executor: fix big_unsigned_int union decimal sign bug (#7202)

executor/executor_test.go

@@ -2946,4 +2946,16 @@ func (s *testSuite) TestUnionAutoSignedCast(c *C) {
 		Check(testkit.Rows("1 -1", "2 1"))
 	tk.MustQuery("select id, v from t4 union select id, v from t3 order by id").
 		Check(testkit.Rows("1 0", "2 1"))
+
+	tk.MustExec("drop table if exists t5,t6,t7")
+	tk.MustExec("create table t5 (id int, v bigint unsigned)")
+	tk.MustExec("create table t6 (id int, v decimal)")
+	tk.MustExec("create table t7 (id int, v bigint)")
+	tk.MustExec("insert into t5 values (1, 1)")
+	tk.MustExec("insert into t6 values (2, -1)")
+	tk.MustExec("insert into t7 values (3, -1)")
+	tk.MustQuery("select id, v from t5 union select id, v from t6 order by id").
+		Check(testkit.Rows("1 1", "2 -1"))
+	tk.MustQuery("select id, v from t5 union select id, v from t7 union select id, v from t6 order by id").
+		Check(testkit.Rows("1 1", "2 -1", "3 -1"))
 }

plan/logical_plan_builder.go

@@ -615,8 +615,8 @@ func (b *planBuilder) buildDistinct(child LogicalPlan, length int) LogicalPlan {
 func unionJoinFieldType(a, b *types.FieldType) *types.FieldType {
 	resultTp := types.NewFieldType(types.MergeFieldType(a.Tp, b.Tp))
 	// This logic will be intelligible when it is associated with the buildProjection4Union logic.
-	if a.Tp == mysql.TypeNewDecimal {
-		// The decimal type will be unsigned only when all the decimals to be united are unsigned.
+	if resultTp.Tp == mysql.TypeNewDecimal {
+		// The decimal result type will be unsigned only when all the decimals to be united are unsigned.
 		resultTp.Flag &= b.Flag & mysql.UnsignedFlag
 	} else {
 		// Non-decimal results will be unsigned when the first SQL statement result in the union is unsigned.