Add SupportRequestInlineInFrom planner support request.

This request allows a support function to replace a function call
appearing in FROM (typically a set-returning function) with an
equivalent SELECT subquery.  The subquery will then be subject
to the planner's usual optimizations, potentially allowing a much
better plan to be generated.  While the planner has long done this
automatically for simple SQL-language functions, it's now possible
for extensions to do it for functions outside that group.
Notably, this could be useful for functions that are presently
implemented in PL/pgSQL and work by generating and then EXECUTE'ing
a SQL query.

Author: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/09de6afa-c33d-4d94-a5cb-afc6cea0d2bb@illuminatedcomputing.com

1 files changed, 25 insertions, 0 deletions

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 55a99c0ff34..537ee6fa254 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -4160,6 +4160,31 @@ supportfn(internal) returns internal
    </para>
 
    <para>
+    <literal>SupportRequestSimplify</literal> is not used
+    for <link linkend="queries-tablefunctions">set-returning
+    functions</link>.  Instead, support functions can implement
+    the <literal>SupportRequestInlineInFrom</literal> request to expand
+    function calls appearing in the <literal>FROM</literal> clause of a
+    query.  (It's also allowed to support this request for
+    non-set-returning functions, although
+    typically <literal>SupportRequestSimplify</literal> would serve as
+    well.)  For this request type, a successful result must be
+    a <literal>SELECT</literal> Query tree, which will replace
+    the <literal>FROM</literal> item as though a sub-select had been
+    written instead.  The Query tree must appear as it would after parse
+    analysis and rewrite processing.  One way to ensure that that's true
+    is to build a SQL string then feed it
+    through <function>pg_parse_query</function>
+    and <function>pg_analyze_and_rewrite</function>, or related
+    functions.  <literal>PARAM_EXTERN</literal> <structname>Param</structname>
+    nodes can appear within the Query to represent the function's
+    arguments; they will be replaced by the actual argument expressions.
+    As with <literal>SupportRequestSimplify</literal>, it is the support
+    function's responsibility that the replacement Query be equivalent to
+    normal execution of the target function.
+   </para>
+
+   <para>
     For target functions that return <type>boolean</type>, it is often useful to estimate
     the fraction of rows that will be selected by a <literal>WHERE</literal> clause using that
     function.  This can be done by a support function that implements