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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FDW-CALLBACKS"
>55.2. 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 callback functions described below.
The scan-related functions are required, the rest are optional.
</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
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-SCAN"
>55.2.1. FDW Routines For Scanning Foreign Tables</A
></H2
><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 that scans 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 55.4</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 55.4</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,
Plan *outer_plan);</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
>, <CODE
CLASS="FUNCTION"
>GetForeignJoinPaths</CODE
>,
or <CODE
CLASS="FUNCTION"
>GetForeignUpperPaths</CODE
>),
the target list to be emitted by the plan node,
the restriction clauses to be enforced by the plan node,
and the outer subplan of the <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
>,
which is used for rechecks performed by <CODE
CLASS="FUNCTION"
>RecheckForeignScan</CODE
>.
(If the path is for a join rather than a base
relation, <TT
CLASS="LITERAL"
>foreigntableid</TT
> is <TT
CLASS="LITERAL"
>InvalidOid</TT
>.)
</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 55.4</A
> for additional information.
</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
>).
<TT
CLASS="LITERAL"
>eflags</TT
> contains flag bits describing the executor's
operating mode for this plan node.
</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 <TT
CLASS="STRUCTFIELD"
>fdw_scan_tlist</TT
> target
list if one was supplied, otherwise they must match the row type 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, or else generate a <TT
CLASS="STRUCTFIELD"
>fdw_scan_tlist</TT
> list with
those columns omitted.
</P
><P
> Note that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s executor doesn't care
whether the rows returned violate any constraints that were defined on
the foreign table — but the planner does care, and may optimize
queries incorrectly if there are rows visible in the foreign table that
do not satisfy a declared constraint. If a constraint is violated when
the user has declared that the constraint should hold true, 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
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-JOIN-SCAN"
>55.2.2. FDW Routines For Scanning Foreign Joins</A
></H2
><P
> If an FDW supports performing foreign joins remotely (rather than
by fetching both tables' data and doing the join locally), it should
provide this callback function:
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
GetForeignJoinPaths (PlannerInfo *root,
RelOptInfo *joinrel,
RelOptInfo *outerrel,
RelOptInfo *innerrel,
JoinType jointype,
JoinPathExtraData *extra);</PRE
><P>
Create possible access paths for a join of two (or more) foreign tables
that all belong to the same foreign server. This optional
function is called during query planning. As
with <CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
>, this function should
generate <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> path(s) for the
supplied <TT
CLASS="LITERAL"
>joinrel</TT
>, and call <CODE
CLASS="FUNCTION"
>add_path</CODE
> to add these
paths to the set of paths considered for the join. But unlike
<CODE
CLASS="FUNCTION"
>GetForeignPaths</CODE
>, it is not necessary that this function
succeed in creating at least one path, since paths involving local
joining are always possible.
</P
><P
> Note that this function will be invoked repeatedly for the same join
relation, with different combinations of inner and outer relations; it is
the responsibility of the FDW to minimize duplicated work.
</P
><P
> If a <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> path is chosen for the join, it will
represent the entire join process; paths generated for the component
tables and subsidiary joins will not be used. Subsequent processing of
the join path proceeds much as it does for a path scanning a single
foreign table. One difference is that the <TT
CLASS="STRUCTFIELD"
>scanrelid</TT
> of
the resulting <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node should be set to zero,
since there is no single relation that it represents; instead,
the <TT
CLASS="STRUCTFIELD"
>fs_relids</TT
> field of the <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
>
node represents the set of relations that were joined. (The latter field
is set up automatically by the core planner code, and need not be filled
by the FDW.) Another difference is that, because the column list for a
remote join cannot be found from the system catalogs, the FDW must
fill <TT
CLASS="STRUCTFIELD"
>fdw_scan_tlist</TT
> with an appropriate list
of <TT
CLASS="STRUCTFIELD"
>TargetEntry</TT
> nodes, representing the set of columns
it will supply at run time in the tuples it returns.
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 55.4</A
> for additional information.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-UPPER-PLANNING"
>55.2.3. FDW Routines For Planning Post-Scan/Join Processing</A
></H2
><P
> If an FDW supports performing remote post-scan/join processing, such as
remote aggregation, it should provide this callback function:
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
GetForeignUpperPaths (PlannerInfo *root,
UpperRelationKind stage,
RelOptInfo *input_rel,
RelOptInfo *output_rel);</PRE
><P>
Create possible access paths for <I
CLASS="FIRSTTERM"
>upper relation</I
> processing,
which is the planner's term for all post-scan/join query processing, such
as aggregation, window functions, sorting, and table updates. This
optional function is called during query planning. Currently, it is
called only if all base relation(s) involved in the query belong to the
same FDW. This function should generate <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
>
path(s) for any post-scan/join processing that the FDW knows how to
perform remotely, and call <CODE
CLASS="FUNCTION"
>add_path</CODE
> to add these paths to
the indicated upper relation. As with <CODE
CLASS="FUNCTION"
>GetForeignJoinPaths</CODE
>,
it is not necessary that this function succeed in creating any paths,
since paths involving local processing are always possible.
</P
><P
> The <TT
CLASS="LITERAL"
>stage</TT
> parameter identifies which post-scan/join step is
currently being considered. <TT
CLASS="LITERAL"
>output_rel</TT
> is the upper relation
that should receive paths representing computation of this step,
and <TT
CLASS="LITERAL"
>input_rel</TT
> is the relation representing the input to this
step. (Note that <TT
CLASS="STRUCTNAME"
>ForeignPath</TT
> paths added
to <TT
CLASS="LITERAL"
>output_rel</TT
> would typically not have any direct dependency
on paths of the <TT
CLASS="LITERAL"
>input_rel</TT
>, since their processing is expected
to be done externally. However, examining paths previously generated for
the previous processing step can be useful to avoid redundant planning
work.)
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 55.4</A
> for additional information.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-UPDATE"
>55.2.4. FDW Routines For Updating Foreign Tables</A
></H2
><P
> If an FDW supports writable foreign tables, it should provide
some or all of the following callback functions depending on
the needs and capabilities of the FDW:
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
AddForeignUpdateTargets (Query *parsetree,
RangeTblEntry *target_rte,
Relation target_relation);</PRE
><P>
<TT
CLASS="COMMAND"
>UPDATE</TT
> and <TT
CLASS="COMMAND"
>DELETE</TT
> operations are performed
against rows previously fetched by the table-scanning functions. The
FDW may need extra information, such as a row ID or the values of
primary-key columns, to ensure that it can identify the exact row to
update or delete. To support that, this function can add extra hidden,
or <SPAN
CLASS="QUOTE"
>"junk"</SPAN
>, target columns to the list of columns that are to be
retrieved from the foreign table during an <TT
CLASS="COMMAND"
>UPDATE</TT
> or
<TT
CLASS="COMMAND"
>DELETE</TT
>.
</P
><P
> To do that, add <TT
CLASS="STRUCTNAME"
>TargetEntry</TT
> items to
<TT
CLASS="LITERAL"
>parsetree->targetList</TT
>, containing expressions for the
extra values to be fetched. Each such entry must be marked
<TT
CLASS="STRUCTFIELD"
>resjunk</TT
> = <TT
CLASS="LITERAL"
>true</TT
>, and must have a distinct
<TT
CLASS="STRUCTFIELD"
>resname</TT
> that will identify it at execution time.
Avoid using names matching <TT
CLASS="LITERAL"
>ctid<TT
CLASS="REPLACEABLE"
><I
>N</I
></TT
></TT
>,
<TT
CLASS="LITERAL"
>wholerow</TT
>, or
<TT
CLASS="LITERAL"
>wholerow<TT
CLASS="REPLACEABLE"
><I
>N</I
></TT
></TT
>, as the core system can
generate junk columns of these names.
If the extra expressions are more complex than simple Vars, they
must be run through <CODE
CLASS="FUNCTION"
>eval_const_expressions</CODE
>
before adding them to the targetlist.
</P
><P
> Although this function is called during planning, the
information provided is a bit different from that available to other
planning routines.
<TT
CLASS="LITERAL"
>parsetree</TT
> is the parse tree for the <TT
CLASS="COMMAND"
>UPDATE</TT
> or
<TT
CLASS="COMMAND"
>DELETE</TT
> command, while <TT
CLASS="LITERAL"
>target_rte</TT
> and
<TT
CLASS="LITERAL"
>target_relation</TT
> describe the target foreign table.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>AddForeignUpdateTargets</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no extra target expressions are added.
(This will make it impossible to implement <TT
CLASS="COMMAND"
>DELETE</TT
>
operations, though <TT
CLASS="COMMAND"
>UPDATE</TT
> may still be feasible if the FDW
relies on an unchanging primary key to identify rows.)
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>List *
PlanForeignModify (PlannerInfo *root,
ModifyTable *plan,
Index resultRelation,
int subplan_index);</PRE
><P>
Perform any additional planning actions needed for an insert, update, or
delete on a foreign table. This function generates the FDW-private
information that will be attached to the <TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan
node that performs the update action. This private information must
have the form of a <TT
CLASS="LITERAL"
>List</TT
>, and will be delivered to
<CODE
CLASS="FUNCTION"
>BeginForeignModify</CODE
> during the execution stage.
</P
><P
> <TT
CLASS="LITERAL"
>root</TT
> is the planner's global information about the query.
<TT
CLASS="LITERAL"
>plan</TT
> is the <TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node, which is
complete except for the <TT
CLASS="STRUCTFIELD"
>fdwPrivLists</TT
> field.
<TT
CLASS="LITERAL"
>resultRelation</TT
> identifies the target foreign table by its
range table index. <TT
CLASS="LITERAL"
>subplan_index</TT
> identifies which target of
the <TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node this is, counting from zero;
use this if you want to index into <TT
CLASS="LITERAL"
>plan->plans</TT
> or other
substructure of the <TT
CLASS="LITERAL"
>plan</TT
> node.
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 55.4</A
> for additional information.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>PlanForeignModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no additional plan-time actions are taken, and the
<TT
CLASS="LITERAL"
>fdw_private</TT
> list delivered to
<CODE
CLASS="FUNCTION"
>BeginForeignModify</CODE
> will be NIL.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
BeginForeignModify (ModifyTableState *mtstate,
ResultRelInfo *rinfo,
List *fdw_private,
int subplan_index,
int eflags);</PRE
><P>
Begin executing a foreign table modification operation. This routine is
called during executor startup. It should perform any initialization
needed prior to the actual table modifications. Subsequently,
<CODE
CLASS="FUNCTION"
>ExecForeignInsert</CODE
>, <CODE
CLASS="FUNCTION"
>ExecForeignUpdate</CODE
> or
<CODE
CLASS="FUNCTION"
>ExecForeignDelete</CODE
> will be called for each tuple to be
inserted, updated, or deleted.
</P
><P
> <TT
CLASS="LITERAL"
>mtstate</TT
> is the overall state of the
<TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node being executed; global data about
the plan and execution state is available via this structure.
<TT
CLASS="LITERAL"
>rinfo</TT
> is the <TT
CLASS="STRUCTNAME"
>ResultRelInfo</TT
> struct describing
the target foreign table. (The <TT
CLASS="STRUCTFIELD"
>ri_FdwState</TT
> field of
<TT
CLASS="STRUCTNAME"
>ResultRelInfo</TT
> is available for the FDW to store any
private state it needs for this operation.)
<TT
CLASS="LITERAL"
>fdw_private</TT
> contains the private data generated by
<CODE
CLASS="FUNCTION"
>PlanForeignModify</CODE
>, if any.
<TT
CLASS="LITERAL"
>subplan_index</TT
> identifies which target of
the <TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node this is.
<TT
CLASS="LITERAL"
>eflags</TT
> contains flag bits describing the executor's
operating mode for this plan node.
</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"
>ExplainForeignModify</CODE
> and <CODE
CLASS="FUNCTION"
>EndForeignModify</CODE
>.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>BeginForeignModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no action is taken during executor startup.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *
ExecForeignInsert (EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot *slot,
TupleTableSlot *planSlot);</PRE
><P>
Insert one tuple into the foreign table.
<TT
CLASS="LITERAL"
>estate</TT
> is global execution state for the query.
<TT
CLASS="LITERAL"
>rinfo</TT
> is the <TT
CLASS="STRUCTNAME"
>ResultRelInfo</TT
> struct describing
the target foreign table.
<TT
CLASS="LITERAL"
>slot</TT
> contains the tuple to be inserted; it will match the
row-type definition of the foreign table.
<TT
CLASS="LITERAL"
>planSlot</TT
> contains the tuple that was generated by the
<TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node's subplan; it differs from
<TT
CLASS="LITERAL"
>slot</TT
> in possibly containing additional <SPAN
CLASS="QUOTE"
>"junk"</SPAN
>
columns. (The <TT
CLASS="LITERAL"
>planSlot</TT
> is typically of little interest
for <TT
CLASS="COMMAND"
>INSERT</TT
> cases, but is provided for completeness.)
</P
><P
> The return value is either a slot containing the data that was actually
inserted (this might differ from the data supplied, for example as a
result of trigger actions), or NULL if no row was actually inserted
(again, typically as a result of triggers). The passed-in
<TT
CLASS="LITERAL"
>slot</TT
> can be re-used for this purpose.
</P
><P
> The data in the returned slot is used only if the <TT
CLASS="COMMAND"
>INSERT</TT
>
query has a <TT
CLASS="LITERAL"
>RETURNING</TT
> clause or the foreign table has
an <TT
CLASS="LITERAL"
>AFTER ROW</TT
> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <TT
CLASS="LITERAL"
>RETURNING</TT
> clause. Regardless, some
slot must be returned to indicate success, or the query's reported row
count will be wrong.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>ExecForeignInsert</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, attempts to insert into the foreign table will fail
with an error message.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *
ExecForeignUpdate (EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot *slot,
TupleTableSlot *planSlot);</PRE
><P>
Update one tuple in the foreign table.
<TT
CLASS="LITERAL"
>estate</TT
> is global execution state for the query.
<TT
CLASS="LITERAL"
>rinfo</TT
> is the <TT
CLASS="STRUCTNAME"
>ResultRelInfo</TT
> struct describing
the target foreign table.
<TT
CLASS="LITERAL"
>slot</TT
> contains the new data for the tuple; it will match the
row-type definition of the foreign table.
<TT
CLASS="LITERAL"
>planSlot</TT
> contains the tuple that was generated by the
<TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node's subplan; it differs from
<TT
CLASS="LITERAL"
>slot</TT
> in possibly containing additional <SPAN
CLASS="QUOTE"
>"junk"</SPAN
>
columns. In particular, any junk columns that were requested by
<CODE
CLASS="FUNCTION"
>AddForeignUpdateTargets</CODE
> will be available from this slot.
</P
><P
> The return value is either a slot containing the row as it was actually
updated (this might differ from the data supplied, for example as a
result of trigger actions), or NULL if no row was actually updated
(again, typically as a result of triggers). The passed-in
<TT
CLASS="LITERAL"
>slot</TT
> can be re-used for this purpose.
</P
><P
> The data in the returned slot is used only if the <TT
CLASS="COMMAND"
>UPDATE</TT
>
query has a <TT
CLASS="LITERAL"
>RETURNING</TT
> clause or the foreign table has
an <TT
CLASS="LITERAL"
>AFTER ROW</TT
> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <TT
CLASS="LITERAL"
>RETURNING</TT
> clause. Regardless, some
slot must be returned to indicate success, or the query's reported row
count will be wrong.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>ExecForeignUpdate</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, attempts to update the foreign table will fail
with an error message.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *
ExecForeignDelete (EState *estate,
ResultRelInfo *rinfo,
TupleTableSlot *slot,
TupleTableSlot *planSlot);</PRE
><P>
Delete one tuple from the foreign table.
<TT
CLASS="LITERAL"
>estate</TT
> is global execution state for the query.
<TT
CLASS="LITERAL"
>rinfo</TT
> is the <TT
CLASS="STRUCTNAME"
>ResultRelInfo</TT
> struct describing
the target foreign table.
<TT
CLASS="LITERAL"
>slot</TT
> contains nothing useful upon call, but can be used to
hold the returned tuple.
<TT
CLASS="LITERAL"
>planSlot</TT
> contains the tuple that was generated by the
<TT
CLASS="STRUCTNAME"
>ModifyTable</TT
> plan node's subplan; in particular, it will
carry any junk columns that were requested by
<CODE
CLASS="FUNCTION"
>AddForeignUpdateTargets</CODE
>. The junk column(s) must be used
to identify the tuple to be deleted.
</P
><P
> The return value is either a slot containing the row that was deleted,
or NULL if no row was deleted (typically as a result of triggers). The
passed-in <TT
CLASS="LITERAL"
>slot</TT
> can be used to hold the tuple to be returned.
</P
><P
> The data in the returned slot is used only if the <TT
CLASS="COMMAND"
>DELETE</TT
>
query has a <TT
CLASS="LITERAL"
>RETURNING</TT
> clause or the foreign table has
an <TT
CLASS="LITERAL"
>AFTER ROW</TT
> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <TT
CLASS="LITERAL"
>RETURNING</TT
> clause. Regardless, some
slot must be returned to indicate success, or the query's reported row
count will be wrong.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>ExecForeignDelete</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, attempts to delete from the foreign table will fail
with an error message.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
EndForeignModify (EState *estate,
ResultRelInfo *rinfo);</PRE
><P>
End the table update 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
> If the <CODE
CLASS="FUNCTION"
>EndForeignModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no action is taken during executor shutdown.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>int
IsForeignRelUpdatable (Relation rel);</PRE
><P>
Report which update operations the specified foreign table supports.
The return value should be a bit mask of rule event numbers indicating
which operations are supported by the foreign table, using the
<TT
CLASS="LITERAL"
>CmdType</TT
> enumeration; that is,
<TT
CLASS="LITERAL"
>(1 << CMD_UPDATE) = 4</TT
> for <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="LITERAL"
>(1 << CMD_INSERT) = 8</TT
> for <TT
CLASS="COMMAND"
>INSERT</TT
>, and
<TT
CLASS="LITERAL"
>(1 << CMD_DELETE) = 16</TT
> for <TT
CLASS="COMMAND"
>DELETE</TT
>.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>IsForeignRelUpdatable</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, foreign tables are assumed to be insertable, updatable,
or deletable if the FDW provides <CODE
CLASS="FUNCTION"
>ExecForeignInsert</CODE
>,
<CODE
CLASS="FUNCTION"
>ExecForeignUpdate</CODE
>, or <CODE
CLASS="FUNCTION"
>ExecForeignDelete</CODE
>
respectively. This function is only needed if the FDW supports some
tables that are updatable and some that are not. (Even then, it's
permissible to throw an error in the execution routine instead of
checking in this function. However, this function is used to determine
updatability for display in the <TT
CLASS="LITERAL"
>information_schema</TT
> views.)
</P
><P
> Some inserts, updates, and deletes to foreign tables can be optimized
by implementing an alternative set of interfaces. The ordinary
interfaces for inserts, updates, and deletes fetch rows from the remote
server and then modify those rows one at a time. In some cases, this
row-by-row approach is necessary, but it can be inefficient. If it is
possible for the foreign server to determine which rows should be
modified without actually retrieving them, and if there are no local
triggers which would affect the operation, then it is possible to
arrange things so that the entire operation is performed on the remote
server. The interfaces described below make this possible.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>bool
PlanDirectModify (PlannerInfo *root,
ModifyTable *plan,
Index resultRelation,
int subplan_index);</PRE
><P>
Decide whether it is safe to execute a direct modification
on the remote server. If so, return <TT
CLASS="LITERAL"
>true</TT
> after performing
planning actions needed for that. Otherwise, return <TT
CLASS="LITERAL"
>false</TT
>.
This optional function is called during query planning.
If this function succeeds, <CODE
CLASS="FUNCTION"
>BeginDirectModify</CODE
>,
<CODE
CLASS="FUNCTION"
>IterateDirectModify</CODE
> and <CODE
CLASS="FUNCTION"
>EndDirectModify</CODE
> will
be called at the execution stage, instead. Otherwise, the table
modification will be executed using the table-updating functions
described above.
The parameters are the same as for <CODE
CLASS="FUNCTION"
>PlanForeignModify</CODE
>.
</P
><P
> To execute the direct modification on the remote server, this function
must rewrite the target subplan with a <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan
node that executes the direct modification on the remote server. The
<TT
CLASS="STRUCTFIELD"
>operation</TT
> field of the <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> must
be set to the <TT
CLASS="LITERAL"
>CmdType</TT
> enumeration appropriately; that is,
<TT
CLASS="LITERAL"
>CMD_UPDATE</TT
> for <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="LITERAL"
>CMD_INSERT</TT
> for <TT
CLASS="COMMAND"
>INSERT</TT
>, and
<TT
CLASS="LITERAL"
>CMD_DELETE</TT
> for <TT
CLASS="COMMAND"
>DELETE</TT
>.
</P
><P
> See <A
HREF="fdw-planning.html"
>Section 55.4</A
> for additional information.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>PlanDirectModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no attempts to execute a direct modification on the
remote server are taken.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
BeginDirectModify (ForeignScanState *node,
int eflags);</PRE
><P>
Prepare to execute a direct modification on the remote server.
This is called during executor startup. It should perform any
initialization needed prior to the direct modification (that should be
done upon the first call to <CODE
CLASS="FUNCTION"
>IterateDirectModify</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 modify 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"
>PlanDirectModify</CODE
>).
<TT
CLASS="LITERAL"
>eflags</TT
> contains flag bits describing the executor's
operating mode for this plan node.
</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"
>ExplainDirectModify</CODE
> and <CODE
CLASS="FUNCTION"
>EndDirectModify</CODE
>.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>BeginDirectModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no attempts to execute a direct modification on the
remote server are taken.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *
IterateDirectModify (ForeignScanState *node);</PRE
><P>
When the <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
> or <TT
CLASS="COMMAND"
>DELETE</TT
>
query doesn't have a <TT
CLASS="LITERAL"
>RETURNING</TT
> clause, just return NULL
after a direct modification on the remote server.
When the query has the clause, fetch one result containing the data
needed for the <TT
CLASS="LITERAL"
>RETURNING</TT
> calculation, returning it in a
tuple table slot (the node's <TT
CLASS="STRUCTFIELD"
>ScanTupleSlot</TT
> should be
used for this purpose). The data that was actually inserted, updated
or deleted must be stored in the
<TT
CLASS="LITERAL"
>es_result_relation_info->ri_projectReturning->pi_exprContext->ecxt_scantuple</TT
>
of the node's <TT
CLASS="STRUCTNAME"
>EState</TT
>.
Return NULL if no more rows are available.
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"
>BeginDirectModify</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 <TT
CLASS="STRUCTFIELD"
>fdw_scan_tlist</TT
> target
list if one was supplied, otherwise they must match the row type of the
foreign table being updated. If you choose to optimize away fetching
columns that are not needed for the <TT
CLASS="LITERAL"
>RETURNING</TT
> calculation,
you should insert nulls in those column positions, or else generate a
<TT
CLASS="STRUCTFIELD"
>fdw_scan_tlist</TT
> list with those columns omitted.
</P
><P
> Whether the query has the clause or not, the query's reported row count
must be incremented by the FDW itself. When the query doesn't have the
clause, the FDW must also increment the row count for the
<TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node in the <TT
CLASS="COMMAND"
>EXPLAIN ANALYZE</TT
>
case.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>IterateDirectModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no attempts to execute a direct modification on the
remote server are taken.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
EndDirectModify (ForeignScanState *node);</PRE
><P>
Clean up following a direct modification on the remote server. It is
normally not important to release palloc'd memory, but for example open
files and connections to the remote server should be cleaned up.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>EndDirectModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no attempts to execute a direct modification on the
remote server are taken.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-ROW-LOCKING"
>55.2.5. FDW Routines For Row Locking</A
></H2
><P
> If an FDW wishes to support <I
CLASS="FIRSTTERM"
>late row locking</I
> (as described
in <A
HREF="fdw-row-locking.html"
>Section 55.5</A
>), it must provide the following
callback functions:
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>RowMarkType
GetForeignRowMarkType (RangeTblEntry *rte,
LockClauseStrength strength);</PRE
><P>
Report which row-marking option to use for a foreign table.
<TT
CLASS="LITERAL"
>rte</TT
> is the <TT
CLASS="STRUCTNAME"
>RangeTblEntry</TT
> node for the table
and <TT
CLASS="LITERAL"
>strength</TT
> describes the lock strength requested by the
relevant <TT
CLASS="LITERAL"
>FOR UPDATE/SHARE</TT
> clause, if any. The result must be
a member of the <TT
CLASS="LITERAL"
>RowMarkType</TT
> enum type.
</P
><P
> This function is called during query planning for each foreign table that
appears in an <TT
CLASS="COMMAND"
>UPDATE</TT
>, <TT
CLASS="COMMAND"
>DELETE</TT
>, or <TT
CLASS="COMMAND"
>SELECT
FOR UPDATE/SHARE</TT
> query and is not the target of <TT
CLASS="COMMAND"
>UPDATE</TT
>
or <TT
CLASS="COMMAND"
>DELETE</TT
>.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>GetForeignRowMarkType</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, the <TT
CLASS="LITERAL"
>ROW_MARK_COPY</TT
> option is always used.
(This implies that <CODE
CLASS="FUNCTION"
>RefetchForeignRow</CODE
> will never be called,
so it need not be provided either.)
</P
><P
> See <A
HREF="fdw-row-locking.html"
>Section 55.5</A
> for more information.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>HeapTuple
RefetchForeignRow (EState *estate,
ExecRowMark *erm,
Datum rowid,
bool *updated);</PRE
><P>
Re-fetch one tuple from the foreign table, after locking it if required.
<TT
CLASS="LITERAL"
>estate</TT
> is global execution state for the query.
<TT
CLASS="LITERAL"
>erm</TT
> is the <TT
CLASS="STRUCTNAME"
>ExecRowMark</TT
> struct describing
the target foreign table and the row lock type (if any) to acquire.
<TT
CLASS="LITERAL"
>rowid</TT
> identifies the tuple to be fetched.
<TT
CLASS="LITERAL"
>updated</TT
> is an output parameter.
</P
><P
> This function should return a palloc'ed copy of the fetched tuple,
or <TT
CLASS="LITERAL"
>NULL</TT
> if the row lock couldn't be obtained. The row lock
type to acquire is defined by <TT
CLASS="LITERAL"
>erm->markType</TT
>, which is the
value previously returned by <CODE
CLASS="FUNCTION"
>GetForeignRowMarkType</CODE
>.
(<TT
CLASS="LITERAL"
>ROW_MARK_REFERENCE</TT
> means to just re-fetch the tuple without
acquiring any lock, and <TT
CLASS="LITERAL"
>ROW_MARK_COPY</TT
> will never be seen by
this routine.)
</P
><P
> In addition, <TT
CLASS="LITERAL"
>*updated</TT
> should be set to <TT
CLASS="LITERAL"
>true</TT
>
if what was fetched was an updated version of the tuple rather than
the same version previously obtained. (If the FDW cannot be sure about
this, always returning <TT
CLASS="LITERAL"
>true</TT
> is recommended.)
</P
><P
> Note that by default, failure to acquire a row lock should result in
raising an error; a <TT
CLASS="LITERAL"
>NULL</TT
> return is only appropriate if
the <TT
CLASS="LITERAL"
>SKIP LOCKED</TT
> option is specified
by <TT
CLASS="LITERAL"
>erm->waitPolicy</TT
>.
</P
><P
> The <TT
CLASS="LITERAL"
>rowid</TT
> is the <TT
CLASS="STRUCTFIELD"
>ctid</TT
> value previously read
for the row to be re-fetched. Although the <TT
CLASS="LITERAL"
>rowid</TT
> value is
passed as a <TT
CLASS="TYPE"
>Datum</TT
>, it can currently only be a <TT
CLASS="TYPE"
>tid</TT
>. The
function API is chosen in hopes that it may be possible to allow other
data types for row IDs in future.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>RefetchForeignRow</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, attempts to re-fetch rows will fail
with an error message.
</P
><P
> See <A
HREF="fdw-row-locking.html"
>Section 55.5</A
> for more information.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>bool
RecheckForeignScan (ForeignScanState *node, TupleTableSlot *slot);</PRE
><P>
Recheck that a previously-returned tuple still matches the relevant
scan and join qualifiers, and possibly provide a modified version of
the tuple. For foreign data wrappers which do not perform join pushdown,
it will typically be more convenient to set this to <TT
CLASS="LITERAL"
>NULL</TT
> and
instead set <TT
CLASS="STRUCTFIELD"
>fdw_recheck_quals</TT
> appropriately.
When outer joins are pushed down, however, it isn't sufficient to
reapply the checks relevant to all the base tables to the result tuple,
even if all needed attributes are present, because failure to match some
qualifier might result in some attributes going to NULL, rather than in
no tuple being returned. <TT
CLASS="LITERAL"
>RecheckForeignScan</TT
> can recheck
qualifiers and return true if they are still satisfied and false
otherwise, but it can also store a replacement tuple into the supplied
slot.
</P
><P
> To implement join pushdown, a foreign data wrapper will typically
construct an alternative local join plan which is used only for
rechecks; this will become the outer subplan of the
<TT
CLASS="LITERAL"
>ForeignScan</TT
>. When a recheck is required, this subplan
can be executed and the resulting tuple can be stored in the slot.
This plan need not be efficient since no base table will return more
than one row; for example, it may implement all joins as nested loops.
The function <TT
CLASS="LITERAL"
>GetExistingLocalJoinPath</TT
> may be used to search
existing paths for a suitable local join path, which can be used as the
alternative local join plan. <TT
CLASS="LITERAL"
>GetExistingLocalJoinPath</TT
>
searches for an unparameterized path in the path list of the specified
join relation. (If it does not find such a path, it returns NULL, in
which case a foreign data wrapper may build the local path by itself or
may choose not to create access paths for that join.)
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-EXPLAIN"
>55.2.6. FDW Routines for <TT
CLASS="COMMAND"
>EXPLAIN</TT
></A
></H2
><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 function can 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
> If the <CODE
CLASS="FUNCTION"
>ExplainForeignScan</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no additional information is printed during
<TT
CLASS="COMMAND"
>EXPLAIN</TT
>.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
ExplainForeignModify (ModifyTableState *mtstate,
ResultRelInfo *rinfo,
List *fdw_private,
int subplan_index,
struct ExplainState *es);</PRE
><P>
Print additional <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output for a foreign table update.
This function can 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"
>ModifyTableState</TT
> node
can be inspected to provide run-time statistics in the <TT
CLASS="COMMAND"
>EXPLAIN
ANALYZE</TT
> case. The first four arguments are the same as for
<CODE
CLASS="FUNCTION"
>BeginForeignModify</CODE
>.
</P
><P
> If the <CODE
CLASS="FUNCTION"
>ExplainForeignModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no additional information is printed during
<TT
CLASS="COMMAND"
>EXPLAIN</TT
>.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
ExplainDirectModify (ForeignScanState *node,
ExplainState *es);</PRE
><P>
Print additional <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output for a direct modification
on the remote server.
This function can 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
> If the <CODE
CLASS="FUNCTION"
>ExplainDirectModify</CODE
> pointer is set to
<TT
CLASS="LITERAL"
>NULL</TT
>, no additional information is printed during
<TT
CLASS="COMMAND"
>EXPLAIN</TT
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-ANALYZE"
>55.2.7. FDW Routines for <TT
CLASS="COMMAND"
>ANALYZE</TT
></A
></H2
><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
>.
</P
><P
> 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
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-IMPORT"
>55.2.8. FDW Routines For <TT
CLASS="COMMAND"
>IMPORT FOREIGN SCHEMA</TT
></A
></H2
><P
></P><PRE
CLASS="PROGRAMLISTING"
>List *
ImportForeignSchema (ImportForeignSchemaStmt *stmt, Oid serverOid);</PRE
><P>
Obtain a list of foreign table creation commands. This function is
called when executing <A
HREF="sql-importforeignschema.html"
>IMPORT FOREIGN SCHEMA</A
>, and is
passed the parse tree for that statement, as well as the OID of the
foreign server to use. It should return a list of C strings, each of
which must contain a <A
HREF="sql-createforeigntable.html"
>CREATE FOREIGN TABLE</A
> command.
These strings will be parsed and executed by the core server.
</P
><P
> Within the <TT
CLASS="STRUCTNAME"
>ImportForeignSchemaStmt</TT
> struct,
<TT
CLASS="STRUCTFIELD"
>remote_schema</TT
> is the name of the remote schema from
which tables are to be imported.
<TT
CLASS="STRUCTFIELD"
>list_type</TT
> identifies how to filter table names:
<TT
CLASS="LITERAL"
>FDW_IMPORT_SCHEMA_ALL</TT
> means that all tables in the remote
schema should be imported (in this case <TT
CLASS="STRUCTFIELD"
>table_list</TT
> is
empty), <TT
CLASS="LITERAL"
>FDW_IMPORT_SCHEMA_LIMIT_TO</TT
> means to include only
tables listed in <TT
CLASS="STRUCTFIELD"
>table_list</TT
>,
and <TT
CLASS="LITERAL"
>FDW_IMPORT_SCHEMA_EXCEPT</TT
> means to exclude the tables
listed in <TT
CLASS="STRUCTFIELD"
>table_list</TT
>.
<TT
CLASS="STRUCTFIELD"
>options</TT
> is a list of options used for the import process.
The meanings of the options are up to the FDW.
For example, an FDW could use an option to define whether the
<TT
CLASS="LITERAL"
>NOT NULL</TT
> attributes of columns should be imported.
These options need not have anything to do with those supported by the
FDW as database object options.
</P
><P
> The FDW may ignore the <TT
CLASS="STRUCTFIELD"
>local_schema</TT
> field of
the <TT
CLASS="STRUCTNAME"
>ImportForeignSchemaStmt</TT
>, because the core server
will automatically insert that name into the parsed <TT
CLASS="COMMAND"
>CREATE
FOREIGN TABLE</TT
> commands.
</P
><P
> The FDW does not have to concern itself with implementing the filtering
specified by <TT
CLASS="STRUCTFIELD"
>list_type</TT
> and <TT
CLASS="STRUCTFIELD"
>table_list</TT
>,
either, as the core server will automatically skip any returned commands
for tables excluded according to those options. However, it's often
useful to avoid the work of creating commands for excluded tables in the
first place. The function <CODE
CLASS="FUNCTION"
>IsImportableForeignTable()</CODE
> may be
useful to test whether a given foreign-table name will pass the filter.
</P
><P
> If the FDW does not support importing table definitions, the
<CODE
CLASS="FUNCTION"
>ImportForeignSchema</CODE
> pointer can be set to <TT
CLASS="LITERAL"
>NULL</TT
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FDW-CALLBACKS-PARALLEL"
>55.2.9. FDW Routines for Parallel Execution</A
></H2
><P
> A <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> node can, optionally, support parallel
execution. A parallel <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> will be executed
in multiple processes and should return each row only once across
all cooperating processes. To do this, processes can coordinate through
fixed size chunks of dynamic shared memory. This shared memory is not
guaranteed to be mapped at the same address in every process, so pointers
may not be used. The following callbacks are all optional in general,
but required if parallel execution is to be supported.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>bool
IsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
RangeTblEntry *rte);</PRE
><P>
Test whether a scan can be performed within a parallel worker. This
function will only be called when the planner believes that a parallel
plan might be possible, and should return true if it is safe for that scan
to run within a parallel worker. This will generally not be the case if
the remote data source has transaction semantics, unless the worker's
connection to the data can somehow be made to share the same transaction
context as the leader.
</P
><P
> If this callback is not defined, it is assumed that the scan must take
place within the parallel leader. Note that returning true does not mean
that the scan itself can be done in parallel, only that the scan can be
performed within a parallel worker. Therefore, it can be useful to define
this method even when parallel execution is not supported.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>Size
EstimateDSMForeignScan(ForeignScanState *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.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
InitializeDSMForeignScan(ForeignScanState *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"
>EstimateDSMForeignScan</CODE
>.
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
InitializeWorkerForeignScan(ForeignScanState *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"
>InitializeDSMForeignScan</TT
>.
This callback is optional, and needs only be supplied if this
custom path supports parallel execution.
</P
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