Update comment about set_join_pathlist_hook().

Etsuro Fujita · Etsuro Fujita · commit c994d06f105a · 2023-09-21T19:45:11.000+09:00
The comment introduced by commit e7cb7ee was a bit too terse, which could lead to extensions doing different things within the hook function than we intend to allow. Extend the comment to explain what they can do within the hook function. Back-patch to all supported branches. In passing, I rephrased a nearby comment that I recently added to the back branches. Reviewed by David Rowley and Andrei Lepikhov. Discussion: https://postgr.es/m/CAPmGK15SBPA1nr3Aqsdm%2BYyS-ay0Ayo2BRYQ8_A2To9eLqwopQ%40mail.gmail.com
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
@@ -312,7 +312,8 @@ add_paths_to_joinrel(PlannerInfo *root,
 	/*
 	 * createplan.c does not currently support handling of pseudoconstant
 	 * clauses assigned to joins pushed down by extensions; check if the
-	 * restrictlist has such clauses, and if so, disallow pushing down joins.
+	 * restrictlist has such clauses, and if not, allow them to consider
+	 * pushing down joins.
 	 */
 	if ((joinrel->fdwroutine &&
 		 joinrel->fdwroutine->GetForeignJoinPaths) ||
@@ -333,7 +334,10 @@ add_paths_to_joinrel(PlannerInfo *root,
 												 jointype, &extra);
 
 	/*
-	 * 6. Finally, give extensions a chance to manipulate the path list.
+	 * 6. Finally, give extensions a chance to manipulate the path list.  They
+	 * could add new paths (such as CustomPaths) by calling add_path(), or
+	 * add_partial_path() if parallel aware.  They could also delete or modify
+	 * paths added by the core code.
 	 */
 	if (set_join_pathlist_hook &&
 		consider_join_pushdown)
