Get rid of duplicate child RTE for a partitioned table.

We've been creating duplicate RTEs for partitioned tables just
because we do so for regular inheritance parent tables.  But unlike
regular-inheritance parents which are themselves regular tables
and thus need to be scanned, partitioned tables don't need the
extra RTE.

This makes the conditions for building a child RTE the same as those
for building an AppendRelInfo, allowing minor simplification in
expand_single_inheritance_child.  Since the planner's actual processing
is driven off the AppendRelInfo list, nothing much changes beyond that,
we just have one fewer useless RTE entry.

Amit Langote, reviewed and hacked a bit by me

Discussion: https://postgr.es/m/9d7c5112-cb99-6a47-d3be-cf1ee6862a1d@lab.ntt.co.jp

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -8435,7 +8435,7 @@ SELECT t1.a,t2.b,t2.c FROM fprt1 t1 LEFT JOIN (SELECT * FROM fprt2 WHERE a < 10)
  Foreign Scan
    Output: t1.a, ftprt2_p1.b, ftprt2_p1.c
    Relations: (public.ftprt1_p1 t1) LEFT JOIN (public.ftprt2_p1 fprt2)
-   Remote SQL: SELECT r6.a, r9.b, r9.c FROM (public.fprt1_p1 r6 LEFT JOIN public.fprt2_p1 r9 ON (((r6.a = r9.b)) AND ((r6.b = r9.a)) AND ((r9.a < 10)))) WHERE ((r6.a < 10)) ORDER BY r6.a ASC NULLS LAST, r9.b ASC NULLS LAST, r9.c ASC NULLS LAST
+   Remote SQL: SELECT r5.a, r7.b, r7.c FROM (public.fprt1_p1 r5 LEFT JOIN public.fprt2_p1 r7 ON (((r5.a = r7.b)) AND ((r5.b = r7.a)) AND ((r7.a < 10)))) WHERE ((r5.a < 10)) ORDER BY r5.a ASC NULLS LAST, r7.b ASC NULLS LAST, r7.c ASC NULLS LAST
 (4 rows)
 
 SELECT t1.a,t2.b,t2.c FROM fprt1 t1 LEFT JOIN (SELECT * FROM fprt2 WHERE a < 10) t2 ON (t1.a = t2.b and t1.b = t2.a) WHERE t1.a < 10 ORDER BY 1,2,3;

src/backend/optimizer/util/inherit.c

@@ -90,9 +90,10 @@ expand_inherited_tables(PlannerInfo *root)
  * A childless table is never considered to be an inheritance set. For
  * regular inheritance, a parent RTE must always have at least two associated
  * AppendRelInfos: one corresponding to the parent table as a simple member of
- * inheritance set and one or more corresponding to the actual children.
- * Since a partitioned table is not scanned, it might have only one associated
- * AppendRelInfo.
+ * the inheritance set and one or more corresponding to the actual children.
+ * (But a partitioned table might have only one associated AppendRelInfo,
+ * since it's not itself scanned and hence doesn't need a second RTE to
+ * represent itself as a member of the set.)
  */
 static void
 expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
@@ -145,6 +146,9 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 	/* Scan the inheritance set and expand it */
 	if (oldrelation->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
 	{
+		/*
+		 * Partitioned table, so set up for partitioning.
+		 */
 		Assert(rte->relkind == RELKIND_PARTITIONED_TABLE);
 
 		if (root->glob->partition_directory == NULL)
@@ -161,6 +165,11 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 	}
 	else
 	{
+		/*
+		 * Ordinary table, so process traditional-inheritance children.  (Note
+		 * that partitioned tables are not allowed to have inheritance
+		 * children, so it's not possible for both cases to apply.)
+		 */
 		List	   *appinfos = NIL;
 		RangeTblEntry *childrte;
 		Index		childRTindex;
@@ -182,8 +191,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 		}
 
 		/*
-		 * This table has no partitions.  Expand any plain inheritance
-		 * children in the order the OIDs were returned by
+		 * Expand inheritance children in the order the OIDs were returned by
 		 * find_all_inheritors.
 		 */
 		foreach(l, inhOIDs)
@@ -273,11 +281,6 @@ expand_partitioned_rtentry(PlannerInfo *root, RangeTblEntry *parentrte,
 		root->partColsUpdated =
 			has_partition_attrs(parentrel, parentrte->updatedCols, NULL);
 
-	/* First expand the partitioned table itself. */
-	expand_single_inheritance_child(root, parentrte, parentRTindex, parentrel,
-									top_parentrc, parentrel,
-									appinfos, &childrte, &childRTindex);
-
 	/*
 	 * If the partitioned table has no partitions, treat this as the
 	 * non-inheritance case.
@@ -288,6 +291,11 @@ expand_partitioned_rtentry(PlannerInfo *root, RangeTblEntry *parentrte,
 		return;
 	}
 
+	/*
+	 * Create a child RTE for each partition.  Note that unlike traditional
+	 * inheritance, we don't need a child RTE for the partitioned table
+	 * itself, because it's not going to be scanned.
+	 */
 	for (i = 0; i < partdesc->nparts; i++)
 	{
 		Oid			childOID = partdesc->oids[i];
@@ -321,8 +329,7 @@ expand_partitioned_rtentry(PlannerInfo *root, RangeTblEntry *parentrte,
 
 /*
  * expand_single_inheritance_child
- *		Build a RangeTblEntry and an AppendRelInfo, if appropriate, plus
- *		maybe a PlanRowMark.
+ *		Build a RangeTblEntry and an AppendRelInfo, plus maybe a PlanRowMark.
  *
  * We now expand the partition hierarchy level by level, creating a
  * corresponding hierarchy of AppendRelInfos and RelOptInfos, where each
@@ -371,9 +378,11 @@ expand_single_inheritance_child(PlannerInfo *root, RangeTblEntry *parentrte,
 	childrte->relid = childOID;
 	childrte->relkind = childrel->rd_rel->relkind;
 	/* A partitioned child will need to be expanded further. */
-	if (childOID != parentOID &&
-		childrte->relkind == RELKIND_PARTITIONED_TABLE)
+	if (childrte->relkind == RELKIND_PARTITIONED_TABLE)
+	{
+		Assert(childOID != parentOID);
 		childrte->inh = true;
+	}
 	else
 		childrte->inh = false;
 	childrte->requiredPerms = 0;
@@ -383,22 +392,16 @@ expand_single_inheritance_child(PlannerInfo *root, RangeTblEntry *parentrte,
 	*childRTindex_p = childRTindex;
 
 	/*
-	 * We need an AppendRelInfo if paths will be built for the child RTE. If
-	 * childrte->inh is true, then we'll always need to generate append paths
-	 * for it.  If childrte->inh is false, we must scan it if it's not a
-	 * partitioned table; but if it is a partitioned table, then it never has
-	 * any data of its own and need not be scanned.
+	 * Build an AppendRelInfo struct for each parent/child pair.
 	 */
-	if (childrte->relkind != RELKIND_PARTITIONED_TABLE || childrte->inh)
-	{
 	appinfo = make_append_rel_info(parentrel, childrel,
 								   parentRTindex, childRTindex);
 	*appinfos = lappend(*appinfos, appinfo);
 
 	/*
 	 * Translate the column permissions bitmaps to the child's attnums (we
-		 * have to build the translated_vars list before we can do this). But
-		 * if this is the parent table, leave copyObject's result alone.
+	 * have to build the translated_vars list before we can do this).  But if
+	 * this is the parent table, we can leave copyObject's result alone.
 	 *
 	 * Note: we need to do this even though the executor won't run any
 	 * permissions checks on the child RTE.  The insertedCols/updatedCols
@@ -413,7 +416,6 @@ expand_single_inheritance_child(PlannerInfo *root, RangeTblEntry *parentrte,
 		childrte->updatedCols = translate_col_privs(parentrte->updatedCols,
 													appinfo->translated_vars);
 	}
-	}
 
 	/*
 	 * Build a PlanRowMark if parent is marked FOR UPDATE/SHARE.