linkend="glossary-relation">relation</glossterm> that contains
     data derived from a <glossterm linkend="glossary-table">table</glossterm>
     or <glossterm linkend="glossary-materialized-view">materialized view</glossterm>.
     Its internal structure supports fast retrieval of and access to the original
     data.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-createindex"/>.
    </para>
   </glossdef>
  </glossentry>
  <glossentry id="glossary-incremental-backup">
   <glossterm>Incremental backup</glossterm>
   <glossdef>
    <para>
     A special <glossterm linkend="glossary-basebackup">base backup</glossterm>
     that for some files may contain only those pages that were modified since
     a previous backup, as opposed to the full contents of every file. Like
     base backups, it is generated by the tool <xref linkend="app-pgbasebackup"/>.
    </para>
    <para>
     To restore incremental backups the tool <xref linkend="app-pgcombinebackup"/>
     is used, which combines incremental backups with a base backup.
     Afterwards, recovery can use
     <glossterm linkend="glossary-wal">WAL</glossterm> to bring the
     <glossterm linkend="glossary-db-cluster">database cluster</glossterm> to
     a consistent state.
    </para>
    <para>
     For more information, see <xref linkend="backup-incremental-backup"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-io">
   <glossterm>Input/Output</glossterm>
   <acronym>I/O</acronym>
   <glossdef>
    <para>
     Input/Output (<acronym>I/O</acronym>) describes the communication between
     a program and peripheral devices. In the context of database systems,
     <acronym>I/O</acronym> commonly, but not exclusively, refers to
     interaction with storage devices or the network.
    </para>
    <glossseealso otherterm="glossary-aio" />
   </glossdef>
  </glossentry>

  <glossentry id="glossary-insert">
   <glossterm>Insert</glossterm>
   <glossdef>
    <para>
     An <acronym>SQL</acronym> command used to add new data into a
     <glossterm linkend="glossary-table">table</glossterm>.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-insert"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-instance">
   <glossterm>Instance</glossterm>
   <glossdef>
    <para>
     A group of <glossterm linkend="glossary-backend">backend</glossterm> and
     <glossterm linkend="glossary-auxiliary-proc">auxiliary processes</glossterm>
     that communicate using a common shared memory area.  One
     <glossterm linkend="glossary-postmaster">postmaster process</glossterm>
     manages the instance; one instance manages exactly one
     <glossterm linkend="glossary-db-cluster">database cluster</glossterm>
     with all its databases.  Many instances can run on the same
     <glossterm linkend="glossary-server">server</glossterm>
     as long as their <acronym>TCP</acronym> ports do not conflict.
    </para>
    <para>
     The instance handles all key features of a <acronym>DBMS</acronym>:
     read and write access to files and shared memory,
     assurance of the <acronym>ACID</acronym> properties,
     <glossterm linkend="glossary-connection">connections</glossterm> to
     <glossterm linkend="glossary-client">client processes</glossterm>,
     privilege verification, crash recovery, replication, etc.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-isolation">
   <glossterm>Isolation</glossterm>
   <glossdef>
    <para>
     The property that the effects of a transaction are not visible to
     <glossterm linkend="glossary-concurrency">concurrent transactions</glossterm>
     before it commits.
     This is one of the <acronym>ACID</acronym> properties.
    </para>
    <para>
     For more information, see <xref linkend="transaction-iso" />.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-join">
   <glossterm>Join</glossterm>
   <glossdef>