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postgresql
9th chunk of `doc/src/sgml/glossary.sgml`
3ce1517f173faaa17d6b04289ddecad47a62c9b181a8b5940000000100000fa0
 <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>
Title: SQL Commands and Data Concepts in PostgreSQL
Summary
This section covers several important SQL and data concepts in PostgreSQL. It defines the DELETE command as an SQL operation that removes rows from tables or relations. It explains domains as user-defined data types based on other types, potentially with value restrictions. Durability, an ACID property, is described as the assurance that committed transactions persist even after system failures. The text also introduces extensions as add-on packages for extra features, file segments as physical storage units for relations, foreign data wrappers as a means to represent external data as local tables, and foreign keys as constraints that ensure referential integrity between tables. The explanation begins to touch on foreign servers, which are likely related to accessing external data sources.