Optimizer: derive TopN from filter on row number

expression/util.go

@@ -177,6 +177,31 @@ func ExtractEquivalenceColumns(result [][]Expression, exprs []Expression) [][]Ex
 	return result
 }
 
+// FindUpperBound looks for column < constant or column <= constant and returns both the column
+// and constant. It return nil, 0 if the expression is not of this form.
+// It is used by derived Top N pattern and it is put here since it looks like
+// a general purpose routine. Similar routines can be added to find lower bound as well.
+func FindUpperBound(expr Expression) (*Column, int64) {
+	scalarFunction, scalarFunctionOk := expr.(*ScalarFunction)
+	if scalarFunctionOk {
+		args := scalarFunction.GetArgs()
+		if len(args) == 2 {
+			col, colOk := args[0].(*Column)
+			constant, constantOk := args[1].(*Constant)
+			if colOk && constantOk && (scalarFunction.FuncName.L == ast.LT || scalarFunction.FuncName.L == ast.LE) {
+				value, valueOk := constant.Value.GetValue().(int64)
+				if valueOk {
+					if scalarFunction.FuncName.L == ast.LT {
+						return col, value - 1
+					}
+					return col, value
+				}
+			}
+		}
+	}
+	return nil, 0
+}
+
 func extractEquivalenceColumns(result [][]Expression, expr Expression) [][]Expression {
 	switch v := expr.(type) {
 	case *ScalarFunction:

planner/core/BUILD.bazel

@@ -50,6 +50,7 @@ go_library(
         "rule_build_key_info.go",
         "rule_column_pruning.go",
         "rule_decorrelate.go",
+        "rule_derive_topn_from_window.go",
         "rule_eliminate_projection.go",
         "rule_generate_column_substitute.go",
         "rule_inject_extra_projection.go",
@@ -201,6 +202,7 @@ go_test(
         "point_get_plan_test.go",
         "prepare_test.go",
         "preprocess_test.go",
+        "rule_derive_topn_from_window_test.go",
         "rule_inject_extra_projection_test.go",
         "rule_join_reorder_dp_test.go",
         "rule_join_reorder_test.go",

planner/core/logical_plan_builder.go

@@ -1031,6 +1031,7 @@ func (b *PlanBuilder) coalesceCommonColumns(p *LogicalJoin, leftPlan, rightPlan
 
 func (b *PlanBuilder) buildSelection(ctx context.Context, p LogicalPlan, where ast.ExprNode, aggMapper map[*ast.AggregateFuncExpr]int) (LogicalPlan, error) {
 	b.optFlag |= flagPredicatePushDown
+	b.optFlag |= flagDeriveTopNFromWindow
 	if b.curClause != havingClause {
 		b.curClause = whereClause
 	}

planner/core/main_test.go

@@ -51,6 +51,7 @@ func TestMain(m *testing.M) {
 	testDataMap.LoadTestSuiteData("testdata", "flat_plan_suite")
 	testDataMap.LoadTestSuiteData("testdata", "binary_plan_suite")
 	testDataMap.LoadTestSuiteData("testdata", "json_plan_suite")
+	testDataMap.LoadTestSuiteData("testdata", "derive_topn_from_window")
 
 	indexMergeSuiteData = testDataMap["index_merge_suite"]
 	planSuiteUnexportedData = testDataMap["plan_suite_unexported"]
@@ -139,3 +140,7 @@ func GetIndexMergeSuiteData() testdata.TestData {
 func GetJSONPlanSuiteData() testdata.TestData {
 	return testDataMap["json_plan_suite"]
 }
+
+func GetDerivedTopNSuiteData() testdata.TestData {
+	return testDataMap["derive_topn_from_window"]
+}

planner/core/optimizer.go

@@ -73,6 +73,7 @@ const (
 	flagPartitionProcessor
 	flagCollectPredicateColumnsPoint
 	flagPushDownAgg
+	flagDeriveTopNFromWindow
 	flagPushDownTopN
 	flagSyncWaitStatsLoadPoint
 	flagJoinReOrder
@@ -95,6 +96,7 @@ var optRuleList = []logicalOptRule{
 	&partitionProcessor{},
 	&collectPredicateColumnsPoint{},
 	&aggregationPushDownSolver{},
+	&deriveTopNFromWindow{},
 	&pushDownTopNOptimizer{},
 	&syncWaitStatsLoadPoint{},
 	&joinReOrderSolver{},

planner/core/plan.go

@@ -277,6 +277,9 @@ type LogicalPlan interface {
 	// pushDownTopN will push down the topN or limit operator during logical optimization.
 	pushDownTopN(topN *LogicalTopN, opt *logicalOptimizeOp) LogicalPlan
 
+	// deriveTopN derives an implicit TopN from a filter on row_number window function..
+	deriveTopN(opt *logicalOptimizeOp) LogicalPlan
+
 	// recursiveDeriveStats derives statistic info between plans.
 	recursiveDeriveStats(colGroups [][]*expression.Column) (*property.StatsInfo, error)
 

planner/core/rule_derive_topn_from_window.go

@@ -0,0 +1,122 @@
+// Copyright 2023 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package core
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/pingcap/tidb/expression"
+	"github.com/pingcap/tidb/parser/ast"
+	"github.com/pingcap/tidb/planner/util"
+)
+
+// deriveTopNFromWindow pushes down the topN or limit. In the future we will remove the limit from `requiredProperty` in CBO phase.
+type deriveTopNFromWindow struct {
+}
+
+func appendDerivedTopNTrace(topN LogicalPlan, opt *logicalOptimizeOp) {
+	child := topN.Children()[0]
+	action := func() string {
+		return fmt.Sprintf("%v_%v top N added below  %v_%v ", topN.TP(), topN.ID(), child.TP(), child.ID())
+	}
+	reason := func() string {
+		return fmt.Sprintf("%v filter on row number", topN.TP())
+	}
+	opt.appendStepToCurrent(topN.ID(), topN.TP(), reason, action)
+}
+
+/*
+		Check the following pattern of filter over row number window function:
+	  - Filter is simple condition of row_number < value or row_number <= value
+	  - The window function is a simple row number
+	  - With default frame: rows between current row and current row. Check is not necessary since
+	    current row is only frame applicable to row number
+	  - No partition
+	  - Child is a data source.
+*/
+func windowIsTopN(p *LogicalSelection) (bool, uint64) {
+	// Check if child is window function.
+	child, isLogicalWindow := p.Children()[0].(*LogicalWindow)
+	if !isLogicalWindow {
+		return false, 0
+	}
+
+	if len(p.Conditions) != 1 {
+		return false, 0
+	}
+
+	// Check if filter is column < constant or column <= constant. If it is in this form find column and constant.
+	column, limitValue := expression.FindUpperBound(p.Conditions[0])
+	if column == nil || limitValue <= 0 {
+		return false, 0
+	}
+
+	// Check if filter on window function
+	windowColumns := child.GetWindowResultColumns()
+	if len(windowColumns) != 1 || !(column.Equal(p.ctx, windowColumns[0])) {
+		return false, 0
+	}
+
+	grandChild := child.Children()[0]
+	_, isDataSource := grandChild.(*DataSource)
+	if !isDataSource {
+		return false, 0
+	}
+	if len(child.WindowFuncDescs) == 1 && child.WindowFuncDescs[0].Name == "row_number" && len(child.PartitionBy) == 0 &&
+		child.Frame.Type == ast.Rows && child.Frame.Start.Type == ast.CurrentRow && child.Frame.End.Type == ast.CurrentRow {
+		return true, uint64(limitValue)
+	}
+	return false, 0
+}
+
+func (s *deriveTopNFromWindow) optimize(_ context.Context, p LogicalPlan, opt *logicalOptimizeOp) (LogicalPlan, error) {
+	return p.deriveTopN(opt), nil
+}
+
+func (s *baseLogicalPlan) deriveTopN(opt *logicalOptimizeOp) LogicalPlan {
+	p := s.self
+	for i, child := range p.Children() {
+		newChild := child.deriveTopN(opt)
+		p.SetChild(i, newChild)
+	}
+	return p
+}
+
+func (s *LogicalSelection) deriveTopN(opt *logicalOptimizeOp) LogicalPlan {
+	p := s.self.(*LogicalSelection)
+	windowIsTopN, limitValue := windowIsTopN(p)
+	if windowIsTopN {
+		child := p.Children()[0].(*LogicalWindow)
+		grandChild := child.Children()[0].(*DataSource)
+		// Build order by for derived Limit
+		byItems := make([]*util.ByItems, 0, len(child.OrderBy))
+		for _, col := range child.OrderBy {
+			byItems = append(byItems, &util.ByItems{Expr: col.Col, Desc: col.Desc})
+		}
+		// Build derived Limit
+		derivedTopN := LogicalTopN{Count: limitValue, ByItems: byItems}.Init(grandChild.ctx, grandChild.blockOffset)
+		derivedTopN.SetChildren(grandChild)
+		/* return datasource->topN->window */
+		child.SetChildren(derivedTopN)
+		appendDerivedTopNTrace(child, opt)
+		return child
+	}
+	return p
+}
+
+func (*deriveTopNFromWindow) name() string {
+	return "derive_topn_from_window"
+}

planner/core/rule_derive_topn_from_window_test.go

@@ -0,0 +1,85 @@
+// Copyright 2023 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package core_test
+
+import (
+	"testing"
+
+	plannercore "github.com/pingcap/tidb/planner/core"
+	"github.com/pingcap/tidb/testkit"
+	"github.com/pingcap/tidb/testkit/testdata"
+)
+
+// Rule should bot be applied
+func TestPushDerivedTopnNegative(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists employee")
+	tk.MustExec("create table t(a int, b int)")
+	tk.MustExec("insert into t values(1,1)")
+	tk.MustExec("insert into t values(2,1)")
+	tk.MustExec("insert into t values(3,2)")
+	tk.MustExec("insert into t values(4,2)")
+	tk.MustExec("insert into t values(5,2)")
+	var input Input
+	var output []struct {
+		SQL  string
+		Plan []string
+	}
+	suiteData := plannercore.GetDerivedTopNSuiteData()
+	suiteData.LoadTestCases(t, &input, &output)
+	for i, sql := range input {
+		plan := tk.MustQuery("explain format = 'brief' " + sql)
+		testdata.OnRecord(func() {
+			output[i].SQL = sql
+			output[i].Plan = testdata.ConvertRowsToStrings(plan.Rows())
+		})
+		plan.Check(testkit.Rows(output[i].Plan...))
+	}
+}
+
+// Rule should be applied
+func TestPushDerivedTopnPositive(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists employee")
+	tk.MustExec("create table t(a int, b int)")
+	tk.MustExec("insert into t values(1,1)")
+	tk.MustExec("insert into t values(2,1)")
+	tk.MustExec("insert into t values(3,2)")
+	tk.MustExec("insert into t values(4,2)")
+	tk.MustExec("insert into t values(5,2)")
+	var input Input
+	var output []struct {
+		SQL  string
+		Plan []string
+		Res  []string
+	}
+	suiteData := plannercore.GetDerivedTopNSuiteData()
+	suiteData.LoadTestCases(t, &input, &output)
+	for i, sql := range input {
+		plan := tk.MustQuery("explain format = 'brief' " + sql)
+		res := tk.MustQuery(sql)
+		testdata.OnRecord(func() {
+			output[i].SQL = sql
+			output[i].Plan = testdata.ConvertRowsToStrings(plan.Rows())
+			output[i].Res = testdata.ConvertRowsToStrings(res.Rows())
+		})
+		plan.Check(testkit.Rows(output[i].Plan...))
+		res.Check(testkit.Rows(output[i].Res...))
+	}
+}

planner/core/testdata/derive_topn_from_window_in.json

@@ -0,0 +1,29 @@
+[
+  {
+    "name": "TestPushDerivedTopnNegative",
+    "cases":[
+      "select row_number() over (partition by a) from t -- pattern missing filter on row number",
+      "select * from (select rank() over () as rank_order from t) DT where rank_order <= 3 -- rank not supported in pattern yet",
+      "select * from (select row_number() over (partition by a) as rownumber from t) DT where rownumber <= 3 -- pattern is applicable but partition by is not supported yet",
+      "select * from (select row_number() over () as rownumber1, row_number() over (partition by a) as rownumber2 from t) DT where rownumber1 <= 3 -- pattern not applicable with multiple window functions",
+      "select * from (select b, row_number() over () as rownumber from t) DT where rownumber <= 3 and b > 5 -- pattern is not applicable with complex filter on top of window",
+      "select * from (select b, row_number() over () as rownumber from t) DT where rownumber > 3  -- pattern is not applicable with filter is not < or <=",
+      "select * from (select a,b, row_number() over () as rownumber from t) DT where  a > b  -- pattern is not applicable with filter is not < or <=",
+      "select * from (select a,b, row_number() over () as rownumber from t) DT where  a <= 3 -- pattern is not applicable with filter is not on row number",
+      "select * from (select a,b, row_number() over () as rownumber from t) DT where 3 >= rownumber  -- pattern is not applicable with filter is not < or <=",
+      "select * from (select a,b, row_number() over () as rownumber from t) DT where rownumber <= -4   -- pattern is not applicable with filter constant negative",
+      "select * from (select row_number() over () as rownumber from t) DT where rownumber <= 3 and rownumber >= 2 -- pattern is not applicable with complex filter"
+    ]
+  },
+  {
+    "name": "TestPushDerivedTopnPositive",
+    "cases":[
+      "select * from (select a,b, row_number() over (order by a) as rownumber from t) DT where rownumber <= 3.5  -- pattern is applicable with N rounded down to an integer",
+      "select * from (select row_number() over (order by a) as rownumber from t) DT where rownumber <= 3 -- pattern is applicable",
+      "select * from (select row_number() over (order by a) as rownumber from t) DT where rownumber < 3 -- pattern is applicable",
+      "select * from (select row_number() over(rows between 1 preceding and 1 following) as rownumber from t) DT where rownumber <= 3 -- pattern is applicable",
+      "select * from (select a,row_number() over (order by a desc) as rownumber,b from t) DT where rownumber <= 3 -- pattern is applicable",
+      "select count(*) from (select * from (select a,row_number() over (order by b) as rownumber,b from t) DT1 where rownumber <= 1) DT2  -- pattern is applicable"
+    ]
+  }
+]