ORCA: Fix window function cost model producing zero local cost when no ORDER BY

When a window function has no ORDER BY clause (empty Order Spec),
ulSortCols is 0, causing the local cost of SequenceProject operators
to be zero. This makes CPhysicalParallelSequenceProject and
CPhysicalSequenceProject have identical costs, and the optimizer
falls back to insertion order to break the tie, always preferring
the non-parallel variant.

Fix by using max(ulSortCols, 1) so the window function evaluation
cost is never zero, allowing the parallel operator to demonstrate
its cost advantage through the parallelization divisor.

Author: yjhjstz

src/backend/gporca/libgpdbcost/src/CCostModelGPDB.cpp

@@ -1622,21 +1622,24 @@ CCostModelGPDB::CostSequenceProject(CMemoryPool *mp, CExpressionHandle &exprhdl,
 	}
 
 	// we process (sorted window of) input tuples to compute window function values
+	// Use at least 1 to account for the base cost of evaluating window functions
+	// even when there are no sort columns (e.g., no ORDER BY in the window spec)
+	ULONG ulCostFactor = std::max(ulSortCols, (ULONG) 1);
 	CCost costLocal =
-		CCost(pci->NumRebinds() * (ulSortCols * num_rows_outer * dWidthOuter *
+		CCost(pci->NumRebinds() * (ulCostFactor * num_rows_outer * dWidthOuter *
 								   dTupDefaultProcCostUnit));
 	CCost costChild =
 		CostChildren(mp, exprhdl, pci, pcmgpdb->GetCostModelParams());
-	
+
 	return costLocal + costChild;
 }
 
 //---------------------------------------------------------------------------
 //	@function:
-//		CCostModelGPDB::CostSequenceProject
+//		CCostModelGPDB::CostHashSequenceProject
 //
 //	@doc:
-//		Cost of sequence project
+//		Cost of hash sequence project
 //
 //---------------------------------------------------------------------------
 CCost
@@ -1675,8 +1678,11 @@ CCostModelGPDB::CostHashSequenceProject(CMemoryPool *mp, CExpressionHandle &expr
 	}
 
 	// we process (sorted window of) input tuples to compute window function values
+	// Use at least 1 to account for the base cost of evaluating window functions
+	// even when there are no sort columns (e.g., no ORDER BY in the window spec)
+	ULONG ulCostFactor = std::max(ulSortCols, (ULONG) 1);
 	CCost costLocal =
-		CCost(pci->NumRebinds() * (ulSortCols * num_rows_outer * dWidthOuter *
+		CCost(pci->NumRebinds() * (ulCostFactor * num_rows_outer * dWidthOuter *
 								   dTupDefaultProcCostUnit));
 	CCost costChild =
 		CostChildren(mp, exprhdl, pci, pcmgpdb->GetCostModelParams());