fix: ROWNUM = N only optimized for N=1

ROWNUM = N predicates can only be transformed to LIMIT when N=1.
For N>1, the predicate must remain in WHERE clause to correctly
return 0 rows per Oracle semantics. Added test cases for ROWNUM = 2
and ROWNUM = 3 to verify correct behavior.

Author: rophy

src/backend/optimizer/plan/planner.c

@@ -738,9 +738,16 @@ transform_rownum_to_limit(Query *parse)
 		}
 		else if (strcmp(opname, "=") == 0)
 		{
-			/* ROWNUM = N  ->  LIMIT N (only makes sense for N=1) */
-			limit_value = n;
-			rownum_qual = qual;
+			/*
+			 * ROWNUM = N can only be optimized for N=1.
+			 * For N>1, the predicate must remain in WHERE clause,
+			 * which will correctly return 0 rows (Oracle semantics).
+			 */
+			if (n == 1)
+			{
+				limit_value = n;
+				rownum_qual = qual;
+			}
 			pfree(opname);
 			break;
 		}

src/oracle_test/regress/expected/rownum.out

@@ -203,6 +203,18 @@ SELECT id, name FROM rownum_test WHERE ROWNUM = 0;
  10 | Jack
 (10 rows)
 
+-- ROWNUM = 2 (not optimizable, returns empty - Oracle semantics)
+SELECT id, name FROM rownum_test WHERE ROWNUM = 2;
+ id | name 
+----+------
+(0 rows)
+
+-- ROWNUM = 3 (not optimizable, returns empty - Oracle semantics)
+SELECT id, name FROM rownum_test WHERE ROWNUM = 3;
+ id | name 
+----+------
+(0 rows)
+
 -- ROWNUM with negative number
 SELECT id, name FROM rownum_test WHERE ROWNUM <= -1;
  id | name 
@@ -308,6 +320,15 @@ EXPLAIN (COSTS OFF) SELECT id, name FROM rownum_test WHERE ROWNUM > 5;
    ->  Seq Scan on rownum_test
 (3 rows)
 
+-- ROWNUM = 2 should NOT be optimized to LIMIT (keep ROWNUM in WHERE)
+EXPLAIN (COSTS OFF) SELECT id, name FROM rownum_test WHERE ROWNUM = 2;
+           QUERY PLAN            
+---------------------------------
+ Result
+   One-Time Filter: (ROWNUM = 2)
+   ->  Seq Scan on rownum_test
+(3 rows)
+
 --
 -- ROWNUM with other clauses
 --

src/oracle_test/regress/sql/rownum.sql

@@ -125,6 +125,12 @@ SELECT id, name FROM rownum_test WHERE ROWNUM >= 2;
 -- ROWNUM = 0 (always false)
 SELECT id, name FROM rownum_test WHERE ROWNUM = 0;
 
+-- ROWNUM = 2 (not optimizable, returns empty - Oracle semantics)
+SELECT id, name FROM rownum_test WHERE ROWNUM = 2;
+
+-- ROWNUM = 3 (not optimizable, returns empty - Oracle semantics)
+SELECT id, name FROM rownum_test WHERE ROWNUM = 3;
+
 -- ROWNUM with negative number
 SELECT id, name FROM rownum_test WHERE ROWNUM <= -1;
 
@@ -174,6 +180,9 @@ SELECT * FROM (
 -- Non-optimizable pattern (no Limit)
 EXPLAIN (COSTS OFF) SELECT id, name FROM rownum_test WHERE ROWNUM > 5;
 
+-- ROWNUM = 2 should NOT be optimized to LIMIT (keep ROWNUM in WHERE)
+EXPLAIN (COSTS OFF) SELECT id, name FROM rownum_test WHERE ROWNUM = 2;
+
 --
 -- ROWNUM with other clauses
 --