2011-02-20 06:17:18 +01:00
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<!-- doc/src/sgml/fdwhandler.sgml -->
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<chapter id="fdwhandler">
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<title>Writing A Foreign Data Wrapper</title>
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<indexterm zone="fdwhandler">
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<primary>foreign data wrapper</primary>
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<secondary>handler for</secondary>
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</indexterm>
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<para>
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All operations on a foreign table are handled through its foreign data
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2012-04-06 21:02:35 +02:00
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wrapper, which consists of a set of functions that the core server
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calls. The foreign data wrapper is responsible for fetching
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2011-02-20 06:17:18 +01:00
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data from the remote data source and returning it to the
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<productname>PostgreSQL</productname> executor. This chapter outlines how
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to write a new foreign data wrapper.
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</para>
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<para>
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The foreign data wrappers included in the standard distribution are good
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references when trying to write your own. Look into the
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<filename>contrib/file_fdw</> subdirectory of the source tree.
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The <xref linkend="sql-createforeigndatawrapper"> reference page also has
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some useful details.
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</para>
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<note>
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<para>
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The SQL standard specifies an interface for writing foreign data wrappers.
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However, PostgreSQL does not implement that API, because the effort to
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accommodate it into PostgreSQL would be large, and the standard API hasn't
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gained wide adoption anyway.
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</para>
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</note>
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2011-07-05 21:54:00 +02:00
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<sect1 id="fdw-functions">
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<title>Foreign Data Wrapper Functions</title>
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<para>
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The FDW author needs to implement a handler function, and optionally
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a validator function. Both functions must be written in a compiled
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language such as C, using the version-1 interface.
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For details on C language calling conventions and dynamic loading,
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see <xref linkend="xfunc-c">.
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</para>
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<para>
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The handler function simply returns a struct of function pointers to
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2012-04-06 21:02:35 +02:00
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callback functions that will be called by the planner, executor, and
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various maintenance commands.
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2011-07-05 21:54:00 +02:00
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Most of the effort in writing an FDW is in implementing these callback
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functions.
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The handler function must be registered with
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<productname>PostgreSQL</productname> as taking no arguments and
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returning the special pseudo-type <type>fdw_handler</type>. The
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callback functions are plain C functions and are not visible or
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callable at the SQL level. The callback functions are described in
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<xref linkend="fdw-callbacks">.
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</para>
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<para>
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The validator function is responsible for validating options given in
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<command>CREATE</command> and <command>ALTER</command> commands for its
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foreign data wrapper, as well as foreign servers, user mappings, and
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foreign tables using the wrapper.
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The validator function must be registered as taking two arguments, a
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text array containing the options to be validated, and an OID
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representing the type of object the options are associated with (in
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the form of the OID of the system catalog the object would be stored
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in, either
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<literal>ForeignDataWrapperRelationId</>,
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<literal>ForeignServerRelationId</>,
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<literal>UserMappingRelationId</>,
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or <literal>ForeignTableRelationId</>).
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If no validator function is supplied, options are not checked at object
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creation time or object alteration time.
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</para>
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</sect1>
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<sect1 id="fdw-callbacks">
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2011-02-20 06:17:18 +01:00
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<title>Foreign Data Wrapper Callback Routines</title>
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<para>
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The FDW handler function returns a palloc'd <structname>FdwRoutine</>
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struct containing pointers to the following callback functions:
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</para>
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<para>
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<programlisting>
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2012-03-05 22:15:59 +01:00
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void
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Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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GetForeignRelSize (PlannerInfo *root,
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RelOptInfo *baserel,
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Oid foreigntableid);
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2011-02-20 06:17:18 +01:00
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</programlisting>
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Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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Obtain relation size estimates for a foreign table. This is called
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at the beginning of planning for a query involving a foreign table.
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<literal>root</> is the planner's global information about the query;
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<literal>baserel</> is the planner's information about this table; and
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2011-02-20 06:17:18 +01:00
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<literal>foreigntableid</> is the <structname>pg_class</> OID of the
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Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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foreign table. (<literal>foreigntableid</> could be obtained from the
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planner data structures, but it's passed explicitly to save effort.)
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2012-03-05 22:15:59 +01:00
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</para>
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<para>
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Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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This function should update <literal>baserel->rows</> to be the
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expected number of rows returned by the table scan, after accounting for
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the filtering done by the restriction quals. The initial value of
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<literal>baserel->rows</> is just a constant default estimate, which
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should be replaced if at all possible. The function may also choose to
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update <literal>baserel->width</> if it can compute a better estimate
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of the average result row width.
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2011-02-20 06:17:18 +01:00
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</para>
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<para>
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Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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See <xref linkend="fdw-planning"> for additional information.
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</para>
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<para>
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<programlisting>
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void
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GetForeignPaths (PlannerInfo *root,
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RelOptInfo *baserel,
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Oid foreigntableid);
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</programlisting>
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Create possible access paths for a scan on a foreign table.
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This is called during query planning.
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The parameters are the same as for <function>GetForeignRelSize</>,
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which has already been called.
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</para>
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<para>
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This function must generate at least one access path
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(<structname>ForeignPath</> node) for a scan on the foreign table and
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must call <function>add_path</> to add each such path to
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<literal>baserel->pathlist</>. It's recommended to use
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<function>create_foreignscan_path</> to build the
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<structname>ForeignPath</> nodes. The function can generate multiple
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access paths, e.g., a path which has valid <literal>pathkeys</> to
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represent a pre-sorted result. Each access path must contain cost
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estimates, and can contain any FDW-private information that is needed to
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identify the specific scan method intended.
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</para>
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<para>
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See <xref linkend="fdw-planning"> for additional information.
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</para>
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<para>
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<programlisting>
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ForeignScan *
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GetForeignPlan (PlannerInfo *root,
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RelOptInfo *baserel,
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Oid foreigntableid,
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ForeignPath *best_path,
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List *tlist,
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List *scan_clauses);
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</programlisting>
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Create a <structname>ForeignScan</> plan node from the selected foreign
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access path. This is called at the end of query planning.
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The parameters are as for <function>GetForeignRelSize</>, plus
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the selected <structname>ForeignPath</> (previously produced by
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<function>GetForeignPaths</>), the target list to be emitted by the
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plan node, and the restriction clauses to be enforced by the plan node.
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2011-02-20 06:17:18 +01:00
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</para>
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<para>
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Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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This function must create and return a <structname>ForeignScan</> plan
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node; it's recommended to use <function>make_foreignscan</> to build the
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<structname>ForeignScan</> node.
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</para>
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<para>
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See <xref linkend="fdw-planning"> for additional information.
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2011-02-20 06:17:18 +01:00
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</para>
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<para>
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<programlisting>
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void
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ExplainForeignScan (ForeignScanState *node,
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ExplainState *es);
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</programlisting>
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Print additional <command>EXPLAIN</> output for a foreign table scan.
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This can just return if there is no need to print anything.
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Otherwise, it should call <function>ExplainPropertyText</> and
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related functions to add fields to the <command>EXPLAIN</> output.
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The flag fields in <literal>es</> can be used to determine what to
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print, and the state of the <structname>ForeignScanState</> node
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2012-06-07 23:06:20 +02:00
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can be inspected to provide run-time statistics in the <command>EXPLAIN
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2011-02-20 06:17:18 +01:00
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ANALYZE</> case.
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</para>
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<para>
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<programlisting>
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void
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BeginForeignScan (ForeignScanState *node,
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int eflags);
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</programlisting>
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Begin executing a foreign scan. This is called during executor startup.
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2011-07-25 15:37:06 +02:00
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It should perform any initialization needed before the scan can start,
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but not start executing the actual scan (that should be done upon the
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first call to <function>IterateForeignScan</>).
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2011-02-20 06:17:18 +01:00
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The <structname>ForeignScanState</> node has already been created, but
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its <structfield>fdw_state</> field is still NULL. Information about
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the table to scan is accessible through the
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<structname>ForeignScanState</> node (in particular, from the underlying
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2012-03-05 22:15:59 +01:00
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<structname>ForeignScan</> plan node, which contains any FDW-private
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
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information provided by <function>GetForeignPlan</>).
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2011-02-20 06:17:18 +01:00
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</para>
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<para>
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Note that when <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</> is
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true, this function should not perform any externally-visible actions;
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it should only do the minimum required to make the node state valid
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|
|
for <function>ExplainForeignScan</> and <function>EndForeignScan</>.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
TupleTableSlot *
|
|
|
|
IterateForeignScan (ForeignScanState *node);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
Fetch one row from the foreign source, returning it in a tuple table slot
|
|
|
|
(the node's <structfield>ScanTupleSlot</> 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 <function>BeginForeignScan</> if you need longer-lived storage, or use
|
|
|
|
the <structfield>es_query_cxt</> of the node's <structname>EState</>.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
The rows returned must match the column signature of the foreign table
|
|
|
|
being scanned. If you choose to optimize away fetching columns that
|
|
|
|
are not needed, you should insert nulls in those column positions.
|
|
|
|
</para>
|
|
|
|
|
2011-06-13 14:28:41 +02:00
|
|
|
<para>
|
|
|
|
Note that <productname>PostgreSQL</productname>'s executor doesn't care
|
|
|
|
whether the rows returned violate the <literal>NOT NULL</literal>
|
|
|
|
constraints which were defined on the foreign table columns - but the
|
|
|
|
planner does care, and may optimize queries incorrectly if
|
|
|
|
<literal>NULL</> values are present in a column declared not to contain
|
|
|
|
them. If a <literal>NULL</> value is encountered when the user has
|
|
|
|
declared that none should be present, it may be appropriate to raise an
|
|
|
|
error (just as you would need to do in the case of a data type mismatch).
|
|
|
|
</para>
|
|
|
|
|
2011-02-20 06:17:18 +01:00
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
void
|
|
|
|
ReScanForeignScan (ForeignScanState *node);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
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.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
void
|
|
|
|
EndForeignScan (ForeignScanState *node);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
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.
|
|
|
|
</para>
|
|
|
|
|
2012-04-06 21:02:35 +02:00
|
|
|
<para>
|
|
|
|
<programlisting>
|
2012-04-06 22:04:10 +02:00
|
|
|
bool
|
|
|
|
AnalyzeForeignTable (Relation relation,
|
|
|
|
AcquireSampleRowsFunc *func,
|
|
|
|
BlockNumber *totalpages);
|
2012-04-06 21:02:35 +02:00
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function is called when <xref linkend="sql-analyze"> is executed on
|
2012-04-06 22:04:10 +02:00
|
|
|
a foreign table. If the FDW can collect statistics for this
|
|
|
|
foreign table, it should return <literal>true</>, and provide a pointer
|
|
|
|
to a function that will collect sample rows from the table in
|
|
|
|
<parameter>func</>, plus the estimated size of the table in pages in
|
|
|
|
<parameter>totalpages</>. Otherwise, return <literal>false</>.
|
|
|
|
If the FDW does not support collecting statistics for any tables, the
|
2012-04-06 21:02:35 +02:00
|
|
|
<function>AnalyzeForeignTable</> pointer can be set to <literal>NULL</>.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
If provided, the sample collection function must have the signature
|
|
|
|
<programlisting>
|
|
|
|
int
|
|
|
|
AcquireSampleRowsFunc (Relation relation, int elevel,
|
|
|
|
HeapTuple *rows, int targrows,
|
|
|
|
double *totalrows,
|
2012-04-06 22:04:10 +02:00
|
|
|
double *totaldeadrows);
|
2012-04-06 21:02:35 +02:00
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
A random sample of up to <parameter>targrows</> rows should be collected
|
|
|
|
from the table and stored into the caller-provided <parameter>rows</>
|
|
|
|
array. The actual number of rows collected must be returned. In
|
2012-04-06 22:04:10 +02:00
|
|
|
addition, store estimates of the total numbers of live and dead rows in
|
|
|
|
the table into the output parameters <parameter>totalrows</> and
|
|
|
|
<parameter>totaldeadrows</>. (Set <parameter>totaldeadrows</> to zero
|
|
|
|
if the FDW does not have any concept of dead rows.)
|
2012-04-06 21:02:35 +02:00
|
|
|
</para>
|
|
|
|
|
2011-02-20 06:17:18 +01:00
|
|
|
<para>
|
2012-03-05 22:15:59 +01:00
|
|
|
The <structname>FdwRoutine</> struct type is declared in
|
|
|
|
<filename>src/include/foreign/fdwapi.h</>, which see for additional
|
|
|
|
details.
|
2011-02-20 06:17:18 +01:00
|
|
|
</para>
|
|
|
|
|
|
|
|
</sect1>
|
|
|
|
|
2012-03-08 00:20:58 +01:00
|
|
|
<sect1 id="fdw-helpers">
|
|
|
|
<title>Foreign Data Wrapper Helper Functions</title>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
Several helper functions are exported from the core server so that
|
|
|
|
authors of foreign data wrappers can get easy access to attributes of
|
|
|
|
FDW-related objects, such as FDW options.
|
|
|
|
To use any of these functions, you need to include the header file
|
|
|
|
<filename>foreign/foreign.h</filename> in your source file.
|
|
|
|
That header also defines the struct types that are returned by
|
|
|
|
these functions.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
ForeignDataWrapper *
|
|
|
|
GetForeignDataWrapper(Oid fdwid);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns a <structname>ForeignDataWrapper</structname>
|
|
|
|
object for the foreign-data wrapper with the given OID. A
|
|
|
|
<structname>ForeignDataWrapper</structname> object contains properties
|
|
|
|
of the FDW (see <filename>foreign/foreign.h</filename> for details).
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
ForeignServer *
|
|
|
|
GetForeignServer(Oid serverid);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns a <structname>ForeignServer</structname> object
|
|
|
|
for the foreign server with the given OID. A
|
|
|
|
<structname>ForeignServer</structname> object contains properties
|
|
|
|
of the server (see <filename>foreign/foreign.h</filename> for details).
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
UserMapping *
|
|
|
|
GetUserMapping(Oid userid, Oid serverid);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns a <structname>UserMapping</structname> object for
|
|
|
|
the user mapping of the given role on the given server. (If there is no
|
|
|
|
mapping for the specific user, it will return the mapping for
|
|
|
|
<literal>PUBLIC</>, or throw error if there is none.) A
|
|
|
|
<structname>UserMapping</structname> object contains properties of the
|
|
|
|
user mapping (see <filename>foreign/foreign.h</filename> for details).
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
ForeignTable *
|
|
|
|
GetForeignTable(Oid relid);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns a <structname>ForeignTable</structname> object for
|
|
|
|
the foreign table with the given OID. A
|
|
|
|
<structname>ForeignTable</structname> object contains properties of the
|
|
|
|
foreign table (see <filename>foreign/foreign.h</filename> for details).
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
List *
|
|
|
|
GetForeignTableColumnOptions(Oid relid, AttrNumber attnum);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns the per-column FDW options for the column with the
|
|
|
|
given foreign table OID and attribute number, in the form of a list of
|
|
|
|
<structname>DefElem</structname>. NIL is returned if the column has no
|
|
|
|
options.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
Some object types have name-based lookup functions in addition to the
|
|
|
|
OID-based ones:
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
ForeignDataWrapper *
|
|
|
|
GetForeignDataWrapperByName(const char *name, bool missing_ok);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns a <structname>ForeignDataWrapper</structname>
|
|
|
|
object for the foreign-data wrapper with the given name. If the wrapper
|
|
|
|
is not found, return NULL if missing_ok is true, otherwise raise an
|
|
|
|
error.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<programlisting>
|
|
|
|
ForeignServer *
|
|
|
|
GetForeignServerByName(const char *name, bool missing_ok);
|
|
|
|
</programlisting>
|
|
|
|
|
|
|
|
This function returns a <structname>ForeignServer</structname> object
|
|
|
|
for the foreign server with the given name. If the server is not found,
|
|
|
|
return NULL if missing_ok is true, otherwise raise an error.
|
|
|
|
</para>
|
|
|
|
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
</sect1>
|
|
|
|
|
|
|
|
<sect1 id="fdw-planning">
|
|
|
|
<title>Foreign Data Wrapper Query Planning</title>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
The FDW callback functions <function>GetForeignRelSize</>,
|
|
|
|
<function>GetForeignPaths</>, and <function>GetForeignPlan</> must fit
|
|
|
|
into the workings of the <productname>PostgreSQL</> planner. Here are
|
|
|
|
some notes about what they must do.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
The information in <literal>root</> and <literal>baserel</> can be used
|
|
|
|
to reduce the amount of information that has to be fetched from the
|
|
|
|
foreign table (and therefore reduce the cost).
|
|
|
|
<literal>baserel->baserestrictinfo</> is particularly interesting, as
|
|
|
|
it contains restriction quals (<literal>WHERE</> clauses) that should be
|
|
|
|
used to filter the rows to be fetched. (The FDW itself is not required
|
|
|
|
to enforce these quals, as the core executor can check them instead.)
|
|
|
|
<literal>baserel->reltargetlist</> can be used to determine which
|
|
|
|
columns need to be fetched; but note that it only lists columns that
|
|
|
|
have to be emitted by the <structname>ForeignScan</> plan node, not
|
|
|
|
columns that are used in qual evaluation but not output by the query.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
Various private fields are available for the FDW planning functions to
|
|
|
|
keep information in. Generally, whatever you store in FDW private fields
|
|
|
|
should be palloc'd, so that it will be reclaimed at the end of planning.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<literal>baserel->fdw_private</> is a <type>void</> pointer that is
|
|
|
|
available for FDW planning functions to store information relevant to
|
|
|
|
the particular foreign table. The core planner does not touch it except
|
|
|
|
to initialize it to NULL when the <literal>baserel</> node is created.
|
|
|
|
It is useful for passing information forward from
|
|
|
|
<function>GetForeignRelSize</> to <function>GetForeignPaths</> and/or
|
|
|
|
<function>GetForeignPaths</> to <function>GetForeignPlan</>, thereby
|
|
|
|
avoiding recalculation.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<function>GetForeignPaths</> can identify the meaning of different
|
|
|
|
access paths by storing private information in the
|
|
|
|
<structfield>fdw_private</> field of <structname>ForeignPath</> nodes.
|
|
|
|
<structfield>fdw_private</> is declared as a <type>List</> pointer, but
|
|
|
|
could actually contain anything since the core planner does not touch
|
|
|
|
it. However, best practice is to use a representation that's dumpable
|
|
|
|
by <function>nodeToString</>, for use with debugging support available
|
|
|
|
in the backend.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<function>GetForeignPlan</> can examine the <structfield>fdw_private</>
|
|
|
|
field of the selected <structname>ForeignPath</> node, and can generate
|
|
|
|
<structfield>fdw_exprs</> and <structfield>fdw_private</> lists to be
|
|
|
|
placed in the <structname>ForeignScan</> plan node, where they will be
|
|
|
|
available at execution time. Both of these lists must be
|
|
|
|
represented in a form that <function>copyObject</> knows how to copy.
|
|
|
|
The <structfield>fdw_private</> list has no other restrictions and is
|
|
|
|
not interpreted by the core backend in any way. The
|
|
|
|
<structfield>fdw_exprs</> list, if not NIL, is expected to contain
|
2012-06-07 23:06:20 +02:00
|
|
|
expression trees that are intended to be executed at run time. These
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
trees will undergo post-processing by the planner to make them fully
|
|
|
|
executable.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
2012-06-07 23:06:20 +02:00
|
|
|
In <function>GetForeignPlan</>, generally the passed-in target list can
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
be copied into the plan node as-is. The passed scan_clauses list
|
|
|
|
contains the same clauses as <literal>baserel->baserestrictinfo</>,
|
|
|
|
but may be re-ordered for better execution efficiency. In simple cases
|
|
|
|
the FDW can just strip <structname>RestrictInfo</> nodes from the
|
|
|
|
scan_clauses list (using <function>extract_actual_clauses</>) and put
|
|
|
|
all the clauses into the plan node's qual list, which means that all the
|
2012-06-07 23:06:20 +02:00
|
|
|
clauses will be checked by the executor at run time. More complex FDWs
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
may be able to check some of the clauses internally, in which case those
|
|
|
|
clauses can be removed from the plan node's qual list so that the
|
|
|
|
executor doesn't waste time rechecking them.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
As an example, the FDW might identify some restriction clauses of the
|
|
|
|
form <replaceable>foreign_variable</> <literal>=</>
|
|
|
|
<replaceable>sub_expression</>, which it determines can be executed on
|
|
|
|
the remote server given the locally-evaluated value of the
|
|
|
|
<replaceable>sub_expression</>. The actual identification of such a
|
|
|
|
clause should happen during <function>GetForeignPaths</>, since it would
|
|
|
|
affect the cost estimate for the path. The path's
|
|
|
|
<structfield>fdw_private</> field would probably include a pointer to
|
|
|
|
the identified clause's <structname>RestrictInfo</> node. Then
|
|
|
|
<function>GetForeignPlan</> would remove that clause from scan_clauses,
|
|
|
|
but add the <replaceable>sub_expression</> to <structfield>fdw_exprs</>
|
|
|
|
to ensure that it gets massaged into executable form. It would probably
|
|
|
|
also put control information into the plan node's
|
|
|
|
<structfield>fdw_private</> field to tell the execution functions what
|
2012-06-07 23:06:20 +02:00
|
|
|
to do at run time. The query transmitted to the remote server would
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
involve something like <literal>WHERE <replaceable>foreign_variable</> =
|
2012-06-07 23:06:20 +02:00
|
|
|
$1</literal>, with the parameter value obtained at run time from
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
evaluation of the <structfield>fdw_exprs</> expression tree.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
The FDW should always construct at least one path that depends only on
|
|
|
|
the table's restriction clauses. In join queries, it might also choose
|
|
|
|
to construct path(s) that depend on join clauses, for example
|
|
|
|
<replaceable>foreign_variable</> <literal>=</>
|
|
|
|
<replaceable>local_variable</>. Such clauses will not be found in
|
|
|
|
<literal>baserel->baserestrictinfo</> but must be sought in the
|
|
|
|
relation's join lists. A path using such a clause is called a
|
2012-10-10 19:54:38 +02:00
|
|
|
<quote>parameterized path</>. It must identify the other relations
|
|
|
|
used in the selected join clause(s) with a suitable value of
|
|
|
|
<literal>param_info</>; use <function>get_baserel_parampathinfo</>
|
|
|
|
to compute that value. In <function>GetForeignPlan</>, the
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
<replaceable>local_variable</> portion of the join clause would be added
|
2012-06-07 23:06:20 +02:00
|
|
|
to <structfield>fdw_exprs</>, and then at run time the case works the
|
Revise FDW planning API, again.
Further reflection shows that a single callback isn't very workable if we
desire to let FDWs generate multiple Paths, because that forces the FDW to
do all work necessary to generate a valid Plan node for each Path. Instead
split the former PlanForeignScan API into three steps: GetForeignRelSize,
GetForeignPaths, GetForeignPlan. We had already bit the bullet of breaking
the 9.1 FDW API for 9.2, so this shouldn't cause very much additional pain,
and it's substantially more flexible for complex FDWs.
Add an fdw_private field to RelOptInfo so that the new functions can save
state there rather than possibly having to recalculate information two or
three times.
In addition, we'd not thought through what would be needed to allow an FDW
to set up subexpressions of its choice for runtime execution. We could
treat ForeignScan.fdw_private as an executable expression but that seems
likely to break existing FDWs unnecessarily (in particular, it would
restrict the set of node types allowable in fdw_private to those supported
by expression_tree_walker). Instead, invent a separate field fdw_exprs
which will receive the postprocessing appropriate for expression trees.
(One field is enough since it can be a list of expressions; also, we assume
the corresponding expression state tree(s) will be held within fdw_state,
so we don't need to add anything to ForeignScanState.)
Per review of Hanada Shigeru's pgsql_fdw patch. We may need to tweak this
further as we continue to work on that patch, but to me it feels a lot
closer to being right now.
2012-03-09 18:48:48 +01:00
|
|
|
same as for an ordinary restriction clause.
|
|
|
|
</para>
|
|
|
|
|
2012-03-08 00:20:58 +01:00
|
|
|
</sect1>
|
|
|
|
|
2011-02-20 06:17:18 +01:00
|
|
|
</chapter>
|