feat: ROWNUM Implementation

src/backend/executor/execExpr.c

@@ -377,6 +377,41 @@ ExecInitExprList(List *nodes, PlanState *parent)
  * Caution: before PG v10, the targetList was a list of ExprStates; now it
  * should be the planner-created targetlist, since we do the compilation here.
  */
+
+/*
+ * Helper function to check if an expression contains ROWNUM
+ */
+static bool
+expression_contains_rownum_walker(Node *node, void *context)
+{
+	if (node == NULL)
+		return false;
+
+	if (IsA(node, RownumExpr))
+		return true;
+
+	return expression_tree_walker(node, expression_contains_rownum_walker, context);
+}
+
+/*
+ * Check if a target list contains ROWNUM expressions
+ */
+static bool
+targetlist_contains_rownum(List *targetList)
+{
+	ListCell   *lc;
+
+	foreach(lc, targetList)
+	{
+		TargetEntry *tle = lfirst_node(TargetEntry, lc);
+
+		if (expression_contains_rownum_walker((Node *) tle->expr, NULL))
+			return true;
+	}
+
+	return false;
+}
+
 ProjectionInfo *
 ExecBuildProjectionInfo(List *targetList,
 						ExprContext *econtext,
@@ -519,6 +554,13 @@ ExecBuildProjectionInfo(List *targetList,
 
 	ExecReadyExpr(state);
 
+	/*
+	 * Check if the target list contains ROWNUM expressions.
+	 * If so, we need to materialize the result tuple to preserve the
+	 * ROWNUM values and prevent re-evaluation in outer queries.
+	 */
+	projInfo->pi_needsMaterialization = targetlist_contains_rownum(targetList);
+
 	return projInfo;
 }
 
@@ -2646,6 +2688,14 @@ 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;
+	int64		rownum_value = 1;  /* default */
+
+	/* Safely get the PlanState and EState */
+	if (state && state->parent)
+	{
+		planstate = state->parent;
+		if (planstate)
+			estate = planstate->state;
+	}
+
+	/*
+	 * Use the estate-level ROWNUM counter.
+	 * When ROWNUM appears in a SELECT list, materialization (handled in
+	 * ExecScanExtended) ensures the value is captured and not re-evaluated.
+	 */
+	if (estate && estate->es_rownum > 0)
+		rownum_value = estate->es_rownum;
+
+	*op->resvalue = Int64GetDatum(rownum_value);
+	*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 */
+	estate->es_rownum = 0;
+
 	estate->es_jit_flags = 0;
 	estate->es_jit = NULL;
 

src/backend/executor/nodeAppend.c

@@ -290,6 +290,9 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 	/* For parallel query, this will be overridden later. */
 	appendstate->choose_next_subplan = choose_next_subplan_locally;
 
+	/* Copy is_union flag for ROWNUM reset handling (Oracle compatibility) */
+	appendstate->as_is_union = node->is_union;
+
 	return appendstate;
 }
 
@@ -386,6 +389,13 @@ ExecAppend(PlanState *pstate)
 		/* choose new sync subplan; if no sync/async subplans, we're done */
 		if (!node->choose_next_subplan(node) && node->as_nasyncremain == 0)
 			return ExecClearTuple(node->ps.ps_ResultTupleSlot);
+
+		/*
+		 * For UNION queries, reset ROWNUM when switching to a new branch.
+		 * In Oracle, each UNION branch has its own independent ROWNUM counter.
+		 */
+		if (node->as_is_union)
+			node->ps.state->es_rownum = 0;
 	}
 }
 

src/backend/executor/nodeMergeAppend.c

@@ -202,6 +202,9 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 	 */
 	mergestate->ms_initialized = false;
 
+	/* Copy is_union flag for ROWNUM reset handling (Oracle compatibility) */
+	mergestate->ms_is_union = node->is_union;
+
 	return mergestate;
 }
 
@@ -238,10 +241,22 @@ ExecMergeAppend(PlanState *pstate)
 		/*
 		 * First time through: pull the first tuple from each valid subplan,
 		 * and set up the heap.
+		 *
+		 * For UNION queries, reset ROWNUM before each subplan starts.
+		 * This ensures each UNION branch has independent ROWNUM counting
+		 * (Oracle compatibility).
 		 */
 		i = -1;
 		while ((i = bms_next_member(node->ms_valid_subplans, i)) >= 0)
 		{
+			/*
+			 * For UNION, reset ROWNUM before each branch executes.
+			 * Each child's Sort will buffer all tuples from its scan,
+			 * so ROWNUM needs to start fresh for each branch.
+			 */
+			if (node->ms_is_union)
+				node->ps.state->es_rownum = 0;
+
 			node->ms_slots[i] = ExecProcNode(node->mergeplans[i]);
 			if (!TupIsNull(node->ms_slots[i]))
 				binaryheap_add_unordered(node->ms_heap, Int32GetDatum(i));

src/backend/executor/nodeResult.c

@@ -132,7 +132,18 @@ ExecResult(PlanState *pstate)
 		}
 
 		/* form the result tuple using ExecProject(), and return it */
-		return ExecProject(node->ps.ps_ProjInfo);
+		TupleTableSlot *result = ExecProject(node->ps.ps_ProjInfo);
+
+		/*
+		 * If the projection contains ROWNUM expressions, materialize
+		 * the virtual tuple to preserve the ROWNUM values as constants.
+		 */
+		if (node->ps.ps_ProjInfo && node->ps.ps_ProjInfo->pi_needsMaterialization)
+		{
+			ExecMaterializeSlot(result);
+		}
+
+		return result;
 	}
 
 	return NULL;

src/backend/executor/nodeSubplan.c

@@ -66,6 +66,7 @@ ExecSubPlan(SubPlanState *node,
 	SubPlan    *subplan = node->subplan;
 	EState	   *estate = node->planstate->state;
 	ScanDirection dir = estate->es_direction;
+	int64		save_rownum = estate->es_rownum;
 	Datum		retval;
 
 	CHECK_FOR_INTERRUPTS();
@@ -82,14 +83,22 @@ ExecSubPlan(SubPlanState *node,
 	/* Force forward-scan mode for evaluation */
 	estate->es_direction = ForwardScanDirection;
 
+	/*
+	 * Reset ROWNUM counter for Oracle compatibility.
+	 * Each correlated subquery invocation should start with ROWNUM=0,
+	 * matching Oracle's behavior.
+	 */
+	estate->es_rownum = 0;
+
 	/* Select appropriate evaluation strategy */
 	if (subplan->useHashTable)
 		retval = ExecHashSubPlan(node, econtext, isNull);
 	else
 		retval = ExecScanSubPlan(node, econtext, isNull);
 
-	/* restore scan direction */
+	/* restore scan direction and ROWNUM counter */
 	estate->es_direction = dir;
+	estate->es_rownum = save_rownum;
 
 	return retval;
 }
@@ -262,6 +271,12 @@ ExecScanSubPlan(SubPlanState *node,
 	/* with that done, we can reset the subplan */
 	ExecReScan(planstate);
 
+	/*
+	 * Reset ROWNUM counter for Oracle compatibility.
+	 * This ensures correlated subqueries start fresh for each outer row.
+	 */
+	planstate->state->es_rownum = 0;
+
 	/*
 	 * For all sublink types except EXPR_SUBLINK and ARRAY_SUBLINK, the result
 	 * is boolean as are the results of the combining operators. We combine
@@ -1104,6 +1119,7 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	SubLinkType subLinkType = subplan->subLinkType;
 	EState	   *estate = planstate->state;
 	ScanDirection dir = estate->es_direction;
+	int64		save_rownum = estate->es_rownum;
 	MemoryContext oldcontext;
 	TupleTableSlot *slot;
 	ListCell   *l;
@@ -1124,6 +1140,12 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	 */
 	estate->es_direction = ForwardScanDirection;
 
+	/*
+	 * Reset ROWNUM counter for Oracle compatibility.
+	 * InitPlans should start with ROWNUM=0, matching Oracle's behavior.
+	 */
+	estate->es_rownum = 0;
+
 	/* Initialize ArrayBuildStateAny in caller's context, if needed */
 	if (subLinkType == ARRAY_SUBLINK)
 		astate = initArrayResultAny(subplan->firstColType,
@@ -1257,8 +1279,9 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 
 	MemoryContextSwitchTo(oldcontext);
 
-	/* restore scan direction */
+	/* restore scan direction and ROWNUM counter */
 	estate->es_direction = dir;
+	estate->es_rownum = save_rownum;
 }
 
 /*

src/backend/executor/nodeSubqueryscan.c

@@ -46,12 +46,26 @@ static TupleTableSlot *
 SubqueryNext(SubqueryScanState *node)
 {
 	TupleTableSlot *slot;
+	EState	   *estate = node->ss.ps.state;
+	int64		save_rownum = estate->es_rownum;
+
+	/*
+	 * For Oracle ROWNUM compatibility: each subquery maintains its own
+	 * local ROWNUM counter. Save the outer query's counter, swap in
+	 * this subquery's counter, execute the subplan, then restore.
+	 * This allows nested subqueries to have independent ROWNUM sequences.
+	 */
+	estate->es_rownum = node->sub_rownum;
 
 	/*
 	 * Get the next tuple from the sub-query.
 	 */
 	slot = ExecProcNode(node->subplan);
 
+	/* Update local counter and restore outer query's counter */
+	node->sub_rownum = estate->es_rownum;
+	estate->es_rownum = save_rownum;
+
 	/*
 	 * We just return the subplan's result slot, rather than expending extra
 	 * cycles for ExecCopySlot().  (Our own ScanTupleSlot is used only for
@@ -112,6 +126,7 @@ ExecInitSubqueryScan(SubqueryScan *node, EState *estate, int eflags)
 	subquerystate->ss.ps.plan = (Plan *) node;
 	subquerystate->ss.ps.state = estate;
 	subquerystate->ss.ps.ExecProcNode = ExecSubqueryScan;
+	subquerystate->sub_rownum = 0;
 
 	/*
 	 * Miscellaneous initialization
@@ -182,6 +197,12 @@ ExecEndSubqueryScan(SubqueryScanState *node)
 void
 ExecReScanSubqueryScan(SubqueryScanState *node)
 {
+	/*
+	 * Reset local ROWNUM counter for Oracle compatibility.
+	 * Each rescan starts with ROWNUM = 1.
+	 */
+	node->sub_rownum = 0;
+
 	ExecScanReScan(&node->ss);
 
 	/*