Fix: cursor pagination with null in columns and support for order by with nulls first or last

Author: dosco

core/internal/qcode/qcode.go

@@ -898,12 +898,32 @@ func (co *Compiler) addSeekPredicate(sel *Select) {
 			switch {
 			case i > 0 && n != i:
 				f.Op = OpEquals
-			case ob.Order == OrderDesc:
+			case ob.Order == OrderDesc ||
+				ob.Order == OrderDescNullsFirst || ob.Order == OrderDescNullsLast:
 				f.Op = OpLesserThan
+			case ob.Order == OrderAsc ||
+				ob.Order == OrderAscNullsLast || ob.Order == OrderAscNullsFirst:
+				f.Op = OpGreaterThan
 			default:
 				f.Op = OpGreaterThan
 			}
 
+			// could be null needs to be handled
+			if !ob.Col.NotNull {
+				isnull1 := newExpOp(OpIsNull)
+				isnull1.Left.Table = "__cur"
+				isnull1.Left.Col = ob.Col
+
+				isnull2 := newExpOp(OpIsNull)
+				isnull2.Left.Col = ob.Col
+
+				or1 := newExpOp(OpOr)
+				or1.Children = append(or.Children, isnull1, isnull2, f)
+
+				// now that f is added to the above or1 we can set f to or1
+				f = or1
+			}
+
 			if and != nil {
 				and.Children = append(and.Children, f)
 			} else {