plan: fix a bug when applying top-n push down rule. (#6187)

plan/physical_plan_test.go

@@ -184,6 +184,10 @@ func (s *testPlanSuite) TestDAGPlanBuilderSimpleCase(c *C) {
 			sql:  "select * from t use index(e_d_c_str_prefix) where t.e_str = b'1110000'",
 			best: "IndexLookUp(Index(t.e_d_c_str_prefix)[[p,p]], Table(t))",
 		},
+		{
+			sql:  "select * from (select * from t use index() order by b) t left join t t1 on t.a=t1.a limit 10",
+			best: "IndexJoin{TableReader(Table(t)->TopN([test.t.b],0,10))->TopN([test.t.b],0,10)->TableReader(Table(t))}(test.t.a,t1.a)->Limit",
+		},
 	}
 	for _, tt := range tests {
 		comment := Commentf("for %s", tt.sql)

plan/topn_push_down.go

@@ -61,6 +61,8 @@ func (ls *LogicalSort) pushDownTopN(topN *LogicalTopN) LogicalPlan {
 	} else if topN.isLimit() {
 		topN.ByItems = ls.ByItems
 		// If a Limit is pushed down, the LogicalSort should be converted to topN and be pushed again.
+		// Since the limit is combined with the sort operator. It's not a partial one any more.
+		topN.partial = false
 		return ls.children[0].pushDownTopN(topN)
 	}
 	// If a TopN is pushed down, this sort is useless.