glossary.sgml

<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 process 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.
    </para>
    <para>
     For more information, see
     <xref linkend="autovacuum"/>.
    </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>
     A process 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-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-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>
     A specialized process responsible for executing checkpoints.
    </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-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
     <!-- XXX Should have term "domain". Need term "type" for that. -->
     <firstterm>domain</firstterm>.
    </para>
    <para>
     For more information, see
     <xref linkend="ddl-constraints"/>.
    </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>.
    </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>
   </glossdef>
  </glossentry>

  <glossentry>
   <glossterm>Database server</glossterm>
   <glosssee otherterm="glossary-instance" />
  </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-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-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 backend and auxiliary processes 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>
     If activated, the process
     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>
   <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>, or
     <command>DELETE</command> operations.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-creatematerializedview"/>.
    </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.