Partially undo commit 94da732.

On closer inspection, mcv.c isn't as broken for ScalarArrayOpExpr
as I thought.  The Var-on-right issue is real enough, but actually
it does cope fine with a NULL array constant --- I was misled by
an XXX comment suggesting it didn't.  Undo that part of the code
change, and replace the XXX comment with something less misleading.

Author: tglsfdc

src/backend/statistics/extended_stats.c

@@ -1051,19 +1051,18 @@ statext_is_compatible_clause_internal(PlannerInfo *root, Node *clause,
 		RangeTblEntry *rte = root->simple_rte_array[relid];
 		ScalarArrayOpExpr *expr = (ScalarArrayOpExpr *) clause;
 		Var		   *var;
-		Const	   *cst;
 		bool		expronleft;
 
 		/* Only expressions with two arguments are considered compatible. */
 		if (list_length(expr->args) != 2)
 			return false;
 
 		/* Check if the expression has the right shape (one Var, one Const) */
-		if (!examine_clause_args(expr->args, &var, &cst, &expronleft))
+		if (!examine_clause_args(expr->args, &var, NULL, &expronleft))
 			return false;
 
-		/* We only support Var on left and non-null array constants */
-		if (!expronleft || cst->constisnull)
+		/* We only support Var on left, Const on right */
+		if (!expronleft)
 			return false;
 
 		/*

src/backend/statistics/mcv.c

@@ -1679,17 +1679,24 @@ mcv_get_match_bitmap(PlannerInfo *root, List *clauses,
 				Datum	   *elem_values;
 				bool	   *elem_nulls;
 
-				/* We expect Var on left and non-null constant on right */
-				if (!varonleft || cst->constisnull)
+				/* We expect Var on left */
+				if (!varonleft)
 					elog(ERROR, "incompatible clause");
 
-				arrayval = DatumGetArrayTypeP(cst->constvalue);
-				get_typlenbyvalalign(ARR_ELEMTYPE(arrayval),
-									 &elmlen, &elmbyval, &elmalign);
-				deconstruct_array(arrayval,
-								  ARR_ELEMTYPE(arrayval),
-								  elmlen, elmbyval, elmalign,
-								  &elem_values, &elem_nulls, &num_elems);
+				/*
+				 * Deconstruct the array constant, unless it's NULL (we'll
+				 * cover that case below)
+				 */
+				if (!cst->constisnull)
+				{
+					arrayval = DatumGetArrayTypeP(cst->constvalue);
+					get_typlenbyvalalign(ARR_ELEMTYPE(arrayval),
+										 &elmlen, &elmbyval, &elmalign);
+					deconstruct_array(arrayval,
+									  ARR_ELEMTYPE(arrayval),
+									  elmlen, elmbyval, elmalign,
+									  &elem_values, &elem_nulls, &num_elems);
+				}
 
 				/* match the attribute to a dimension of the statistic */
 				idx = bms_member_index(keys, var->varattno);