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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>57.2. Foreign Data Wrapper Callback Routines</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="fdw-functions.html" title="57.1. Foreign Data Wrapper Functions" /><link rel="next" href="fdw-helpers.html" title="57.3. Foreign Data Wrapper Helper Functions" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">57.2. Foreign Data Wrapper Callback Routines</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="fdw-functions.html" title="57.1. Foreign Data Wrapper Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="fdwhandler.html" title="Chapter 57. Writing A Foreign Data Wrapper">Up</a></td><th width="60%" align="center">Chapter 57. Writing A Foreign Data Wrapper</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="fdw-helpers.html" title="57.3. Foreign Data Wrapper Helper Functions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="FDW-CALLBACKS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">57.2. Foreign Data Wrapper Callback Routines</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-SCAN">57.2.1. FDW Routines For Scanning Foreign Tables</a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-JOIN-SCAN">57.2.2. FDW Routines For Scanning Foreign Joins</a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-UPPER-PLANNING">57.2.3. FDW Routines For Planning Post-Scan/Join Processing</a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-UPDATE">57.2.4. FDW Routines For Updating Foreign Tables</a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-ROW-LOCKING">57.2.5. FDW Routines For Row Locking</a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-EXPLAIN">57.2.6. FDW Routines for <code class="command">EXPLAIN</code></a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-ANALYZE">57.2.7. FDW Routines for <code class="command">ANALYZE</code></a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-IMPORT">57.2.8. FDW Routines For <code class="command">IMPORT FOREIGN SCHEMA</code></a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-PARALLEL">57.2.9. FDW Routines for Parallel Execution</a></span></dt><dt><span class="sect2"><a href="fdw-callbacks.html#FDW-CALLBACKS-REPARAMETERIZE-PATHS">57.2.10. FDW Routines For reparameterization of paths</a></span></dt></dl></div><p>
The FDW handler function returns a palloc'd <code class="structname">FdwRoutine</code>
struct containing pointers to the callback functions described below.
The scan-related functions are required, the rest are optional.
</p><p>
The <code class="structname">FdwRoutine</code> struct type is declared in
<code class="filename">src/include/foreign/fdwapi.h</code>, which see for additional
details.
</p><div class="sect2" id="FDW-CALLBACKS-SCAN"><div class="titlepage"><div><div><h3 class="title">57.2.1. FDW Routines For Scanning Foreign Tables</h3></div></div></div><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.
<code class="literal">root</code> is the planner's global information about the query;
<code class="literal">baserel</code> is the planner's information about this table; and
<code class="literal">foreigntableid</code> is the <code class="structname">pg_class</code> OID of the
foreign table. (<code class="literal">foreigntableid</code> could be obtained from the
planner data structures, but it's passed explicitly to save effort.)
</p><p>
This function should update <code class="literal">baserel->rows</code> 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
<code class="literal">baserel->rows</code> is just a constant default estimate, which
should be replaced if at all possible. The function may also choose to
update <code class="literal">baserel->width</code> if it can compute a better estimate
of the average result row width.
</p><p>
See <a class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</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
(<code class="structname">ForeignPath</code> node) for a scan on the foreign table and
must call <code class="function">add_path</code> to add each such path to
<code class="literal">baserel->pathlist</code>. It's recommended to use
<code class="function">create_foreignscan_path</code> to build the
<code class="structname">ForeignPath</code> nodes. The function can generate multiple
access paths, e.g., a path which has valid <code class="literal">pathkeys</code> 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 class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</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 <code class="structname">ForeignScan</code> 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 <code class="structname">ForeignPath</code> (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 <code class="structname">ForeignScan</code>,
which is used for rechecks performed by <code class="function">RecheckForeignScan</code>.
(If the path is for a join rather than a base
relation, <code class="literal">foreigntableid</code> is <code class="literal">InvalidOid</code>.)
</p><p>
This function must create and return a <code class="structname">ForeignScan</code> plan
node; it's recommended to use <code class="function">make_foreignscan</code> to build the
<code class="structname">ForeignScan</code> node.
</p><p>
See <a class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</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 <code class="structname">ForeignScanState</code> node has already been created, but
its <code class="structfield">fdw_state</code> field is still NULL. Information about
the table to scan is accessible through the
<code class="structname">ForeignScanState</code> node (in particular, from the underlying
<code class="structname">ForeignScan</code> plan node, which contains any FDW-private
information provided by <code class="function">GetForeignPlan</code>).
<code class="literal">eflags</code> contains flag bits describing the executor's
operating mode for this plan node.
</p><p>
Note that when <code class="literal">(eflags & EXEC_FLAG_EXPLAIN_ONLY)</code> 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 <code class="structfield">ScanTupleSlot</code> 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 <code class="structfield">es_query_cxt</code> of the node's <code class="structname">EState</code>.
</p><p>
The rows returned must match the <code class="structfield">fdw_scan_tlist</code> 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 <code class="structfield">fdw_scan_tlist</code> 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" id="FDW-CALLBACKS-JOIN-SCAN"><div class="titlepage"><div><div><h3 class="title">57.2.2. FDW Routines For Scanning Foreign Joins</h3></div></div></div><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 <code class="structname">ForeignPath</code> path(s) for the
supplied <code class="literal">joinrel</code>, 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 <code class="structname">ForeignPath</code> 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 <code class="structfield">scanrelid</code> of
the resulting <code class="structname">ForeignScan</code> plan node should be set to zero,
since there is no single relation that it represents; instead,
the <code class="structfield">fs_relids</code> field of the <code class="structname">ForeignScan</code>
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 <code class="structfield">fdw_scan_tlist</code> with an appropriate list
of <code class="structfield">TargetEntry</code> nodes, representing the set of columns
it will supply at run time in the tuples it returns.
</p><p>
See <a class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</a> for additional information.
</p></div><div class="sect2" id="FDW-CALLBACKS-UPPER-PLANNING"><div class="titlepage"><div><div><h3 class="title">57.2.3. FDW Routines For Planning Post-Scan/Join Processing</h3></div></div></div><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,
void *extra);
</pre><p>
Create possible access paths for <em class="firstterm">upper relation</em> 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 <code class="structname">ForeignPath</code>
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 <code class="literal">stage</code> parameter identifies which post-scan/join step is
currently being considered. <code class="literal">output_rel</code> is the upper relation
that should receive paths representing computation of this step,
and <code class="literal">input_rel</code> is the relation representing the input to this
step. The <code class="literal">extra</code> parameter provides additional details,
currently, it is set only for <code class="literal">UPPERREL_PARTIAL_GROUP_AGG</code>
or <code class="literal">UPPERREL_GROUP_AGG</code>, in which case it points to a
<code class="literal">GroupPathExtraData</code> structure.
(Note that <code class="structname">ForeignPath</code> paths added
to <code class="literal">output_rel</code> would typically not have any direct dependency
on paths of the <code class="literal">input_rel</code>, 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 class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</a> for additional information.
</p></div><div class="sect2" id="FDW-CALLBACKS-UPDATE"><div class="titlepage"><div><div><h3 class="title">57.2.4. FDW Routines For Updating Foreign Tables</h3></div></div></div><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>
<code class="command">UPDATE</code> and <code class="command">DELETE</code> 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">“<span class="quote">junk</span>”</span>, target columns to the list of columns that are to be
retrieved from the foreign table during an <code class="command">UPDATE</code> or
<code class="command">DELETE</code>.
</p><p>
To do that, add <code class="structname">TargetEntry</code> items to
<code class="literal">parsetree->targetList</code>, containing expressions for the
extra values to be fetched. Each such entry must be marked
<code class="structfield">resjunk</code> = <code class="literal">true</code>, and must have a distinct
<code class="structfield">resname</code> that will identify it at execution time.
Avoid using names matching <code class="literal">ctid<em class="replaceable"><code>N</code></em></code>,
<code class="literal">wholerow</code>, or
<code class="literal">wholerow<em class="replaceable"><code>N</code></em></code>, 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.
<code class="literal">parsetree</code> is the parse tree for the <code class="command">UPDATE</code> or
<code class="command">DELETE</code> command, while <code class="literal">target_rte</code> and
<code class="literal">target_relation</code> describe the target foreign table.
</p><p>
If the <code class="function">AddForeignUpdateTargets</code> pointer is set to
<code class="literal">NULL</code>, no extra target expressions are added.
(This will make it impossible to implement <code class="command">DELETE</code>
operations, though <code class="command">UPDATE</code> 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 <code class="structname">ModifyTable</code> plan
node that performs the update action. This private information must
have the form of a <code class="literal">List</code>, and will be delivered to
<code class="function">BeginForeignModify</code> during the execution stage.
</p><p>
<code class="literal">root</code> is the planner's global information about the query.
<code class="literal">plan</code> is the <code class="structname">ModifyTable</code> plan node, which is
complete except for the <code class="structfield">fdwPrivLists</code> field.
<code class="literal">resultRelation</code> identifies the target foreign table by its
range table index. <code class="literal">subplan_index</code> identifies which target of
the <code class="structname">ModifyTable</code> plan node this is, counting from zero;
use this if you want to index into <code class="literal">plan->plans</code> or other
substructure of the <code class="literal">plan</code> node.
</p><p>
See <a class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</a> for additional information.
</p><p>
If the <code class="function">PlanForeignModify</code> pointer is set to
<code class="literal">NULL</code>, no additional plan-time actions are taken, and the
<code class="literal">fdw_private</code> 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>
<code class="literal">mtstate</code> is the overall state of the
<code class="structname">ModifyTable</code> plan node being executed; global data about
the plan and execution state is available via this structure.
<code class="literal">rinfo</code> is the <code class="structname">ResultRelInfo</code> struct describing
the target foreign table. (The <code class="structfield">ri_FdwState</code> field of
<code class="structname">ResultRelInfo</code> is available for the FDW to store any
private state it needs for this operation.)
<code class="literal">fdw_private</code> contains the private data generated by
<code class="function">PlanForeignModify</code>, if any.
<code class="literal">subplan_index</code> identifies which target of
the <code class="structname">ModifyTable</code> plan node this is.
<code class="literal">eflags</code> contains flag bits describing the executor's
operating mode for this plan node.
</p><p>
Note that when <code class="literal">(eflags & EXEC_FLAG_EXPLAIN_ONLY)</code> 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
<code class="literal">NULL</code>, 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.
<code class="literal">estate</code> is global execution state for the query.
<code class="literal">rinfo</code> is the <code class="structname">ResultRelInfo</code> struct describing
the target foreign table.
<code class="literal">slot</code> contains the tuple to be inserted; it will match the
row-type definition of the foreign table.
<code class="literal">planSlot</code> contains the tuple that was generated by the
<code class="structname">ModifyTable</code> plan node's subplan; it differs from
<code class="literal">slot</code> in possibly containing additional <span class="quote">“<span class="quote">junk</span>”</span>
columns. (The <code class="literal">planSlot</code> is typically of little interest
for <code class="command">INSERT</code> 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
<code class="literal">slot</code> can be re-used for this purpose.
</p><p>
The data in the returned slot is used only if the <code class="command">INSERT</code>
query has a <code class="literal">RETURNING</code> clause or the foreign table has
an <code class="literal">AFTER ROW</code> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <code class="literal">RETURNING</code> 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
<code class="literal">NULL</code>, attempts to insert into the foreign table will fail
with an error message.
</p><p>
Note that this function is also called when inserting routed tuples into
a foreign-table partition or executing <code class="command">COPY FROM</code> on
a foreign table, in which case it is called in a different way than it
is in the <code class="command">INSERT</code> case. See the callback functions
described below that allow the FDW to support that.
</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.
<code class="literal">estate</code> is global execution state for the query.
<code class="literal">rinfo</code> is the <code class="structname">ResultRelInfo</code> struct describing
the target foreign table.
<code class="literal">slot</code> contains the new data for the tuple; it will match the
row-type definition of the foreign table.
<code class="literal">planSlot</code> contains the tuple that was generated by the
<code class="structname">ModifyTable</code> plan node's subplan; it differs from
<code class="literal">slot</code> in possibly containing additional <span class="quote">“<span class="quote">junk</span>”</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
<code class="literal">slot</code> can be re-used for this purpose.
</p><p>
The data in the returned slot is used only if the <code class="command">UPDATE</code>
query has a <code class="literal">RETURNING</code> clause or the foreign table has
an <code class="literal">AFTER ROW</code> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <code class="literal">RETURNING</code> 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
<code class="literal">NULL</code>, 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.
<code class="literal">estate</code> is global execution state for the query.
<code class="literal">rinfo</code> is the <code class="structname">ResultRelInfo</code> struct describing
the target foreign table.
<code class="literal">slot</code> contains nothing useful upon call, but can be used to
hold the returned tuple.
<code class="literal">planSlot</code> contains the tuple that was generated by the
<code class="structname">ModifyTable</code> 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 <code class="literal">slot</code> can be used to hold the tuple to be returned.
</p><p>
The data in the returned slot is used only if the <code class="command">DELETE</code>
query has a <code class="literal">RETURNING</code> clause or the foreign table has
an <code class="literal">AFTER ROW</code> trigger. Triggers require all columns, but the
FDW could choose to optimize away returning some or all columns depending
on the contents of the <code class="literal">RETURNING</code> 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
<code class="literal">NULL</code>, 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
<code class="literal">NULL</code>, no action is taken during executor shutdown.
</p><p>
Tuples inserted into a partitioned table by <code class="command">INSERT</code> or
<code class="command">COPY FROM</code> are routed to partitions. If an FDW
supports routable foreign-table partitions, it should also provide the
following callback functions. These functions are also called when
<code class="command">COPY FROM</code> is executed on a foreign table.
</p><p>
</p><pre class="programlisting">
void
BeginForeignInsert(ModifyTableState *mtstate,
ResultRelInfo *rinfo);
</pre><p>
Begin executing an insert operation on a foreign table. This routine is
called right before the first tuple is inserted into the foreign table
in both cases when it is the partition chosen for tuple routing and the
target specified in a <code class="command">COPY FROM</code> command. It should
perform any initialization needed prior to the actual insertion.
Subsequently, <code class="function">ExecForeignInsert</code> will be called for
each tuple to be inserted into the foreign table.
</p><p>
<code class="literal">mtstate</code> is the overall state of the
<code class="structname">ModifyTable</code> plan node being executed; global data about
the plan and execution state is available via this structure.
<code class="literal">rinfo</code> is the <code class="structname">ResultRelInfo</code> struct describing
the target foreign table. (The <code class="structfield">ri_FdwState</code> field of
<code class="structname">ResultRelInfo</code> is available for the FDW to store any
private state it needs for this operation.)
</p><p>
When this is called by a <code class="command">COPY FROM</code> command, the
plan-related global data in <code class="literal">mtstate</code> is not provided
and the <code class="literal">planSlot</code> parameter of
<code class="function">ExecForeignInsert</code> subsequently called for each
inserted tuple is <code class="literal">NULL</code>, whether the foreign table is
the partition chosen for tuple routing or the target specified in the
command.
</p><p>
If the <code class="function">BeginForeignInsert</code> pointer is set to
<code class="literal">NULL</code>, no action is taken for the initialization.
</p><p>
Note that if the FDW does not support routable foreign-table partitions
and/or executing <code class="command">COPY FROM</code> on foreign tables, this
function or <code class="function">ExecForeignInsert</code> subsequently called
must throw error as needed.
</p><p>
</p><pre class="programlisting">
void
EndForeignInsert(EState *estate,
ResultRelInfo *rinfo);
</pre><p>
End the insert operation 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">EndForeignInsert</code> pointer is set to
<code class="literal">NULL</code>, no action is taken for the termination.
</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
<code class="literal">CmdType</code> enumeration; that is,
<code class="literal">(1 << CMD_UPDATE) = 4</code> for <code class="command">UPDATE</code>,
<code class="literal">(1 << CMD_INSERT) = 8</code> for <code class="command">INSERT</code>, and
<code class="literal">(1 << CMD_DELETE) = 16</code> for <code class="command">DELETE</code>.
</p><p>
If the <code class="function">IsForeignRelUpdatable</code> pointer is set to
<code class="literal">NULL</code>, 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 <code class="literal">information_schema</code> 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 <code class="literal">true</code> after performing
planning actions needed for that. Otherwise, return <code class="literal">false</code>.
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 <code class="structname">ForeignScan</code> plan
node that executes the direct modification on the remote server. The
<code class="structfield">operation</code> field of the <code class="structname">ForeignScan</code> must
be set to the <code class="literal">CmdType</code> enumeration appropriately; that is,
<code class="literal">CMD_UPDATE</code> for <code class="command">UPDATE</code>,
<code class="literal">CMD_INSERT</code> for <code class="command">INSERT</code>, and
<code class="literal">CMD_DELETE</code> for <code class="command">DELETE</code>.
</p><p>
See <a class="xref" href="fdw-planning.html" title="57.4. Foreign Data Wrapper Query Planning">Section 57.4</a> for additional information.
</p><p>
If the <code class="function">PlanDirectModify</code> pointer is set to
<code class="literal">NULL</code>, 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 <code class="structname">ForeignScanState</code> node has already been created, but
its <code class="structfield">fdw_state</code> field is still NULL. Information about
the table to modify is accessible through the
<code class="structname">ForeignScanState</code> node (in particular, from the underlying
<code class="structname">ForeignScan</code> plan node, which contains any FDW-private
information provided by <code class="function">PlanDirectModify</code>).
<code class="literal">eflags</code> contains flag bits describing the executor's
operating mode for this plan node.
</p><p>
Note that when <code class="literal">(eflags & EXEC_FLAG_EXPLAIN_ONLY)</code> 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
<code class="literal">NULL</code>, 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 <code class="command">INSERT</code>, <code class="command">UPDATE</code> or <code class="command">DELETE</code>
query doesn't have a <code class="literal">RETURNING</code> 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 <code class="literal">RETURNING</code> calculation, returning it in a
tuple table slot (the node's <code class="structfield">ScanTupleSlot</code> should be
used for this purpose). The data that was actually inserted, updated
or deleted must be stored in the
<code class="literal">es_result_relation_info->ri_projectReturning->pi_exprContext->ecxt_scantuple</code>
of the node's <code class="structname">EState</code>.
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 <code class="structfield">es_query_cxt</code> of the node's <code class="structname">EState</code>.
</p><p>
The rows returned must match the <code class="structfield">fdw_scan_tlist</code> 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 <code class="literal">RETURNING</code> calculation,
you should insert nulls in those column positions, or else generate a
<code class="structfield">fdw_scan_tlist</code> 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
<code class="structname">ForeignScanState</code> node in the <code class="command">EXPLAIN ANALYZE</code>
case.
</p><p>
If the <code class="function">IterateDirectModify</code> pointer is set to
<code class="literal">NULL</code>, 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
<code class="literal">NULL</code>, no attempts to execute a direct modification on the
remote server are taken.
</p></div><div class="sect2" id="FDW-CALLBACKS-ROW-LOCKING"><div class="titlepage"><div><div><h3 class="title">57.2.5. FDW Routines For Row Locking</h3></div></div></div><p>
If an FDW wishes to support <em class="firstterm">late row locking</em> (as described
in <a class="xref" href="fdw-row-locking.html" title="57.5. Row Locking in Foreign Data Wrappers">Section 57.5</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.
<code class="literal">rte</code> is the <code class="structname">RangeTblEntry</code> node for the table
and <code class="literal">strength</code> describes the lock strength requested by the
relevant <code class="literal">FOR UPDATE/SHARE</code> clause, if any. The result must be
a member of the <code class="literal">RowMarkType</code> enum type.
</p><p>
This function is called during query planning for each foreign table that
appears in an <code class="command">UPDATE</code>, <code class="command">DELETE</code>, or <code class="command">SELECT
FOR UPDATE/SHARE</code> query and is not the target of <code class="command">UPDATE</code>
or <code class="command">DELETE</code>.
</p><p>
If the <code class="function">GetForeignRowMarkType</code> pointer is set to
<code class="literal">NULL</code>, the <code class="literal">ROW_MARK_COPY</code> 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 class="xref" href="fdw-row-locking.html" title="57.5. Row Locking in Foreign Data Wrappers">Section 57.5</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.
<code class="literal">estate</code> is global execution state for the query.
<code class="literal">erm</code> is the <code class="structname">ExecRowMark</code> struct describing
the target foreign table and the row lock type (if any) to acquire.
<code class="literal">rowid</code> identifies the tuple to be fetched.
<code class="literal">updated</code> is an output parameter.
</p><p>
This function should return a palloc'ed copy of the fetched tuple,
or <code class="literal">NULL</code> if the row lock couldn't be obtained. The row lock
type to acquire is defined by <code class="literal">erm->markType</code>, which is the
value previously returned by <code class="function">GetForeignRowMarkType</code>.
(<code class="literal">ROW_MARK_REFERENCE</code> means to just re-fetch the tuple without
acquiring any lock, and <code class="literal">ROW_MARK_COPY</code> will never be seen by
this routine.)
</p><p>
In addition, <code class="literal">*updated</code> should be set to <code class="literal">true</code>
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 <code class="literal">true</code> is recommended.)
</p><p>
Note that by default, failure to acquire a row lock should result in
raising an error; a <code class="literal">NULL</code> return is only appropriate if
the <code class="literal">SKIP LOCKED</code> option is specified
by <code class="literal">erm->waitPolicy</code>.
</p><p>
The <code class="literal">rowid</code> is the <code class="structfield">ctid</code> value previously read
for the row to be re-fetched. Although the <code class="literal">rowid</code> value is
passed as a <code class="type">Datum</code>, it can currently only be a <code class="type">tid</code>. 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
<code class="literal">NULL</code>, attempts to re-fetch rows will fail
with an error message.
</p><p>
See <a class="xref" href="fdw-row-locking.html" title="57.5. Row Locking in Foreign Data Wrappers">Section 57.5</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 <code class="literal">NULL</code> and
instead set <code class="structfield">fdw_recheck_quals</code> 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. <code class="literal">RecheckForeignScan</code> 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
<code class="literal">ForeignScan</code>. 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 <code class="literal">GetExistingLocalJoinPath</code> may be used to search
existing paths for a suitable local join path, which can be used as the
alternative local join plan. <code class="literal">GetExistingLocalJoinPath</code>
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" id="FDW-CALLBACKS-EXPLAIN"><div class="titlepage"><div><div><h3 class="title">57.2.6. FDW Routines for <code class="command">EXPLAIN</code></h3></div></div></div><p>
</p><pre class="programlisting">
void
ExplainForeignScan(ForeignScanState *node,
ExplainState *es);
</pre><p>
Print additional <code class="command">EXPLAIN</code> output for a foreign table scan.
This function can call <code class="function">ExplainPropertyText</code> and
related functions to add fields to the <code class="command">EXPLAIN</code> output.
The flag fields in <code class="literal">es</code> can be used to determine what to
print, and the state of the <code class="structname">ForeignScanState</code> node
can be inspected to provide run-time statistics in the <code class="command">EXPLAIN
ANALYZE</code> case.
</p><p>
If the <code class="function">ExplainForeignScan</code> pointer is set to
<code class="literal">NULL</code>, no additional information is printed during
<code class="command">EXPLAIN</code>.
</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 <code class="command">EXPLAIN</code> output for a foreign table update.
This function can call <code class="function">ExplainPropertyText</code> and
related functions to add fields to the <code class="command">EXPLAIN</code> output.
The flag fields in <code class="literal">es</code> can be used to determine what to
print, and the state of the <code class="structname">ModifyTableState</code> node
can be inspected to provide run-time statistics in the <code class="command">EXPLAIN
ANALYZE</code> 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
<code class="literal">NULL</code>, no additional information is printed during
<code class="command">EXPLAIN</code>.
</p><p>
</p><pre class="programlisting">
void
ExplainDirectModify(ForeignScanState *node,
ExplainState *es);
</pre><p>
Print additional <code class="command">EXPLAIN</code> 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 <code class="command">EXPLAIN</code> output.
The flag fields in <code class="literal">es</code> can be used to determine what to
print, and the state of the <code class="structname">ForeignScanState</code> node
can be inspected to provide run-time statistics in the <code class="command">EXPLAIN
ANALYZE</code> case.
</p><p>
If the <code class="function">ExplainDirectModify</code> pointer is set to
<code class="literal">NULL</code>, no additional information is printed during
<code class="command">EXPLAIN</code>.
</p></div><div class="sect2" id="FDW-CALLBACKS-ANALYZE"><div class="titlepage"><div><div><h3 class="title">57.2.7. FDW Routines for <code class="command">ANALYZE</code></h3></div></div></div><p>
</p><pre class="programlisting">
bool
AnalyzeForeignTable(Relation relation,
AcquireSampleRowsFunc *func,
BlockNumber *totalpages);
</pre><p>
This function is called when <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a> is executed on
a foreign table. If the FDW can collect statistics for this
foreign table, it should return <code class="literal">true</code>, and provide a pointer
to a function that will collect sample rows from the table in
<em class="parameter"><code>func</code></em>, plus the estimated size of the table in pages in
<em class="parameter"><code>totalpages</code></em>. Otherwise, return <code class="literal">false</code>.
</p><p>
If the FDW does not support collecting statistics for any tables, the
<code class="function">AnalyzeForeignTable</code> pointer can be set to <code class="literal">NULL</code>.
</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 <em class="parameter"><code>targrows</code></em> rows should be collected
from the table and stored into the caller-provided <em class="parameter"><code>rows</code></em>
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 <em class="parameter"><code>totalrows</code></em> and
<em class="parameter"><code>totaldeadrows</code></em>. (Set <em class="parameter"><code>totaldeadrows</code></em> to zero
if the FDW does not have any concept of dead rows.)
</p></div><div class="sect2" id="FDW-CALLBACKS-IMPORT"><div class="titlepage"><div><div><h3 class="title">57.2.8. FDW Routines For <code class="command">IMPORT FOREIGN SCHEMA</code></h3></div></div></div><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 class="xref" href="sql-importforeignschema.html" title="IMPORT FOREIGN SCHEMA"><span class="refentrytitle">IMPORT FOREIGN SCHEMA</span></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 class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a> command.
These strings will be parsed and executed by the core server.
</p><p>
Within the <code class="structname">ImportForeignSchemaStmt</code> struct,
<code class="structfield">remote_schema</code> is the name of the remote schema from
which tables are to be imported.
<code class="structfield">list_type</code> identifies how to filter table names:
<code class="literal">FDW_IMPORT_SCHEMA_ALL</code> means that all tables in the remote
schema should be imported (in this case <code class="structfield">table_list</code> is
empty), <code class="literal">FDW_IMPORT_SCHEMA_LIMIT_TO</code> means to include only
tables listed in <code class="structfield">table_list</code>,
and <code class="literal">FDW_IMPORT_SCHEMA_EXCEPT</code> means to exclude the tables
listed in <code class="structfield">table_list</code>.
<code class="structfield">options</code> 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
<code class="literal">NOT NULL</code> 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 <code class="structfield">local_schema</code> field of
the <code class="structname">ImportForeignSchemaStmt</code>, because the core server
will automatically insert that name into the parsed <code class="command">CREATE
FOREIGN TABLE</code> commands.
</p><p>
The FDW does not have to concern itself with implementing the filtering
specified by <code class="structfield">list_type</code> and <code class="structfield">table_list</code>,
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 <code class="literal">NULL</code>.
</p></div><div class="sect2" id="FDW-CALLBACKS-PARALLEL"><div class="titlepage"><div><div><h3 class="title">57.2.9. FDW Routines for Parallel Execution</h3></div></div></div><p>
A <code class="structname">ForeignScan</code> node can, optionally, support parallel
execution. A parallel <code class="structname">ForeignScan</code> will be executed
in multiple processes and must return each row exactly 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 it
must not contain pointers. The following functions are all optional,
but most are 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 function 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.
This function is optional, and can be omitted if not needed; but if it
is omitted, the next three functions must be omitted as well, because
no shared memory will be allocated for the FDW's use.
</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. <code class="literal">coordinate</code> points to a shared memory area of
size equal to the return value of <code class="function">EstimateDSMForeignScan</code>.
This function is optional, and can be omitted if not needed.
</p><p>
</p><pre class="programlisting">
void
ReInitializeDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt,
void *coordinate);
</pre><p>
Re-initialize the dynamic shared memory required for parallel operation
when the foreign-scan plan node is about to be re-scanned.
This function is optional, and can be omitted if not needed.
Recommended practice is that this function reset only shared state,
while the <code class="function">ReScanForeignScan</code> function resets only local
state. Currently, this function will be called
before <code class="function">ReScanForeignScan</code>, but it's best not to rely on
that ordering.
</p><p>
</p><pre class="programlisting">
void
InitializeWorkerForeignScan(ForeignScanState *node, shm_toc *toc,
void *coordinate);
</pre><p>
Initialize a parallel worker's local state based on the shared state
set up by the leader during <code class="function">InitializeDSMForeignScan</code>.
This function is optional, and can be omitted if not needed.
</p><p>
</p><pre class="programlisting">
void
ShutdownForeignScan(ForeignScanState *node);
</pre><p>
Release resources when it is anticipated the node will not be executed
to completion. This is not called in all cases; sometimes,
<code class="literal">EndForeignScan</code> 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, foreign data wrappers that
wish to take some action before the DSM segment goes away should implement
this method.
</p></div><div class="sect2" id="FDW-CALLBACKS-REPARAMETERIZE-PATHS"><div class="titlepage"><div><div><h3 class="title">57.2.10. FDW Routines For reparameterization of paths</h3></div></div></div><p>
</p><pre class="programlisting">
List *
ReparameterizeForeignPathByChild(PlannerInfo *root, List *fdw_private,
RelOptInfo *child_rel);
</pre><p>
This function is called while converting a path parameterized by the
top-most parent of the given child relation <code class="literal">child_rel</code> to be
parameterized by the child relation. The function is used to reparameterize
any paths or translate any expression nodes saved in the given
<code class="literal">fdw_private</code> member of a <code class="structname">ForeignPath</code>. The
callback may use <code class="literal">reparameterize_path_by_child</code>,
<code class="literal">adjust_appendrel_attrs</code> or
<code class="literal">adjust_appendrel_attrs_multilevel</code> as required.
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