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 CostHashSequenceProject and
CPhysicalSequenceProject have identical costs.

Fix by using max(ulSortCols, 1) so the window function evaluation
cost is never zero.

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());