feat: implement Oracle-compatible ROWNUM pseudocolumn

Add ROWNUM pseudocolumn with Oracle-compatible semantics:
- ROWNUM <= N optimized to LIMIT for simple queries
- Blocks LIMIT optimization for complex queries (ORDER BY, DISTINCT,
  GROUP BY, aggregation) to preserve Oracle semantics
- Handles special Oracle edge cases: ROWNUM > N, ROWNUM >= N, ROWNUM = N
- Supports tautology cases: ROWNUM > 0, ROWNUM >= 1 return all rows
- Marks ROWNUM as volatile to ensure per-row evaluation

Includes comprehensive regression tests covering all edge cases.

Author: rophy

src/backend/executor/execExpr.c

@@ -2646,6 +2646,14 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				break;
 			}
 
+		case T_RownumExpr:
+			{
+				/* Oracle ROWNUM pseudocolumn */
+				scratch.opcode = EEOP_ROWNUM;
+				ExprEvalPushStep(state, &scratch);
+				break;
+			}
+
 		case T_ReturningExpr:
 			{
 				ReturningExpr *rexpr = (ReturningExpr *) node;

src/backend/executor/execExprInterp.c

@@ -552,6 +552,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 		&&CASE_EEOP_SQLVALUEFUNCTION,
 		&&CASE_EEOP_CURRENTOFEXPR,
 		&&CASE_EEOP_NEXTVALUEEXPR,
+		&&CASE_EEOP_ROWNUM,
 		&&CASE_EEOP_RETURNINGEXPR,
 		&&CASE_EEOP_ARRAYEXPR,
 		&&CASE_EEOP_ARRAYCOERCE,
@@ -1593,6 +1594,18 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			EEO_NEXT();
 		}
 
+		EEO_CASE(EEOP_ROWNUM)
+		{
+			/*
+			 * Oracle ROWNUM pseudocolumn: return the current row number.
+			 * The row number is incremented by the executor for each row
+			 * emitted.
+			 */
+			ExecEvalRownum(state, op);
+
+			EEO_NEXT();
+		}
+
 		EEO_CASE(EEOP_RETURNINGEXPR)
 		{
 			/*
@@ -3322,6 +3335,41 @@ ExecEvalNextValueExpr(ExprState *state, ExprEvalStep *op)
 	*op->resnull = false;
 }
 
+/*
+ * Evaluate Oracle ROWNUM pseudocolumn.
+ *
+ * Returns the current row number from the executor state. The row number
+ * is incremented for each row emitted during query execution.
+ *
+ * ROWNUM starts at 1 and increments before any ORDER BY is applied.
+ */
+void
+ExecEvalRownum(ExprState *state, ExprEvalStep *op)
+{
+	PlanState  *planstate;
+	EState	   *estate = NULL;
+
+	/* Safely get the EState from the parent PlanState */
+	if (state && state->parent)
+	{
+		planstate = state->parent;
+		if (planstate)
+			estate = planstate->state;
+	}
+
+	/*
+	 * Return current row number as INT8 (bigint).
+	 * The counter starts at 1 and is incremented before each ExecProcNode call.
+	 * For standalone expressions without an estate, default to 1.
+	 */
+	if (estate)
+		*op->resvalue = Int64GetDatum(estate->es_rownum);
+	else
+		*op->resvalue = Int64GetDatum(1);
+
+	*op->resnull = false;
+}
+
 /*
  * Evaluate NullTest / IS NULL for rows.
  */

src/backend/executor/execUtils.c

@@ -162,6 +162,9 @@ CreateExecutorState(void)
 	estate->es_parallel_workers_to_launch = 0;
 	estate->es_parallel_workers_launched = 0;
 
+	/* Oracle ROWNUM support: initialize row counter to 0 */
+	estate->es_rownum = 0;
+
 	estate->es_jit_flags = 0;
 	estate->es_jit = NULL;
 

src/backend/nodes/nodeFuncs.c

@@ -291,6 +291,10 @@ exprType(const Node *expr)
 		case T_NextValueExpr:
 			type = ((const NextValueExpr *) expr)->typeId;
 			break;
+		case T_RownumExpr:
+			/* ROWNUM returns a numeric value */
+			type = INT8OID;
+			break;
 		case T_InferenceElem:
 			{
 				const InferenceElem *n = (const InferenceElem *) expr;
@@ -1072,6 +1076,10 @@ exprCollation(const Node *expr)
 			/* NextValueExpr's result is an integer type ... */
 			coll = InvalidOid;	/* ... so it has no collation */
 			break;
+		case T_RownumExpr:
+			/* RownumExpr's result is an integer type ... */
+			coll = InvalidOid;	/* ... so it has no collation */
+			break;
 		case T_InferenceElem:
 			coll = exprCollation((Node *) ((const InferenceElem *) expr)->expr);
 			break;
@@ -1329,6 +1337,10 @@ exprSetCollation(Node *expr, Oid collation)
 			/* NextValueExpr's result is an integer type ... */
 			Assert(!OidIsValid(collation)); /* ... so never set a collation */
 			break;
+		case T_RownumExpr:
+			/* RownumExpr's result is an integer type ... */
+			Assert(!OidIsValid(collation)); /* ... so never set a collation */
+			break;
 		default:
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(expr));
 			break;
@@ -2164,6 +2176,7 @@ expression_tree_walker_impl(Node *node,
 		case T_SetToDefault:
 		case T_CurrentOfExpr:
 		case T_NextValueExpr:
+		case T_RownumExpr:
 		case T_RangeTblRef:
 		case T_SortGroupClause:
 		case T_CTESearchClause:
@@ -3045,6 +3058,7 @@ expression_tree_mutator_impl(Node *node,
 		case T_SetToDefault:
 		case T_CurrentOfExpr:
 		case T_NextValueExpr:
+		case T_RownumExpr:
 		case T_RangeTblRef:
 		case T_SortGroupClause:
 		case T_CTESearchClause: