend="glossary-sql-object">SQL object</glossterm>
     that has a name and a list of attributes 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.
     A relation in this sense is a container or a descriptor for a set of tuples.
    </para>

    <para>
     <firstterm>Class</firstterm> is an alternative but archaic term.
     The system catalog
     <link linkend="catalog-pg-class"><structname>pg_class</structname></link>
     holds an entry for each <productname>PostgreSQL</productname> relation.
    </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>,
     <command>DELETE</command>, or <command>MERGE</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-system-time">
   <glossterm>System time</glossterm>
   <glossdef>
    <para>
     In a <glossterm linkend="glossary-temporal-table">temporal table</glossterm>,
     the dimension of time that represents when the table itself was changed
     (as opposed to the entity the table describes).
     Often used for auditing, compliance, and debugging.
    </para>
    <para>
     For more information, see
     <xref linkend="ddl-temporal-tables"/>.
    </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-temporal-leftovers">
   <glossterm>Temporal leftovers</glossterm>
   <glossdef>
    <para>
     After a temporal update or delete, the portion of history that was not
     updated/deleted. When using ranges to track application time, there may be
     zero, one, or two stretches of history that were not updated/deleted
     (before and/or after the portion that was updated/deleted). New rows are
     automatically inserted into the table to preserve that history. A single
     multirange can accommodate the untouched history before and after the
     update/delete, so there will be only zero or one leftover.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-temporal-table">
   <glossterm>Temporal table</glossterm>
   <glossdef>
    <para>
     <glossterm linkend="glossary-table">Tables</glossterm>
     that track <glossterm linkend="glossary-application-time">application time</glossterm>
     or <glossterm linkend="glossary-system-time">system time</glossterm> (or both).
     Not to be confused with <glossterm linkend="glossary-temporary-table">temporary tables</glossterm>.
    </para>
    <para>
     For more information, see
     <xref linkend="ddl-temporal-tables"/>.
    </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 &mdash; 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-utc">
   <glossterm>UTC</glossterm>
   <glossdef>
    <para>
     Universal Coordinated Time, the primary global time reference,
     approximately the time prevailing at the zero meridian of longitude.
     Often but inaccurately referred to as GMT (Greenwich Mean Time).
    </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-summarizer">
   <glossterm>WAL summarizer (process)</glossterm>
   <glossdef>
    <para>
     An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm>
     that summarizes WAL data for
     <glossterm linkend="glossary-incremental-backup">incremental backups</glossterm>.
    </para>
    <para>
     For more information, see <xref linkend="runtime-config-wal-summarization"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-wal-writer">
   <glossterm>WAL writer (process)</glossterm>
   <glossdef>
    <para>
     An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm>
     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>