Fix 7663 by checking if the limit is null when the limit is computed with respect to the offset.

src/backend/distributed/planner/multi_logical_optimizer.c

@@ -4753,23 +4753,34 @@ WorkerLimitCount(Node *limitCount, Node *limitOffset, OrderByLimitReference
 	if (workerLimitNode != NULL && limitOffset != NULL)
 	{
 		Const *workerLimitConst = (Const *) workerLimitNode;
+		if (!workerLimitConst->constisnull)
+		{
+			/* Only update the worker limit if the const is not null.*/
 			Const *workerOffsetConst = (Const *) limitOffset;
 			int64 workerLimitCount = DatumGetInt64(workerLimitConst->constvalue);
 			int64 workerOffsetCount = DatumGetInt64(workerOffsetConst->constvalue);
-
 			workerLimitCount = workerLimitCount + workerOffsetCount;
 			workerLimitNode = (Node *) MakeIntegerConstInt64(workerLimitCount);
 		}
 		
+	}
+
 	/* display debug message on limit push down */
 	if (workerLimitNode != NULL)
 	{
 		Const *workerLimitConst = (Const *) workerLimitNode;
+		if (!workerLimitConst->constisnull)
+		{	
 			int64 workerLimitCount = DatumGetInt64(workerLimitConst->constvalue);
 
 			ereport(DEBUG1, (errmsg("push down of limit count: " INT64_FORMAT,
 									workerLimitCount)));
 		}
+		else
+		{
+			ereport(DEBUG1, (errmsg("push down of limit count: ALL")));
+		}
+	}
 
 	return workerLimitNode;
 }

src/test/regress/expected/multi_limit_clause.out

@@ -521,6 +521,46 @@ SELECT
           1 |    1
 (1 row)
 
+-- check if we can correctly push the limit when it is null
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey LIMIT null;
+DEBUG:  push down of limit count: ALL
+ l_orderkey
+---------------------------------------------------------------------
+          1
+          1
+          1
+          1
+          1
+          1
+          2
+(7 rows)
+
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey OFFSET 1 LIMIT null;
+DEBUG:  push down of limit count: ALL
+ l_orderkey
+---------------------------------------------------------------------
+          1
+          1
+          1
+          1
+          1
+          2
+(6 rows)
+
+SELECT count(*) FROM lineitem LIMIT null;
+DEBUG:  push down of limit count: ALL
+ count
+---------------------------------------------------------------------
+ 12000
+(1 row)
+
+SELECT count(*) FROM lineitem OFFSET 0 LIMIT null;
+DEBUG:  push down of limit count: ALL
+ count
+---------------------------------------------------------------------
+ 12000
+(1 row)
+
 SET client_min_messages TO NOTICE;
 -- non constants should not push down
 CREATE OR REPLACE FUNCTION my_limit()

src/test/regress/sql/multi_limit_clause.sql

@@ -222,6 +222,15 @@ SELECT
 	ORDER BY 2 DESC, 1
 	LIMIT 5;
 
+-- check if we can correctly push the limit when it is null
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey LIMIT null;
+
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey OFFSET 1 LIMIT null;
+
+SELECT count(*) FROM lineitem LIMIT null;
+
+SELECT count(*) FROM lineitem OFFSET 0 LIMIT null;
+
 SET client_min_messages TO NOTICE;
 
 -- non constants should not push down