Check if the limit is null (#7665)

DESCRIPTION: Add a check to see if the given limit is null. 

Fixes a bug by checking if the limit given in the query is null when the
actual limit is computed with respect to the given offset.
Prior to this change, null is interpreted as 0 during the limit
calculation when both limit and offset are given.

Fixes #7663

src/backend/distributed/planner/multi_logical_optimizer.c

@@ -4753,22 +4753,35 @@ WorkerLimitCount(Node *limitCount, Node *limitOffset, OrderByLimitReference
 	if (workerLimitNode != NULL && limitOffset != NULL)
 	{
 		Const *workerLimitConst = (Const *) workerLimitNode;
-		Const *workerOffsetConst = (Const *) limitOffset;
-		int64 workerLimitCount = DatumGetInt64(workerLimitConst->constvalue);
-		int64 workerOffsetCount = DatumGetInt64(workerOffsetConst->constvalue);
 
-		workerLimitCount = workerLimitCount + workerOffsetCount;
-		workerLimitNode = (Node *) MakeIntegerConstInt64(workerLimitCount);
+		/* Only update the worker limit if the const is not null.*/
+		if (!workerLimitConst->constisnull)
+		{
+			Const *workerOffsetConst = (Const *) limitOffset;
+			int64 workerLimitCount = DatumGetInt64(workerLimitConst->constvalue);
+
+			/* If the offset is null, it defaults to 0 when cast to int64. */
+			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;
-		int64 workerLimitCount = DatumGetInt64(workerLimitConst->constvalue);
+		if (!workerLimitConst->constisnull)
+		{
+			int64 workerLimitCount = DatumGetInt64(workerLimitConst->constvalue);
 
-		ereport(DEBUG1, (errmsg("push down of limit count: " INT64_FORMAT,
-								workerLimitCount)));
+			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 +521,86 @@ 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)
+
+-- check if we push the right limit when both offset and limit are given
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey OFFSET 1 LIMIT 3;
+DEBUG:  push down of limit count: 4
+ l_orderkey
+---------------------------------------------------------------------
+          1
+          1
+          1
+(3 rows)
+
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey OFFSET null LIMIT 1;
+DEBUG:  push down of limit count: 1
+ l_orderkey
+---------------------------------------------------------------------
+          1
+(1 row)
+
+-- check if we can correctly push the limit when it is all
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 2 LIMIT all;
+DEBUG:  push down of limit count: ALL
+ l_orderkey
+---------------------------------------------------------------------
+          1
+          1
+          1
+          1
+          1
+          1
+(6 rows)
+
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 2 OFFSET 2 LIMIT all;
+DEBUG:  push down of limit count: ALL
+ l_orderkey
+---------------------------------------------------------------------
+          1
+          1
+          1
+          1
+(4 rows)
+
 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 +222,25 @@ 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;
+
+-- check if we push the right limit when both offset and limit are given
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey OFFSET 1 LIMIT 3;
+
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 3 ORDER BY l_orderkey OFFSET null LIMIT 1;
+
+-- check if we can correctly push the limit when it is all
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 2 LIMIT all;
+
+SELECT l_orderkey FROM lineitem WHERE l_orderkey < 2 OFFSET 2 LIMIT all;
+
 SET client_min_messages TO NOTICE;
 
 -- non constants should not push down