values
     that are equal to those.
    </para>
    <para>
     Because <glossterm linkend="glossary-null">null values</glossterm> are
     not considered equal to each other, multiple rows with null values are
     allowed to exist without violating the unique constraint.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-unlogged">
   <glossterm>Unlogged</glossterm>
   <glossdef>
    <para>
     The property of certain <glossterm linkend="glossary-relation">relations</glossterm>
     that the changes to them are not reflected in the
     <glossterm linkend="glossary-wal">WAL</glossterm>.
     This disables replication and crash recovery for these relations.
    </para>
    <para>
     The primary use of unlogged tables is for storing
     transient work data that must be shared across processes.
    </para>
    <para>
     <glossterm linkend="glossary-temporary-table">Temporary tables</glossterm>
     are always unlogged.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-update">
   <glossterm>Update</glossterm>
   <glossdef>
    <para>
     An <acronym>SQL</acronym> command used to modify
     <glossterm linkend="glossary-tuple">rows</glossterm>
     that may already exist in a specified <glossterm linkend="glossary-table">table</glossterm>.
     It cannot create or remove rows.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-update"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-user">
   <glossterm>User</glossterm>
   <glossdef>
    <para>
     A <glossterm linkend="glossary-role">role</glossterm> that has the
     <firstterm>login privilege</firstterm>
     (see <xref linkend="role-attributes"/>).
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-user-mapping">
   <glossterm>User mapping</glossterm>
   <glossdef>
    <para>
     The translation of login credentials in the local
     <glossterm linkend="glossary-database">database</glossterm> to credentials
     in a remote data system defined by a
     <glossterm linkend="glossary-foreign-data-wrapper">foreign data wrapper</glossterm>.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-createusermapping"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-utc">
   <glossterm>UTC</glossterm>
   <glossdef>
    <para>
     Universal Coordinated Time, the primary global time reference,
     approximately the time prevailing at the zero meridian of longitude.
     Often but inaccurately referred to as GMT (Greenwich Mean Time).
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-vacuum">
   <glossterm>Vacuum</glossterm>
   <glossdef>
    <para>
     The process of removing outdated
     <glossterm linkend="glossary-tuple">tuple versions</glossterm>
     from tables or materialized views, and other closely related
     processing required by <productname>PostgreSQL</productname>'s
     implementation of <glossterm linkend="glossary-mvcc">MVCC</glossterm>.
     This can be initiated through the use of
     the <command>VACUUM</command> command, but can also be handled automatically
     via <glossterm linkend="glossary-autovacuum">autovacuum</glossterm> processes.
    </para>
    <para>
     For more information, see
     <xref linkend="routine-vacuuming"/> .
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-view">
   <glossterm>View</glossterm>
   <glossdef>
    <para>
     A <glossterm linkend="glossary-relation">relation</glossterm> that is defined by a
     <command>SELECT</command> statement, but has no storage of its own.
     Any time a query references a view, the definition of the view is
     substituted into the query as if the user had typed it as a subquery
     instead of the name of the view.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-createview"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-vm">