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><A
NAME="CUSTOM-SCAN-PLAN"
>57.2. Creating Custom Scan Plans</A
></H1
><P
>    A custom scan is represented in a finished plan tree using the following
    structure:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct CustomScan
{
    Scan      scan;
    uint32    flags;
    List     *custom_plans;
    List     *custom_exprs;
    List     *custom_private;
    List     *custom_scan_tlist;
    Bitmapset *custom_relids;
    const CustomScanMethods *methods;
} CustomScan;</PRE
><P>
  </P
><P
>    <TT
CLASS="STRUCTFIELD"
>scan</TT
> must be initialized as for any other scan, including
    estimated costs, target lists, qualifications, and so on.
    <TT
CLASS="STRUCTFIELD"
>flags</TT
> is a bit mask with the same meaning as in
    <TT
CLASS="STRUCTNAME"
>CustomPath</TT
>.
    <TT
CLASS="STRUCTFIELD"
>custom_plans</TT
> can be used to store child
    <TT
CLASS="STRUCTNAME"
>Plan</TT
> nodes.
    <TT
CLASS="STRUCTFIELD"
>custom_exprs</TT
> should be used to
    store expression trees that will need to be fixed up by
    <TT
CLASS="FILENAME"
>setrefs.c</TT
> and <TT
CLASS="FILENAME"
>subselect.c</TT
>, while
    <TT
CLASS="STRUCTFIELD"
>custom_private</TT
> should be used to store other private data
    that is only used by the custom scan provider itself.
    <TT
CLASS="STRUCTFIELD"
>custom_scan_tlist</TT
> can be NIL when scanning a base
    relation, indicating that the custom scan returns scan tuples that match
    the base relation's row type.  Otherwise it is a target list describing
    the actual scan tuples.  <TT
CLASS="STRUCTFIELD"
>custom_scan_tlist</TT
> must be
    provided for joins, and could be provided for scans if the custom scan
    provider can compute some non-Var expressions.
    <TT
CLASS="STRUCTFIELD"
>custom_relids</TT
> is set by the core code to the set of
    relations (range table indexes) that this scan node handles; except when
    this scan is replacing a join, it will have only one member.
    <TT
CLASS="STRUCTFIELD"
>methods</TT
> must point to a (usually statically allocated)
    object implementing the required custom scan methods, which are further
    detailed below.
  </P
><P
>   When a <TT
CLASS="STRUCTNAME"
>CustomScan</TT
> scans a single relation,
   <TT
CLASS="STRUCTFIELD"
>scan.scanrelid</TT
> must be the range table index of the table
   to be scanned.  When it replaces a join, <TT
CLASS="STRUCTFIELD"
>scan.scanrelid</TT
>
   should be zero.
  </P
><P
>   Plan trees must be able to be duplicated using <CODE
CLASS="FUNCTION"
>copyObject</CODE
>,
   so all the data stored within the <SPAN
CLASS="QUOTE"
>"custom"</SPAN
> fields must consist of
   nodes that that function can handle.  Furthermore, custom scan providers
   cannot substitute a larger structure that embeds
   a <TT
CLASS="STRUCTNAME"
>CustomScan</TT
> for the structure itself, as would be possible
   for a <TT
CLASS="STRUCTNAME"
>CustomPath</TT
> or <TT
CLASS="STRUCTNAME"
>CustomScanState</TT
>.
  </P
><DIV
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><H2
CLASS="SECT2"
><A
NAME="CUSTOM-SCAN-PLAN-CALLBACKS"
>57.2.1. Custom Scan Plan Callbacks</A
></H2
><P
></P><PRE
CLASS="PROGRAMLISTING"
>Node *(*CreateCustomScanState) (CustomScan *cscan);</PRE
><P>
    Allocate a <TT
CLASS="STRUCTNAME"
>CustomScanState</TT
> for this
    <TT
CLASS="STRUCTNAME"
>CustomScan</TT
>.  The actual allocation will often be larger than
    required for an ordinary <TT
CLASS="STRUCTNAME"
>CustomScanState</TT
>, because many
    providers will wish to embed that as the first field of a larger structure.
    The value returned must have the node tag and <TT
CLASS="STRUCTFIELD"
>methods</TT
>
    set appropriately, but other fields should be left as zeroes at this
    stage; after <CODE
CLASS="FUNCTION"
>ExecInitCustomScan</CODE
> performs basic initialization,
    the <CODE
CLASS="FUNCTION"
>BeginCustomScan</CODE
> callback will be invoked to give the
    custom scan provider a chance to do whatever else is needed.
   </P
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