sqlsmith: added support for randomly using NULLS FIRST or NULLS LAST

Prior to this commit, SQLSmith did not produce queries that used the
non-default nulls ordering. This commit adds support for sometimes using
NULLS FIRST or NULLS LAST with ordering clauses.

Release note: None

pkg/internal/sqlsmith/relational.go

@@ -515,6 +515,16 @@ func (s *Smither) randDirection() tree.Direction {
 	return orderDirections[s.rnd.Intn(len(orderDirections))]
 }
 
+var nullsOrders = []tree.NullsOrder{
+	tree.DefaultNullsOrder,
+	tree.NullsFirst,
+	tree.NullsLast,
+}
+
+func (s *Smither) randNullsOrder() tree.NullsOrder {
+	return nullsOrders[s.rnd.Intn(len(nullsOrders))]
+}
+
 var nullabilities = []tree.Nullability{
 	tree.NotNull,
 	tree.Null,
@@ -772,7 +782,8 @@ func makeSelect(s *Smither) (tree.Statement, bool) {
 			}
 			order[i] = &tree.Order{
 				Expr:       expr,
-				NullsOrder: tree.NullsFirst,
+				Direction:  s.randDirection(),
+				NullsOrder: s.randNullsOrder(),
 			}
 		}
 		stmt = &tree.Select{
@@ -1277,8 +1288,9 @@ func (s *Smither) makeOrderBy(refs colRefs) tree.OrderBy {
 			continue
 		}
 		ob = append(ob, &tree.Order{
-			Expr:      ref.item,
-			Direction: s.randDirection(),
+			Expr:       ref.item,
+			Direction:  s.randDirection(),
+			NullsOrder: s.randNullsOrder(),
 		})
 	}
 	return ob

pkg/internal/sqlsmith/scalar.go

@@ -462,7 +462,9 @@ func makeFunc(s *Smither, ctx Context, typ *types.T, refs colRefs) (tree.TypedEx
 			orderTypes []*types.T
 		)
 		addOrdCol := func(expr tree.Expr, typ *types.T) {
-			order = append(order, &tree.Order{Expr: expr, Direction: s.randDirection()})
+			order = append(order, &tree.Order{
+				Expr: expr, Direction: s.randDirection(), NullsOrder: s.randNullsOrder(),
+			})
 			orderTypes = append(orderTypes, typ)
 		}
 		s.sample(len(refs)-len(parts), 2, func(i int) {
@@ -520,7 +522,9 @@ func makeFunc(s *Smither, ctx Context, typ *types.T, refs colRefs) (tree.TypedEx
 			funcExpr.AggType = tree.GeneralAgg
 			funcExpr.OrderBy = make(tree.OrderBy, len(args))
 			for i := range funcExpr.OrderBy {
-				funcExpr.OrderBy[i] = &tree.Order{Expr: args[i], Direction: s.randDirection()}
+				funcExpr.OrderBy[i] = &tree.Order{
+					Expr: args[i], Direction: s.randDirection(), NullsOrder: s.randNullsOrder(),
+				}
 			}
 		}
 	}