used, but it must not be lower.  The return value is in bytes.
    This callback is optional, and need only be supplied if this custom
    scan provider supports parallel execution.
   </para>

   <para>
<programlisting>
void (*InitializeDSMCustomScan) (CustomScanState *node,
                                 ParallelContext *pcxt,
                                 void *coordinate);
</programlisting>
    Initialize the dynamic shared memory that will be required for parallel
    operation.  <literal>coordinate</literal> points to a shared memory area of
    size equal to the return value of <function>EstimateDSMCustomScan</function>.
    This callback is optional, and need only be supplied if this custom
    scan provider supports parallel execution.
   </para>

   <para>
<programlisting>
void (*ReInitializeDSMCustomScan) (CustomScanState *node,
                                   ParallelContext *pcxt,
                                   void *coordinate);
</programlisting>
    Re-initialize the dynamic shared memory required for parallel operation
    when the custom-scan plan node is about to be re-scanned.
    This callback is optional, and need only be supplied if this custom
    scan provider supports parallel execution.
    Recommended practice is that this callback reset only shared state,
    while the <function>ReScanCustomScan</function> callback resets only local
    state.  Currently, this callback will be called
    before <function>ReScanCustomScan</function>, but it's best not to rely on
    that ordering.
   </para>

   <para>
<programlisting>
void (*InitializeWorkerCustomScan) (CustomScanState *node,
                                    shm_toc *toc,
                                    void *coordinate);
</programlisting>
    Initialize a parallel worker's local state based on the shared state
    set up by the leader during <function>InitializeDSMCustomScan</function>.
    This callback is optional, and need only be supplied if this custom
    scan provider supports parallel execution.
   </para>

   <para>
<programlisting>
void (*ShutdownCustomScan) (CustomScanState *node);
</programlisting>
    Release resources when it is anticipated the node will not be executed
    to completion.  This is not called in all cases; sometimes,
    <literal>EndCustomScan</literal> may be called without this function having
    been called first.  Since the DSM segment used by parallel query is
    destroyed just after this callback is invoked, custom scan providers that
    wish to take some action before the DSM segment goes away should implement
    this method.
   </para>

   <para>
<programlisting>
void (*ExplainCustomScan) (CustomScanState *node,
                           List *ancestors,
                           ExplainState *es);
</programlisting>
    Output additional information for <command>EXPLAIN</command> of a custom-scan
    plan node.  This callback is optional.  Common data stored in the
    <structname>ScanState</structname>, such as the target list and scan relation, will
    be shown even without this callback, but the callback allows the display
    of additional, private state.
   </para>
  </sect2>
 </sect1>
</chapter>