planner: don't push down topn to nil table plan side (pingcap#25214)

makemyownlife · Aug 18, 2021 · 1f8f3cc · 1f8f3cc
1 parent b2a1d21
commit 1f8f3cc
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Showing 2 changed files with 22 additions and 11 deletions.
diff --git a/planner/core/physical_plan_test.go b/planner/core/physical_plan_test.go
@@ -1543,6 +1543,20 @@ func (s *testPlanSuite) TestDAGPlanBuilderWindowParallel(c *C) {
 	s.doTestDAGPlanBuilderWindow(c, vars, input, output)
 }
 
+func (s *testPlanSuite) TestTopNPushDownEmpty(c *C) {
+	store, dom, err := newStoreWithBootstrap()
+	c.Assert(err, IsNil)
+	defer func() {
+		dom.Close()
+		store.Close()
+	}()
+	tk := testkit.NewTestKit(c, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int, b int, c int, index idx_a(a))")
+	tk.MustQuery("select extract(day_hour from 'ziy') as res from t order by res limit 1").Check(testkit.Rows())
+}
+
 func (s *testPlanSuite) doTestDAGPlanBuilderWindow(c *C, vars, input []string, output []struct {
 	SQL  string
 	Best string

diff --git a/planner/core/task.go b/planner/core/task.go
@@ -1203,14 +1203,6 @@ func (p *PhysicalTopN) canPushDown(storeTp kv.StoreType) bool {
 	return expression.CanExprsPushDown(p.ctx.GetSessionVars().StmtCtx, exprs, p.ctx.GetClient(), storeTp)
 }
 
-func (p *PhysicalTopN) allColsFromSchema(schema *expression.Schema) bool {
-	cols := make([]*expression.Column, 0, len(p.ByItems))
-	for _, item := range p.ByItems {
-		cols = append(cols, expression.ExtractColumns(item.Expr)...)
-	}
-	return len(schema.ColumnsIndices(cols)) > 0
-}
-
 // GetCost computes the cost of in memory sort.
 func (p *PhysicalSort) GetCost(count float64, schema *expression.Schema) float64 {
 	if count < 2.0 {
@@ -1271,11 +1263,16 @@ func (p *PhysicalTopN) getPushedDownTopN(childPlan PhysicalPlan) *PhysicalTopN {
 func (p *PhysicalTopN) attach2Task(tasks ...task) task {
 	t := tasks[0].copy()
 	inputCount := t.count()
-	if copTask, ok := t.(*copTask); ok && p.canPushDown(copTask.getStoreType()) && len(copTask.rootTaskConds) == 0 {
+	cols := make([]*expression.Column, 0, len(p.ByItems))
+	for _, item := range p.ByItems {
+		cols = append(cols, expression.ExtractColumns(item.Expr)...)
+	}
+	needPushDown := len(cols) > 0
+	if copTask, ok := t.(*copTask); ok && needPushDown && p.canPushDown(copTask.getStoreType()) && len(copTask.rootTaskConds) == 0 {
 		// If all columns in topN are from index plan, we push it to index plan, otherwise we finish the index plan and
 		// push it to table plan.
 		var pushedDownTopN *PhysicalTopN
-		if !copTask.indexPlanFinished && p.allColsFromSchema(copTask.indexPlan.Schema()) {
+		if !copTask.indexPlanFinished && len(copTask.indexPlan.Schema().ColumnsIndices(cols)) > 0 {
 			pushedDownTopN = p.getPushedDownTopN(copTask.indexPlan)
 			copTask.indexPlan = pushedDownTopN
 		} else {
@@ -1284,7 +1281,7 @@ func (p *PhysicalTopN) attach2Task(tasks ...task) task {
 			copTask.tablePlan = pushedDownTopN
 		}
 		copTask.addCost(pushedDownTopN.GetCost(inputCount, false))
-	} else if mppTask, ok := t.(*mppTask); ok && p.canPushDown(kv.TiFlash) {
+	} else if mppTask, ok := t.(*mppTask); ok && needPushDown && p.canPushDown(kv.TiFlash) {
 		pushedDownTopN := p.getPushedDownTopN(mppTask.p)
 		mppTask.p = pushedDownTopN
 	}