Fix run-time partition pruning code to handle NULL values properly.

The previous coding just ignored pruning constraints that compare a
partition key to a null-valued expression.  This is silly, since really
what we can do there is conclude that all partitions are rejected: the
pruning operator is known strict so the comparison must always fail.

This also fixes the logic to not ignore constisnull for a Const comparison
value.  That's probably an unreachable case, since the planner would
normally have simplified away a strict operator with a constant-null input.
But this code has no business assuming that.

David Rowley, per a gripe from me

Discussion: https://postgr.es/m/26279.1528670981@sss.pgh.pa.us

src/backend/partitioning/partprune.c

@@ -170,7 +170,8 @@ static PruneStepResult *perform_pruning_combine_step(PartitionPruneContext *cont
 static bool match_boolean_partition_clause(Oid partopfamily, Expr *clause,
 							   Expr *partkey, Expr **outconst);
 static bool partkey_datum_from_expr(PartitionPruneContext *context,
-						Expr *expr, int stateidx, Datum *value);
+						Expr *expr, int stateidx,
+						Datum *value, bool *isnull);
 
 
 /*
@@ -184,8 +185,9 @@ static bool partkey_datum_from_expr(PartitionPruneContext *context,
  * indexes.
  *
  * If no non-Const expressions are being compared to the partition key in any
- * of the 'partitioned_rels', then we return NIL.  In such a case run-time
- * partition pruning would be useless, since the planner did it already.
+ * of the 'partitioned_rels', then we return NIL to indicate no run-time
+ * pruning should be performed.  Run-time pruning would be useless, since the
+ * pruning done during planning will have pruned everything that can be.
  */
 List *
 make_partition_pruneinfo(PlannerInfo *root, List *partition_rels,
@@ -2835,14 +2837,33 @@ perform_pruning_base_step(PartitionPruneContext *context,
 			Expr	   *expr;
 			int			stateidx;
 			Datum		datum;
+			bool		isnull;
 
 			expr = lfirst(lc1);
 			stateidx = PruneCxtStateIdx(context->partnatts,
 										opstep->step.step_id, keyno);
-			if (partkey_datum_from_expr(context, expr, stateidx, &datum))
+			if (partkey_datum_from_expr(context, expr, stateidx,
+										&datum, &isnull))
 			{
 				Oid			cmpfn;
 
+				/*
+				 * Since we only allow strict operators in pruning steps, any
+				 * null-valued comparison value must cause the comparison to
+				 * fail, so that no partitions could match.
+				 */
+				if (isnull)
+				{
+					PruneStepResult *result;
+
+					result = (PruneStepResult *) palloc(sizeof(PruneStepResult));
+					result->bound_offsets = NULL;
+					result->scan_default = false;
+					result->scan_null = false;
+
+					return result;
+				}
+
 				/*
 				 * If we're going to need a different comparison function than
 				 * the one cached in the PartitionKey, we'll need to look up
@@ -3072,8 +3093,8 @@ match_boolean_partition_clause(Oid partopfamily, Expr *clause, Expr *partkey,
  *		Evaluate expression for potential partition pruning
  *
  * Evaluate 'expr', whose ExprState is stateidx of the context exprstate
- * array; set *value to the resulting Datum.  Return true if evaluation was
- * possible, otherwise false.
+ * array; set *value and *isnull to the resulting Datum and nullflag.
+ * Return true if evaluation was possible, otherwise false.
  *
  * Note that the evaluated result may be in the per-tuple memory context of
  * context->planstate->ps_ExprContext, and we may have leaked other memory
@@ -3082,11 +3103,16 @@ match_boolean_partition_clause(Oid partopfamily, Expr *clause, Expr *partkey,
  */
 static bool
 partkey_datum_from_expr(PartitionPruneContext *context,
-						Expr *expr, int stateidx, Datum *value)
+						Expr *expr, int stateidx,
+						Datum *value, bool *isnull)
 {
 	if (IsA(expr, Const))
 	{
-		*value = ((Const *) expr)->constvalue;
+		/* We can always determine the value of a constant */
+		Const	   *con = (Const *) expr;
+
+		*value = con->constvalue;
+		*isnull = con->constisnull;
 		return true;
 	}
 	else
@@ -3105,14 +3131,10 @@ partkey_datum_from_expr(PartitionPruneContext *context,
 		{
 			ExprState  *exprstate;
 			ExprContext *ectx;
-			bool		isNull;
 
 			exprstate = context->exprstates[stateidx];
 			ectx = context->planstate->ps_ExprContext;
-			*value = ExecEvalExprSwitchContext(exprstate, ectx, &isNull);
-			if (isNull)
-				return false;
-
+			*value = ExecEvalExprSwitchContext(exprstate, ectx, isnull);
 			return true;
 		}
 	}

src/test/regress/expected/partition_prune.out

@@ -2731,6 +2731,20 @@ explain (analyze, costs off, summary off, timing off)  execute q1 (1,2,2,1);
          Filter: ((b = ANY (ARRAY[$1, $2])) AND ($3 <> b) AND ($4 <> b))
 (4 rows)
 
+-- Ensure Params that evaluate to NULL properly prune away all partitions
+explain (analyze, costs off, summary off, timing off)
+select * from listp where a = (select null::int);
+                  QUERY PLAN                  
+----------------------------------------------
+ Append (actual rows=0 loops=1)
+   InitPlan 1 (returns $0)
+     ->  Result (actual rows=1 loops=1)
+   ->  Seq Scan on listp_1_1 (never executed)
+         Filter: (a = $0)
+   ->  Seq Scan on listp_2_1 (never executed)
+         Filter: (a = $0)
+(7 rows)
+
 drop table listp;
 -- Ensure runtime pruning works with initplans params with boolean types
 create table boolvalues (value bool not null);

src/test/regress/sql/partition_prune.sql

@@ -685,6 +685,10 @@ explain (analyze, costs off, summary off, timing off)  execute q1 (1,2,2,0);
 -- One subplan will remain in this case, but it should not be executed.
 explain (analyze, costs off, summary off, timing off)  execute q1 (1,2,2,1);
 
+-- Ensure Params that evaluate to NULL properly prune away all partitions
+explain (analyze, costs off, summary off, timing off)
+select * from listp where a = (select null::int);
+
 drop table listp;
 
 -- Ensure runtime pruning works with initplans params with boolean types