plan: fix predicate push down for UnionScan (#7726)

executor/union_scan_test.go

@@ -78,3 +78,16 @@ func (s *testSuite) TestDirtyTransaction(c *C) {
 	tk.MustQuery("select * from t use index(idx) where c > 1 and d = 4").Check(testkit.Rows("1 2 3 4"))
 	tk.MustExec("commit")
 }
+
+func (s *testSuite) TestUnionScanWithCastCondition(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("create table ta (a varchar(20))")
+	tk.MustExec("insert ta values ('1'), ('2')")
+	tk.MustExec("create table tb (a varchar(20))")
+	tk.MustExec("begin")
+	tk.MustQuery("select * from ta where a = 1").Check(testkit.Rows("1"))
+	tk.MustExec("insert tb values ('0')")
+	tk.MustQuery("select * from ta where a = 1").Check(testkit.Rows("1"))
+	tk.MustExec("rollback")
+}

plan/predicate_push_down.go

@@ -62,12 +62,13 @@ func (p *LogicalSelection) PredicatePushDown(predicates []expression.Expression)
 
 // PredicatePushDown implements LogicalPlan PredicatePushDown interface.
 func (p *LogicalUnionScan) PredicatePushDown(predicates []expression.Expression) ([]expression.Expression, LogicalPlan) {
-	p.children[0].PredicatePushDown(predicates)
+	retainedPredicates, _ := p.children[0].PredicatePushDown(predicates)
 	p.conditions = make([]expression.Expression, 0, len(predicates))
 	for _, cond := range predicates {
 		p.conditions = append(p.conditions, cond.Clone())
 	}
-	return nil, p
+	// The conditions in UnionScan is only used for added rows, so parent Selection should not be removed.
+	return retainedPredicates, p
 }
 
 // PredicatePushDown implements LogicalPlan PredicatePushDown interface.