Skip to content

P
Postgres FD Implementation
Commit 347d2b07 authored by Alvaro Herrera's avatar Alvaro Herrera
Browse files
Options

Add a glossary to the documentation 

More work is still needed, but this is a good start.
Co-authored-by: default avatarCorey Huinker <corey.huinker@gmail.com>
Co-authored-by: default avatarJürgen Purtz <juergen@purtz.de>
Co-authored-by: default avatarRoger Harkavy <rogerharkavy@gmail.com>
Co-authored-by: default avatarÁlvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: default avatarJustin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CADkLM=eP6HOeqDjn0FdXuGRusQu4oWH_LFsKjjafmhvWD=aSpQ@mail.gmail.com
parent 3031440e
Show whitespace changes
Inline Side-by-side
Showing with 1777 additions and 0 deletions
doc/src/sgml/filelist.sgml
View file @ 347d2b07
......@@ -170,6 +170,7 @@
<!ENTITY limits SYSTEM "limits.sgml">
<!ENTITY acronyms SYSTEM "acronyms.sgml">
<!ENTITY glossary SYSTEM "glossary.sgml">
<!ENTITY color SYSTEM "color.sgml">
<!ENTITY features-supported SYSTEM "features-supported.sgml">
......
doc/src/sgml/glossary.sgml 0 → 100644
View file @ 347d2b07
<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-aggregate">
<glossterm>Aggregate Function</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-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>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Analytic Function</glossterm>
<glosssee otherterm="glossary-window-function" />
</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 that 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> or
<glossterm linkend="glossary-table">table</glossterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-autovacuum">
<glossterm>Autovacuum</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 act on behalf of <glossterm linkend="glossary-session">client sessions</glossterm>
and handle their 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 continuously writes dirty pages 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-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-checkpointer">
<glossterm>Checkpointer (process)</glossterm>
<glossdef>
<para>
A specialized process responsible for executing checkpoints.
</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>
<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">
<glossterm>Cluster</glossterm>
<glossdef>
<para>
A group of <glossterm linkend="glossary-database">databases</glossterm> plus their
<glossterm linkend="glossary-global-sql-object">global SQL objects</glossterm>. The
<firstterm>cluster</firstterm> is managed by exactly one
<glossterm linkend="glossary-instance">instance</glossterm>. A newly created
Cluster will have three databases created automatically. They are
<literal>template0</literal>, <literal>template1</literal>, and
<literal>postgres</literal>. It is expected that an application will
create one or more additional database aside from these three.
</para>
<para>
(Don't confuse the <productname>PostgreSQL</productname>-specific term
<glossterm linkend="glossary-cluster">Cluster</glossterm> with the SQL
command <command>CLUSTER</command>).
</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 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>.
</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-data-directory">
<glossterm>Data Directory</glossterm>
<glossdef>
<para>
The base directory on the filesystem of a
<glossterm linkend="glossary-server">server</glossterm> that contains all
data files and subdirectories associated with a
<glossterm linkend="glossary-cluster">cluster</glossterm> with the
exception of <glossterm linkend="glossary-tablespace">tablespaces</glossterm>.
The environment variable <literal>PGDATA</literal> is commonly used to
refer to the
<glossterm linkend="glossary-data-directory">data directory</glossterm>.
</para>
<para>
An <glossterm linkend="glossary-instance">instance</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-database">
<glossterm>Database</glossterm>
<glossdef>
<para>
A named collection of
<glossterm linkend="glossary-sql-object">SQL objects</glossterm>.
</para>
<para>
For more information, see
<xref linkend="manage-ag-overview"/>.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Database Server</glossterm>
<glosssee otherterm="glossary-instance" />
</glossentry>
<glossentry id="glossary-datum">
<glossterm>Datum</glossterm>
<glossdef>
<para>
The internal representation of one value of a <acronym>SQL</acronym>
data type.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-delete">
<glossterm>Delete</glossterm>
<glossdef>
<para>
A <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 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,
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 table</glossterm>s.
</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</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-function">
<glossterm>Function</glossterm>
<glossdef>
<para>
Any defined transformation of data. Many functions are already defined
within <productname>PostgreSQL</productname> itself, but user-defined
ones can also be added.
</para>
<para>
For more information, see
<xref linkend="sql-createfunction"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-global-sql-object">
<glossterm>Global SQL Object</glossterm>
<glossdef>
<para>
<glossterm linkend="glossary-sql-object">SQL objects</glossterm> which do
not belong to a specific
<glossterm linkend="glossary-database">database</glossterm>.
</para>
<para>
These objects are
<glossterm linkend="glossary-role">roles</glossterm>,
<glossterm linkend="glossary-tablespace">tablespaces</glossterm>,
replication origins, and subscriptions for logical replication.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-grant">
<glossterm>Grant</glossterm>
<glossdef>
<para>
A <acronym>SQL</acronym> command that is used to allow
<glossterm linkend="glossary-user">users</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
<glossterm linkend="glossary-file-segment">segment files</glossterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-host">
<glossterm>Host</glossterm>
<glossdef>
<para>
A computer that communicates with other hosts over a network.
This term can be used to refer to either a
<glossterm linkend="glossary-client">client</glossterm>
or a <glossterm linkend="glossary-server">server</glossterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-index">
<glossterm>Index</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-relation">relation</glossterm> types
such as a <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>
A <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>
An <glossterm>instance</glossterm> is a group of processes,
its supporting <glossterm linkend="glossary-data-directory">storage space</glossterm>,
plus their
common <glossterm linkend="glossary-shared-memory">shared memory</glossterm>,
running on a single <glossterm linkend="glossary-server">server</glossterm>.
The <glossterm linkend="glossary-instance">instance</glossterm>
handles all key features of a <acronym>DBMS</acronym>: read and write
access to files and shared memory, assurance of
the <acronym>ACID</acronym> paradigm, <acronym>MVCC</acronym>,
<glossterm linkend="glossary-connection">connections</glossterm> to client programs, backup,
recovery, replication, privileges, etc.
</para>
<para>
An <glossterm>instance</glossterm> manages exactly one
<glossterm linkend="glossary-cluster">cluster</glossterm>.
</para>
<para>
Many <glossterm>instances</glossterm> can run on the same server as
long as their <acronym>TCP/IP</acronym> ports do not conflict.
Different instances on a server may use the
same or different versions of <productname>PostgreSQL</productname>.
</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>
A <acronym>SQL</acronym> keyword used in <command>SELECT</command> statements 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
<glossterm linkend="glossary-relation">relation</glossterm> by
values contained within one or more
<glossterm linkend="glossary-attribute">attributes</glossterm>
in that table.
</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-logger">
<glossterm>Logger (process)</glossterm>
<glossdef>
<para>
If activated, the
<glossterm linkend="glossary-logger">Logger</glossterm> 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 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>
<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</glossterm>
<glossdef>
<para>
A <glossterm linkend="glossary-relation">relation</glossterm> that is
defined in the same way that a a <glossterm linkend="glossary-view">view</glossterm>
is, 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 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>
<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>:
a partition is a user-defined criteria that identifies which neighboring
<glossterm linkend="glossary-tuple">rows</glossterm> can be considered by the
function.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-partitioned-table">
<glossterm>Partitioned Table</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 other auxiliary processes 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-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-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-procedure">
<glossterm>Procedure</glossterm>
<glossdef>
<para>
A defined set of instructions for manipulating data within a
<glossterm linkend="glossary-database">database</glossterm>.
A <glossterm linkend="glossary-procedure">procedure</glossterm> can
be written in a variety of programming languages. They are
similar to <glossterm linkend="glossary-function">functions</glossterm>,
but are different in that they must be invoked via the <command>CALL</command>
command rather than the <command>SELECT</command> or <command>PERFORM</command>
commands, and they are allowed to make transactional statements such
as <command>COMMIT</command> and <command>ROLLBACK</command>.
</para>
<para>
For more information, see
<xref linkend="sql-createprocedure"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-query">
<glossterm>Query</glossterm>
<glossdef>
<para>
A request sent by a client to a <glossterm linkend="glossary-backend">backend</glossterm>,
usually to return results or to modify data on the database.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-planner">
<glossterm>Query Planner</glossterm>
<glossdef>
<para>
The part of <productname>PostgreSQL</productname> that is devoted to
determining (<firstterm>planning</firstterm>) the most efficient way to
execute <glossterm linkend="glossary-query">queries</glossterm>.
Also known as <firstterm>query optimizer</firstterm>,
<firstterm>optimizer</firstterm>, or simply <firstterm>planner</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Record</glossterm>
<glosssee otherterm="glossary-tuple" />
</glossentry>
<glossentry>
<glossterm>Recycling</glossterm>
<glosssee otherterm="glossary-wal-file" />
</glossentry>
<glossentry id="glossary-referential-integrity">
<glossterm>Referential Integrity</glossterm>
<glossdef>
<para>
A means of restricting data in one <glossterm linkend="glossary-relation">relation</glossterm>
by a <glossterm linkend="glossary-foreign-key">foreign key</glossterm>
so that it must have matching data in another
<glossterm linkend="glossary-relation">relation</glossterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-relation">
<glossterm>Relation</glossterm>
<glossdef>
<para>
The generic term for all objects in a
<glossterm linkend="glossary-database">database</glossterm>
that have a name and a list of
<glossterm linkend="glossary-attribute">attributes</glossterm>
defined in a specific order.
<glossterm linkend="glossary-table">Tables</glossterm>,
<glossterm linkend="glossary-view">views</glossterm>,
<glossterm linkend="glossary-foreign-table">foreign tables</glossterm>,
<glossterm linkend="glossary-materialized-view">materialized views</glossterm>, and
<glossterm linkend="glossary-index">indexes</glossterm> are all relations.
</para>
<para>
<firstterm>Class</firstterm> is an archaic synonym for
<firstterm>relation</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-replica">
<glossterm>Replica</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-result-set">
<glossterm>Result Set</glossterm>
<glossdef>
<para>
A data structure transmitted from a
<glossterm linkend="glossary-backend">backend process</glossterm> to
client program upon the completion of a <acronym>SQL</acronym>
command, usually a <command>SELECT</command> but it can be an
<command>INSERT</command>, <command>UPDATE</command>, or
<command>DELETE</command> command if the <literal>RETURNING</literal>
clause is specified. The data structure consists of zero or more
<glossterm linkend="glossary-tuple">rows</glossterm> with the same ordered set of
<glossterm linkend="glossary-attribute">attributes</glossterm>.
</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>.
Roless 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>
<glossterm>Row</glossterm>
<glosssee otherterm="glossary-tuple" />
</glossentry>
<glossentry id="glossary-savepoint">
<glossterm>Savepoint</glossterm>
<glossdef>
<para>
A special mark inside 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>
The names of SQL objects of the same type in the same schema are enforced unique.
There is no restriction on reusing a name in multiple schemas.
</para>
<para>
All system-defined SQL objects reside in schema <literal>pg_catalog</literal>,
and commonly many user-defined SQL objects reside in the default schema
<literal>public</literal>,
but it is common and recommended that other schemas are created to hold
application-specific SQL objects.
</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>
<!-- 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 Virtual Machine.
</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>
<!-- XXX we should have a much more concise definition
<glossentry id="glossary-sequence">
<glossterm>Sequence</glossterm>
<glossdef>
<para>
A <glossterm linkend="glossary-database">Database</glossterm> object which
represents the mathematical concept of a numerical integral sequence.
It can be thought of as a <glossterm linkend="glossary-table">Table</glossterm>
with exactly one <glossterm linkend="glossary-tuple">Row</glossterm> and
one <glossterm linkend="glossary-column">Column</glossterm>. The
value stored is known as the current value. A
<glossterm linkend="glossary-sequence">Sequence</glossterm> has a defined
direction (usually increasing) and an interval step (usually 1).
Whenever the <literal>NEXTVAL</literal> pseudo-column of a
<glossterm linkend="glossary-sequence">Sequence</glossterm> is accessed,
the current value is moved in the defined direction by the defined
interval step, that value is returned to the invoking query,
and the current value of the sequence is updated to reflect the new value.
The value can be updated multiple times in a single query,
the net effect being that each row selected will have a different value.
Values taken from a
<glossterm linkend="glossary-sequence">Sequence</glossterm> are never
reverted, even in the case of a
<glossterm linkend="glossary-rollback">Rollback</glossterm>, which means
that the <glossterm linkend="glossary-sequence">Sequence</glossterm>
will never generate the same number twice, which makes it useful for
generating
<glossterm linkend="glossary-primary-key">Primary Key</glossterm> values.
</para>
<para>
For more information, see
<xref linkend="sql-createsequence"/>.
</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>
A <glossterm linkend="glossary-table">table</glossterm>,
<glossterm linkend="glossary-view">view</glossterm>,
<glossterm linkend="glossary-materialized-view">materialized view</glossterm>,
<glossterm linkend="glossary-index">index</glossterm>,
<glossterm linkend="glossary-constraint">constraint</glossterm>,
<!-- XXX define sequence <glossterm linkend="glossary-sequence"> --> sequence,
<glossterm linkend="glossary-function">function</glossterm>,
<glossterm linkend="glossary-procedure">procedure</glossterm>,
<glossterm linkend="glossary-trigger">trigger</glossterm>,
data type, or operator. Every one of those SQL objects
belong to exactly one <glossterm linkend="glossary-schema">Schema</glossterm>.
</para>
<para>
There also exist SQL objects that do not belong to schemas; those include
<glossterm linkend="glossary-extension">extensions</glossterm>,
<glossterm linkend="glossary-cast">data type cases</glossterm>,
and
<glossterm linkend="glossary-foreign-data-wrapper">foreign data wrappers</glossterm>.
</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 id="glossary-stats-collector">
<glossterm>Stats Collector</glossterm>
<glossdef>
<para>
This process collects statistical information about the
<glossterm linkend="glossary-cluster">Cluster</glossterm>'s activities.
</para>
<para>
For more information, see
<xref linkend="monitoring-stats"/>.
</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 each <glossterm linkend="glossary-database">database</glossterm>
and the <glossterm linkend="glossary-global-sql-object">global SQL objects</glossterm>
of the <glossterm linkend="glossary-cluster">cluster</glossterm>.
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> that expose some
of the same and additional information as mandated by the
<glossterm linkend="glossary-sql-standard">SQL standard</glossterm>.
</para>
<para>
For more information, see
<xref linkend="ddl-schemas"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-table">
<glossterm>Table</glossterm>
<glossdef>
<para>
A collection of <glossterm linkend="glossary-tuple">tuples</glossterm> having
a common data structure (the same number of
<glossterm linkend="glossary-attribute">attributes</glossterm>, in the same
order, having the same name and type per position).
A table is the most common form of
<glossterm linkend="glossary-relation">Relation</glossterm> in
<productname>PostgreSQL</productname>.
</para>
<para>
For more information, see
<xref linkend="sql-createtable"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-tablespace">
<glossterm>Tablespace</glossterm>
<glossdef>
<para>
A named location on the server filesystem.
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, an instance contains a single usable tablespace which is
used as the default one for all SQL objects, called <literal>pg_default</literal>.
</para>
<para>
For more information, see
<xref linkend="manage-ag-tablespaces"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-temporary-table">
<glossterm>Temporary Table</glossterm>
<glossdef>
<para>
<glossterm linkend="glossary-table">Tables</glossterm> that exist either
for the lifetime of a
<glossterm linkend="glossary-session">session</glossterm> or a
<glossterm linkend="glossary-transaction">transaction</glossterm>, as
specified at the time of creation.
The data in them is not visible to other sessions, and is not
<glossterm linkend="glossary-logged">logged</glossterm>.
Temporary tables are often used to store intermediate data for a
multi-step operation.
</para>
<para>
For more information, see
<xref linkend="sql-createtable"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-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-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>
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>
A <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
<literal>LOGIN</literal> privilege.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-user-mapping">
<glossterm>User mapping</glossterm>
<glossdef>
<para>
The translation of login credentials in the local
<glossterm linkend="glossary-database">database</glossterm> to credentials
in a remote data system defined by a
<glossterm linkend="glossary-foreign-data-wrapper">foreign data wrapper</glossterm>.
</para>
<para>
For more information, see
<xref linkend="sql-createusermapping"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="glossary-vacuum">
<glossterm>Vacuum</glossterm>
<glossdef>
<para>
The process of removing outdated <glossterm linkend="glossary-tuple">tuple
versions</glossterm> from tables, and other closely related
garbage-collection-like 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-wal-archiver">
<glossterm>WAL Archiver (process)</glossterm>
<glossdef>
<para>
A process that saves copies of <glossterm linkend="glossary-wal-file">WAL files</glossterm>
for the purposes 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 are replayed to restore the system to the state as it was
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>
<glossterm>WAL</glossterm>
<glosssee otherterm="glossary-wal" />
</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>
<glossterm>WAL Segment</glossterm>
<glosssee otherterm="glossary-wal-file" />
</glossentry>
<glossentry id="glossary-wal-writer">
<glossterm>WAL Writer (process)</glossterm>
<glossdef>
<para>
A process that writes <glossterm linkend="glossary-wal-record">WAL records</glossterm>
from <glossterm id="linkend-shared-memory">shared memory</glossterm> to
<glossterm id="linkend-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</glossterm>
<glossdef>
<para>
A type of <glossterm linkend="glossary-function">function</glossterm> whose
result is based on values found in
<glossterm linkend="glossary-tuple">rows</glossterm> of the same
<glossterm linkend="glossary-partition">partition</glossterm>.
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-instance">instance</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>
doc/src/sgml/postgres.sgml
View file @ 347d2b07
......@@ -278,6 +278,7 @@
&docguide;
&limits;
&acronyms;
&glossary;
&color;
</part>
......
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment