Remove some dead code in selfuncs.c

RelOptInfo.userid is the same for all relations in a given inheritance
tree, so the code in examine_variable() and example_simple_variable()
that repeats the ACL checks on the root parent rel instead of a given
leaf child relations need not recompute userid too.

Author: Amit Langote <amitlangote09@gmail.com>
Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20221210201753.GA27893@telsasoft.com

src/backend/optimizer/util/relnode.c

@@ -500,7 +500,6 @@ find_join_rel(PlannerInfo *root, Relids relids)
  *
  * Otherwise these fields are left invalid, so GetForeignJoinPaths will not be
  * called for the join relation.
- *
  */
 static void
 set_foreign_rel_properties(RelOptInfo *joinrel, RelOptInfo *outer_rel,

src/backend/utils/adt/selfuncs.c

@@ -5080,6 +5080,18 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 		 */
 		ListCell   *ilist;
 		ListCell   *slist;
+		Oid			userid;
+
+		/*
+		 * Determine the user ID to use for privilege checks: either
+		 * onerel->userid if it's set (e.g., in case we're accessing the table
+		 * via a view), or the current user otherwise.
+		 *
+		 * If we drill down to child relations, we keep using the same userid:
+		 * it's going to be the same anyway, due to how we set up the relation
+		 * tree (q.v. build_simple_rel).
+		 */
+		userid = OidIsValid(onerel->userid) ? onerel->userid : GetUserId();
 
 		foreach(ilist, onerel->indexlist)
 		{
@@ -5150,18 +5162,10 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 							{
 								/* Get index's table for permission check */
 								RangeTblEntry *rte;
-								Oid			userid;
 
 								rte = planner_rt_fetch(index->rel->relid, root);
 								Assert(rte->rtekind == RTE_RELATION);
 
-								/*
-								 * Use onerel->userid if it's set, in case
-								 * we're accessing the table via a view.
-								 */
-								userid = OidIsValid(onerel->userid) ?
-									onerel->userid : GetUserId();
-
 								/*
 								 * For simplicity, we insist on the whole
 								 * table being selectable, rather than trying
@@ -5212,9 +5216,6 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 										rte = planner_rt_fetch(varno, root);
 										Assert(rte->rtekind == RTE_RELATION);
 
-										userid = OidIsValid(onerel->userid) ?
-											onerel->userid : GetUserId();
-
 										vardata->acl_ok =
 											rte->securityQuals == NIL &&
 											(pg_class_aclcheck(rte->relid,
@@ -5281,8 +5282,6 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 				/* found a match, see if we can extract pg_statistic row */
 				if (equal(node, expr))
 				{
-					Oid			userid;
-
 					/*
 					 * XXX Not sure if we should cache the tuple somewhere.
 					 * Now we just create a new copy every time.
@@ -5292,13 +5291,6 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 
 					vardata->freefunc = ReleaseDummy;
 
-					/*
-					 * Use onerel->userid if it's set, in case we're accessing
-					 * the table via a view.
-					 */
-					userid = OidIsValid(onerel->userid) ?
-						onerel->userid : GetUserId();
-
 					/*
 					 * For simplicity, we insist on the whole table being
 					 * selectable, rather than trying to identify which
@@ -5345,9 +5337,6 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 							rte = planner_rt_fetch(varno, root);
 							Assert(rte->rtekind == RTE_RELATION);
 
-							userid = OidIsValid(onerel->userid) ?
-								onerel->userid : GetUserId();
-
 							vardata->acl_ok =
 								rte->securityQuals == NIL &&
 								(pg_class_aclcheck(rte->relid,
@@ -5486,9 +5475,10 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 				rte = planner_rt_fetch(varno, root);
 				Assert(rte->rtekind == RTE_RELATION);
 
-				userid = OidIsValid(onerel->userid) ?
-					onerel->userid : GetUserId();
-
+				/*
+				 * Fine to use the same userid as it's the same in all
+				 * relations of a given inheritance tree.
+				 */
 				vardata->acl_ok =
 					rte->securityQuals == NIL &&
 					((pg_class_aclcheck(rte->relid, userid,