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><H1
CLASS="SECT1"
><A
NAME="FDW-CALLBACKS"
>50.2. Foreign Data Wrapper Callback Routines</A
></H1
><P
> The FDW handler function returns a palloc'd <TT
CLASS="STRUCTNAME"
>FdwRoutine</TT
>
struct containing pointers to the following callback functions:
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
GetForeignRelSize (PlannerInfo *root,
RelOptInfo *baserel,
Oid foreigntableid);</PRE
><P>
Obtain relation size estimates for a foreign table. This is called
at the beginning of planning for a query involving a foreign table.
<TT
CLASS="LITERAL"
>root</TT
> is the planner's global information about the query;
<TT
CLASS="LITERAL"
>baserel</TT
> is the planner's information about this table; and
<TT
CLASS="LITERAL"
>foreigntableid</TT
> is the <TT
CLASS="STRUCTNAME"
>pg_class</TT
> OID of the
foreign table. (<TT
CLASS="LITERAL"
>foreigntableid</TT
> could be obtained from the
planner data structures, but it's passed explicitly to save effort.)
</P
><P
> This function should update <TT
CLASS="LITERAL"
>baserel->rows</TT
> to be the
expected number of rows returned by the table scan, after accounting for
the filtering done by the restriction quals. The initial value of
<TT
CLASS="LITERAL"
>baserel->rows</TT
> is just a constant default estimate, which
should be replaced if at all possible. The function may also choose to
update <TT
CLASS="LITERAL"
>baserel->width</TT
> if it can compute a better estimate
of the average result row width.
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 50.4</A
> for additional information.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
GetForeignPaths (PlannerInfo *root,
RelOptInfo *baserel,
Oid foreigntableid);</PRE
><P>
Create possible access paths for a scan on a foreign table.
This is called during query planning.
The parameters are the same as for <CODE
CLASS="FUNCTION"
>GetForeignRelSize</CODE
>,
which has already been called.
</P
><P
> This function must generate at least one access path
(<TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> node) for a scan on the foreign table and
must call <CODE
CLASS="FUNCTION"
>add_path</CODE
> to add each such path to
<TT
CLASS="LITERAL"
>baserel->pathlist</TT
>. It's recommended to use
<CODE
CLASS="FUNCTION"
>create_foreignscan_path</CODE
> to build the
<TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> nodes. The function can generate multiple
access paths, e.g., a path which has valid <TT
CLASS="LITERAL"
>pathkeys</TT
> to
represent a pre-sorted result. Each access path must contain cost
estimates, and can contain any FDW-private information that is needed to
identify the specific scan method intended.
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 50.4</A
> for additional information.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>ForeignScan *
GetForeignPlan (PlannerInfo *root,
RelOptInfo *baserel,
Oid foreigntableid,
ForeignPath *best_path,
List *tlist,
List *scan_clauses);</PRE
><P>
Create a <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node from the selected foreign
access path. This is called at the end of query planning.
The parameters are as for <CODE
CLASS="FUNCTION"
>GetForeignRelSize</CODE
>, plus
the selected <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> (previously produced by
<CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
>), the target list to be emitted by the
plan node, and the restriction clauses to be enforced by the plan node.
</P
><P
> This function must create and return a <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan
node; it's recommended to use <CODE
CLASS="FUNCTION"
>make_foreignscan</CODE
> to build the
<TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> node.
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 50.4</A
> for additional information.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
ExplainForeignScan (ForeignScanState *node,
ExplainState *es);</PRE
><P>
Print additional <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output for a foreign table scan.
This can just return if there is no need to print anything.
Otherwise, it should call <CODE
CLASS="FUNCTION"
>ExplainPropertyText</CODE
> and
related functions to add fields to the <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output.
The flag fields in <TT
CLASS="LITERAL"
>es</TT
> can be used to determine what to
print, and the state of the <TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node
can be inspected to provide run-time statistics in the <TT
CLASS="COMMAND"
>EXPLAIN
ANALYZE</TT
> case.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
BeginForeignScan (ForeignScanState *node,
int eflags);</PRE
><P>
Begin executing a foreign scan. This is called during executor startup.
It should perform any initialization needed before the scan can start,
but not start executing the actual scan (that should be done upon the
first call to <CODE
CLASS="FUNCTION"
>IterateForeignScan</CODE
>).
The <TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node has already been created, but
its <TT
CLASS="STRUCTFIELD"
>fdw_state</TT
> field is still NULL. Information about
the table to scan is accessible through the
<TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node (in particular, from the underlying
<TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node, which contains any FDW-private
information provided by <CODE
CLASS="FUNCTION"
>GetForeignPlan</CODE
>).
</P
><P
> Note that when <TT
CLASS="LITERAL"
>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</TT
> is
true, this function should not perform any externally-visible actions;
it should only do the minimum required to make the node state valid
for <CODE
CLASS="FUNCTION"
>ExplainForeignScan</CODE
> and <CODE
CLASS="FUNCTION"
>EndForeignScan</CODE
>.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *
IterateForeignScan (ForeignScanState *node);</PRE
><P>
Fetch one row from the foreign source, returning it in a tuple table slot
(the node's <TT
CLASS="STRUCTFIELD"
>ScanTupleSlot</TT
> should be used for this
purpose). Return NULL if no more rows are available. The tuple table
slot infrastructure allows either a physical or virtual tuple to be
returned; in most cases the latter choice is preferable from a
performance standpoint. Note that this is called in a short-lived memory
context that will be reset between invocations. Create a memory context
in <CODE
CLASS="FUNCTION"
>BeginForeignScan</CODE
> if you need longer-lived storage, or use
the <TT
CLASS="STRUCTFIELD"
>es_query_cxt</TT
> of the node's <TT
CLASS="STRUCTNAME"
>EState</TT
>.
</P
><P
> The rows returned must match the column signature of the foreign table
being scanned. If you choose to optimize away fetching columns that
are not needed, you should insert nulls in those column positions.
</P
><P
> Note that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s executor doesn't care
whether the rows returned violate the <TT
CLASS="LITERAL"
>NOT NULL</TT
>
constraints which were defined on the foreign table columns - but the
planner does care, and may optimize queries incorrectly if
<TT
CLASS="LITERAL"
>NULL</TT
> values are present in a column declared not to contain
them. If a <TT
CLASS="LITERAL"
>NULL</TT
> value is encountered when the user has
declared that none should be present, it may be appropriate to raise an
error (just as you would need to do in the case of a data type mismatch).
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
ReScanForeignScan (ForeignScanState *node);</PRE
><P>
Restart the scan from the beginning. Note that any parameters the
scan depends on may have changed value, so the new scan does not
necessarily return exactly the same rows.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
EndForeignScan (ForeignScanState *node);</PRE
><P>
End the scan and release resources. It is normally not important
to release palloc'd memory, but for example open files and connections
to remote servers should be cleaned up.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>bool
AnalyzeForeignTable (Relation relation,
AcquireSampleRowsFunc *func,
BlockNumber *totalpages);</PRE
><P>
This function is called when <A
HREF="sql-analyze.html"
>ANALYZE</A
> is executed on
a foreign table. If the FDW can collect statistics for this
foreign table, it should return <TT
CLASS="LITERAL"
>true</TT
>, and provide a pointer
to a function that will collect sample rows from the table in
<TT
CLASS="PARAMETER"
>func</TT
>, plus the estimated size of the table in pages in
<TT
CLASS="PARAMETER"
>totalpages</TT
>. Otherwise, return <TT
CLASS="LITERAL"
>false</TT
>.
If the FDW does not support collecting statistics for any tables, the
<CODE
CLASS="FUNCTION"
>AnalyzeForeignTable</CODE
> pointer can be set to <TT
CLASS="LITERAL"
>NULL</TT
>.
</P
><P
> If provided, the sample collection function must have the signature
</P><PRE
CLASS="PROGRAMLISTING"
>int
AcquireSampleRowsFunc (Relation relation, int elevel,
HeapTuple *rows, int targrows,
double *totalrows,
double *totaldeadrows);</PRE
><P>
A random sample of up to <TT
CLASS="PARAMETER"
>targrows</TT
> rows should be collected
from the table and stored into the caller-provided <TT
CLASS="PARAMETER"
>rows</TT
>
array. The actual number of rows collected must be returned. In
addition, store estimates of the total numbers of live and dead rows in
the table into the output parameters <TT
CLASS="PARAMETER"
>totalrows</TT
> and
<TT
CLASS="PARAMETER"
>totaldeadrows</TT
>. (Set <TT
CLASS="PARAMETER"
>totaldeadrows</TT
> to zero
if the FDW does not have any concept of dead rows.)
</P
><P
> The <TT
CLASS="STRUCTNAME"
>FdwRoutine</TT
> struct type is declared in
<TT
CLASS="FILENAME"
>src/include/foreign/fdwapi.h</TT
>, which see for additional
details.
</P
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