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><A
NAME="CUSTOM-SCAN-EXECUTION"
>57.3. Executing Custom Scans</A
></H1
><P
>   When a <TT
CLASS="STRUCTFIELD"
>CustomScan</TT
> is executed, its execution state is
   represented by a <TT
CLASS="STRUCTFIELD"
>CustomScanState</TT
>, which is declared as
   follows:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct CustomScanState
{
    ScanState ss;
    uint32    flags;
    const CustomExecMethods *methods;
} CustomScanState;</PRE
><P>
  </P
><P
>   <TT
CLASS="STRUCTFIELD"
>ss</TT
> is initialized as for any other scan state,
   except that if the scan is for a join rather than a base relation,
   <TT
CLASS="LITERAL"
>ss.ss_currentRelation</TT
> is left NULL.
   <TT
CLASS="STRUCTFIELD"
>flags</TT
> is a bit mask with the same meaning as in
   <TT
CLASS="STRUCTNAME"
>CustomPath</TT
> and <TT
CLASS="STRUCTNAME"
>CustomScan</TT
>.
   <TT
CLASS="STRUCTFIELD"
>methods</TT
> must point to a (usually statically allocated)
   object implementing the required custom scan state methods, which are
   further detailed below.  Typically, a <TT
CLASS="STRUCTNAME"
>CustomScanState</TT
>, which
   need not support <CODE
CLASS="FUNCTION"
>copyObject</CODE
>, will actually be a larger
   structure embedding the above as its first member.
  </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="CUSTOM-SCAN-EXECUTION-CALLBACKS"
>57.3.1. Custom Scan Execution Callbacks</A
></H2
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*BeginCustomScan) (CustomScanState *node,
                         EState *estate,
                         int eflags);</PRE
><P>
    Complete initialization of the supplied <TT
CLASS="STRUCTNAME"
>CustomScanState</TT
>.
    Standard fields have been initialized by <CODE
CLASS="FUNCTION"
>ExecInitCustomScan</CODE
>,
    but any private fields should be initialized here.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *(*ExecCustomScan) (CustomScanState *node);</PRE
><P>
    Fetch the next scan tuple.  If any tuples remain, it should fill
    <TT
CLASS="LITERAL"
>ps_ResultTupleSlot</TT
> with the next tuple in the current scan
    direction, and then return the tuple slot.  If not,
    <TT
CLASS="LITERAL"
>NULL</TT
> or an empty slot should be returned.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*EndCustomScan) (CustomScanState *node);</PRE
><P>
    Clean up any private data associated with the <TT
CLASS="LITERAL"
>CustomScanState</TT
>.
    This method is required, but it does not need to do anything if there is
    no associated data or it will be cleaned up automatically.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*ReScanCustomScan) (CustomScanState *node);</PRE
><P>
    Rewind the current scan to the beginning and prepare to rescan the
    relation.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*MarkPosCustomScan) (CustomScanState *node);</PRE
><P>
    Save the current scan position so that it can subsequently be restored
    by the <CODE
CLASS="FUNCTION"
>RestrPosCustomScan</CODE
> callback.  This callback is
    optional, and need only be supplied if the
    <TT
CLASS="LITERAL"
>CUSTOMPATH_SUPPORT_MARK_RESTORE</TT
> flag is set.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*RestrPosCustomScan) (CustomScanState *node);</PRE
><P>
    Restore the previous scan position as saved by the
    <CODE
CLASS="FUNCTION"
>MarkPosCustomScan</CODE
> callback.  This callback is optional,
    and need only be supplied if the
    <TT
CLASS="LITERAL"
>CUSTOMPATH_SUPPORT_MARK_RESTORE</TT
> flag is set.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>Size (*EstimateDSMCustomScan) (CustomScanState *node,
                               ParallelContext *pcxt);</PRE
><P>
    Estimate the amount of dynamic shared memory that will be required
    for parallel operation.  This may be higher than the amount that will
    actually be 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.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*InitializeDSMCustomScan) (CustomScanState *node,
                                 ParallelContext *pcxt,
                                 void *coordinate);</PRE
><P>
    Initialize the dynamic shared memory that will be required for parallel
    operation; <TT
CLASS="LITERAL"
>coordinate</TT
> points to an amount of allocated space
    equal to the return value of <CODE
CLASS="FUNCTION"
>EstimateDSMCustomScan</CODE
>.
    This callback is optional, and need only be supplied if this custom
    scan provider supports parallel execution.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*InitializeWorkerCustomScan) (CustomScanState *node,
                                    shm_toc *toc,
                                    void *coordinate);</PRE
><P>
    Initialize a parallel worker's custom state based on the shared state
    set up in the leader by <TT
CLASS="LITERAL"
>InitializeDSMCustomScan</TT
>.
    This callback is optional, and needs only be supplied if this
    custom path supports parallel execution.
   </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void (*ExplainCustomScan) (CustomScanState *node,
                           List *ancestors,
                           ExplainState *es);</PRE
><P>
    Output additional information for <TT
CLASS="COMMAND"
>EXPLAIN</TT
> of a custom-scan
    plan node.  This callback is optional.  Common data stored in the
    <TT
CLASS="STRUCTNAME"
>ScanState</TT
>, 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.
   </P
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