<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>,
     <command>DELETE</command>, or <command>MERGE</command> operations.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-creatematerializedview"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-merge">
   <glossterm>Merge</glossterm>
   <glossdef>
    <para>
     An <acronym>SQL</acronym> command used to conditionally add, modify,
     or remove <glossterm linkend="glossary-tuple">rows</glossterm>
     in a given <glossterm linkend="glossary-table">table</glossterm>,
     using data from a source
     <glossterm linkend="glossary-relation">relation</glossterm>.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-merge"/>.
    </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.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-partitioned-table">
   <glossterm>Partitioned table (relation)</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>.