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Redesign PlanForeignScan API to allow multiple paths for a foreign table.

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The original API specification only allowed an FDW to create a single
access path, which doesn't seem like a terribly good idea in hindsight.
Instead, move the responsibility for building the Path node and calling
add_path() into the FDW's PlanForeignScan function.  Now, it can do that
more than once if appropriate.  There is no longer any need for the
transient FdwPlan struct, so get rid of that.

Etsuro Fujita, Shigeru Hanada, Tom Lane
This commit is contained in:
Tom Lane
2012-03-05 16:15:59 -05:00
parent 3f47e145f1
commit 6b289942bf
12 changed files with 103 additions and 125 deletions
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33
doc/src/sgml/fdwhandler.sgml
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@ -88,21 +88,31 @@
<para>
<programlisting>
FdwPlan *
void
PlanForeignScan (Oid foreigntableid,
PlannerInfo *root,
RelOptInfo *baserel);
</programlisting>
Plan a scan on a foreign table. This is called when a query is planned.
Create possible access paths for a scan on a foreign table. This is
called when a query is planned.
<literal>foreigntableid</> is the <structname>pg_class</> OID of the
foreign table. <literal>root</> is the planner's global information
about the query, and <literal>baserel</> is the planner's information
about this table.
The function must return a palloc'd struct that contains cost estimates
plus any FDW-private information that is needed to execute the foreign
scan at a later time. (Note that the private information must be
represented in a form that <function>copyObject</> knows how to copy.)
</para>
<para>
The function must generate at least one access path (ForeignPath node)
for a scan on the foreign table and must call <function>add_path</> to
add the path to <literal>baserel-&gt;pathlist</>. It's recommended to
use <function>create_foreignscan_path</> to build the ForeignPath node.
The function may generate multiple access paths, e.g., a path which has
valid <literal>pathkeys</> to represent a pre-sorted result. Each access
path must contain cost estimates, and can contain any FDW-private
information that is needed to execute the foreign scan at a later time.
(Note that the private information must be represented in a form that
<function>copyObject</> knows how to copy.)
</para>
<para>
@ -159,9 +169,8 @@ BeginForeignScan (ForeignScanState *node,
its <structfield>fdw_state</> field is still NULL. Information about
the table to scan is accessible through the
<structname>ForeignScanState</> node (in particular, from the underlying
<structname>ForeignScan</> plan node, which contains a pointer to the
<structname>FdwPlan</> structure returned by
<function>PlanForeignScan</>).
<structname>ForeignScan</> plan node, which contains any FDW-private
information provided by <function>PlanForeignScan</>).
</para>
<para>
@ -228,9 +237,9 @@ EndForeignScan (ForeignScanState *node);
</para>
<para>
The <structname>FdwRoutine</> and <structname>FdwPlan</> struct types
are declared in <filename>src/include/foreign/fdwapi.h</>, which see
for additional details.
The <structname>FdwRoutine</> struct type is declared in
<filename>src/include/foreign/fdwapi.h</>, which see for additional
details.
</para>
</sect1>