When removing a relation from the query, drop its RelOptInfo.

In commit b78f626 I opined that it was "too risky" to delete a
relation's RelOptInfo from the planner's data structures when we have
realized that we don't need to join to it; so instead we just marked
it as a dead relation.  In hindsight that judgment seems flawed: any
subsequent access to such a dead relation is arguably a bug in
itself, so leaving the RelOptInfo present just helps to mask bugs.
Let's delete it instead, allowing removal of the whole notion of a
"dead relation".  So far as the regression tests can find, this
requires no other code changes, except for one Assert in equivclass.c
that was very dubiously not complaining about access to a dead rel.

Discussion: https://postgr.es/m/229905.1676062220@sss.pgh.pa.us

src/backend/optimizer/path/equivclass.c

@@ -729,8 +729,7 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 				continue;
 			}
 
-			Assert(rel->reloptkind == RELOPT_BASEREL ||
-				   rel->reloptkind == RELOPT_DEADREL);
+			Assert(rel->reloptkind == RELOPT_BASEREL);
 
 			rel->eclass_indexes = bms_add_member(rel->eclass_indexes,
 												 ec_index);

src/backend/optimizer/plan/analyzejoins.c

@@ -331,12 +331,6 @@ remove_rel_from_query(PlannerInfo *root, int relid, int ojrelid,
 	Index		rti;
 	ListCell   *l;
 
-	/*
-	 * Mark the rel as "dead" to show it is no longer part of the join tree.
-	 * (Removing it from the baserel array altogether seems too risky.)
-	 */
-	rel->reloptkind = RELOPT_DEADREL;
-
 	/*
 	 * Remove references to the rel from other baserels' attr_needed arrays.
 	 */
@@ -487,6 +481,16 @@ remove_rel_from_query(PlannerInfo *root, int relid, int ojrelid,
 	 * There may be references to the rel in root->fkey_list, but if so,
 	 * match_foreign_keys_to_quals() will get rid of them.
 	 */
+
+	/*
+	 * Finally, remove the rel from the baserel array to prevent it from being
+	 * referenced again.  (We can't do this earlier because
+	 * remove_join_clause_from_rels will touch it.)
+	 */
+	root->simple_rel_array[relid] = NULL;
+
+	/* And nuke the RelOptInfo, just in case there's another access path */
+	pfree(rel);
 }
 
 /*

src/backend/optimizer/plan/initsplan.c

@@ -2886,8 +2886,9 @@ match_foreign_keys_to_quals(PlannerInfo *root)
 
 		/*
 		 * Either relid might identify a rel that is in the query's rtable but
-		 * isn't referenced by the jointree so won't have a RelOptInfo.  Hence
-		 * don't use find_base_rel() here.  We can ignore such FKs.
+		 * isn't referenced by the jointree, or has been removed by join
+		 * removal, so that it won't have a RelOptInfo.  Hence don't use
+		 * find_base_rel() here.  We can ignore such FKs.
 		 */
 		if (fkinfo->con_relid >= root->simple_rel_array_size ||
 			fkinfo->ref_relid >= root->simple_rel_array_size)
@@ -2901,8 +2902,7 @@ match_foreign_keys_to_quals(PlannerInfo *root)
 
 		/*
 		 * Ignore FK unless both rels are baserels.  This gets rid of FKs that
-		 * link to inheritance child rels (otherrels) and those that link to
-		 * rels removed by join removal (dead rels).
+		 * link to inheritance child rels (otherrels).
 		 */
 		if (con_rel->reloptkind != RELOPT_BASEREL ||
 			ref_rel->reloptkind != RELOPT_BASEREL)

src/include/nodes/pathnodes.h

@@ -638,9 +638,6 @@ typedef struct PartitionSchemeData *PartitionScheme;
  * Many of the fields in these RelOptInfos are meaningless, but their Path
  * fields always hold Paths showing ways to do that processing step.
  *
- * Lastly, there is a RelOptKind for "dead" relations, which are base rels
- * that we have proven we don't need to join after all.
- *
  * Parts of this data structure are specific to various scan and join
  * mechanisms.  It didn't seem worth creating new node types for them.
  *
@@ -823,8 +820,7 @@ typedef enum RelOptKind
 	RELOPT_OTHER_MEMBER_REL,
 	RELOPT_OTHER_JOINREL,
 	RELOPT_UPPER_REL,
-	RELOPT_OTHER_UPPER_REL,
-	RELOPT_DEADREL
+	RELOPT_OTHER_UPPER_REL
 } RelOptKind;
 
 /*