plan: forbid limit to push across selection.

plan/match_property.go

@@ -231,10 +231,7 @@ func (p *Selection) matchProperty(prop *requiredProperty, childPlanInfo ...*phys
 		res.count = uint64(float64(res.count) * selectionFactor)
 		return res
 	}
-	np := *p
-	np.SetChildren(childPlanInfo[0].p)
-	count := uint64(float64(childPlanInfo[0].count) * selectionFactor)
-	return &physicalPlanInfo{p: &np, cost: childPlanInfo[0].cost, count: count}
+	return childPlanInfo[0]
 }
 
 // matchProperty implements PhysicalPlan matchProperty interface.

plan/physical_plan_builder.go

@@ -807,20 +807,21 @@ func (p *Selection) convert2PhysicalPlan(prop *requiredProperty) (*physicalPlanI
 			info = infoEnforce
 		}
 	}
-	if ds, ok := p.children[0].(*DataSource); !ok {
-		info = p.matchProperty(prop, info)
-	} else {
-		client := p.ctx.GetClient()
-		memDB := infoschema.IsMemoryDB(ds.DBName.L)
-		isDistReq := !memDB && client != nil && client.SupportRequestType(kv.ReqTypeSelect, 0)
-		if !isDistReq {
-			info = p.matchProperty(prop, info)
-		}
-	}
+	info = p.matchProperty(prop, info)
 	p.storePlanInfo(prop, info)
 	return info, nil
 }
 
+func (p *Selection) appendSelToInfo(info *physicalPlanInfo) *physicalPlanInfo {
+	np := *p
+	np.SetChildren(info.p)
+	return &physicalPlanInfo{
+		p:     &np,
+		cost:  info.cost,
+		count: uint64(float64(info.count) * selectionFactor),
+	}
+}
+
 func (p *Selection) convert2PhysicalPlanPushOrder(prop *requiredProperty) (*physicalPlanInfo, error) {
 	child := p.children[0].(LogicalPlan)
 	limit := prop.limit
@@ -838,7 +839,17 @@ func (p *Selection) convert2PhysicalPlanPushOrder(prop *requiredProperty) (*phys
 				info = enforceProperty(&requiredProperty{limit: limit}, info)
 			}
 		} else {
-			info = enforceProperty(&requiredProperty{limit: limit}, info)
+			info = enforceProperty(&requiredProperty{limit: limit}, p.appendSelToInfo(info))
+		}
+	} else if ds, ok := p.children[0].(*DataSource); !ok {
+		// TODO: It's tooooooo tricky here, we will remove all these logic after implementing DAG push down.
+		info = p.appendSelToInfo(info)
+	} else {
+		client := p.ctx.GetClient()
+		memDB := infoschema.IsMemoryDB(ds.DBName.L)
+		isDistReq := !memDB && client != nil && client.SupportRequestType(kv.ReqTypeSelect, 0)
+		if !isDistReq {
+			info = p.appendSelToInfo(info)
 		}
 	}
 	return info, nil
@@ -855,6 +866,8 @@ func (p *Selection) convert2PhysicalPlanEnforce(prop *requiredProperty) (*physic
 	if prop.limit != nil && len(prop.props) > 0 {
 		if t, ok := info.p.(physicalDistSQLPlan); ok {
 			t.addTopN(p.ctx, prop)
+		} else if _, ok := info.p.(*Selection); !ok {
+			info = p.appendSelToInfo(info)
 		}
 		info = enforceProperty(prop, info)
 	} else if len(prop.props) != 0 {

plan/physical_plan_test.go

@@ -417,6 +417,14 @@ func (s *testPlanSuite) TestCBO(c *C) {
 			sql:  "select sum(a.b), sum(b.b) from t a join t b on a.c = b.c group by a.d order by a.d",
 			best: "LeftHashJoin{Table(t)->Table(t)}(a.c,b.c)->HashAgg->Sort->Trim",
 		},
+		{
+			sql:  "select * from t t1 left outer join t t2 on true where least(1,2,3,t1.a,t2.b) > 0 order by t2.a limit 10",
+			best: "LeftHashJoin{Table(t)->Table(t)}->Selection->Sort + Limit(10) + Offset(0)",
+		},
+		{
+			sql:  "select * from t t1 left outer join t t2 on true where least(1,2,3,t1.a,t2.b) > 0 limit 10",
+			best: "LeftHashJoin{Table(t)->Table(t)}->Selection->Limit",
+		},
 		{
 			sql:  "select count(*) from t where concat(a,b) = 'abc' group by c",
 			best: "Index(t.c_d_e)[[<nil>,+inf]]->Selection->StreamAgg",

plan/plan.go

@@ -130,6 +130,19 @@ func (p *requiredProperty) getHashKey() ([]byte, error) {
 	return bytes, errors.Trace(err)
 }
 
+// String implements fmt.Stringer interface. Just for test.
+func (p *requiredProperty) String() string {
+	ret := "Prop{"
+	for _, colProp := range p.props {
+		ret += fmt.Sprintf("col: %s, desc %v, ", colProp.col, colProp.desc)
+	}
+	ret += fmt.Sprintf("}, Len: %d", p.sortKeyLen)
+	if p.limit != nil {
+		ret += fmt.Sprintf(", Limit: %d,%d", p.limit.Offset, p.limit.Count)
+	}
+	return ret
+}
+
 type physicalPlanInfo struct {
 	p     PhysicalPlan
 	cost  float64