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rule_eliminate_projection.go
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/
rule_eliminate_projection.go
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// Copyright 2016 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,
// See the License for the specific language governing permissions and
// limitations under the License.
package core
import (
"github.com/pingcap/tidb/expression"
)
// canProjectionBeEliminatedLoose checks whether a projection can be eliminated,
// returns true if every expression is a single column.
func canProjectionBeEliminatedLoose(p *LogicalProjection) bool {
for _, expr := range p.Exprs {
_, ok := expr.(*expression.Column)
if !ok {
return false
}
}
return true
}
// canProjectionBeEliminatedStrict checks whether a projection can be
// eliminated, returns true if the projection just copy its child's output.
func canProjectionBeEliminatedStrict(p *PhysicalProjection) bool {
if p.Schema().Len() == 0 {
return true
}
child := p.Children()[0]
if p.Schema().Len() != child.Schema().Len() {
return false
}
for i, expr := range p.Exprs {
col, ok := expr.(*expression.Column)
if !ok || !col.Equal(nil, child.Schema().Columns[i]) {
return false
}
}
return true
}
func resolveColumnAndReplace(origin *expression.Column, replace map[string]*expression.Column) {
dst := replace[string(origin.HashCode(nil))]
if dst != nil {
colName, retType := origin.ColName, origin.RetType
*origin = *dst
origin.ColName, origin.RetType = colName, retType
}
}
func resolveExprAndReplace(origin expression.Expression, replace map[string]*expression.Column) {
switch expr := origin.(type) {
case *expression.Column:
resolveColumnAndReplace(expr, replace)
case *expression.CorrelatedColumn:
resolveColumnAndReplace(&expr.Column, replace)
case *expression.ScalarFunction:
for _, arg := range expr.GetArgs() {
resolveExprAndReplace(arg, replace)
}
}
}
func doPhysicalProjectionElimination(p PhysicalPlan) PhysicalPlan {
for i, child := range p.Children() {
p.Children()[i] = doPhysicalProjectionElimination(child)
}
proj, isProj := p.(*PhysicalProjection)
if !isProj || !canProjectionBeEliminatedStrict(proj) {
return p
}
child := p.Children()[0]
return child
}
// eliminatePhysicalProjection should be called after physical optimization to
// eliminate the redundant projection left after logical projection elimination.
func eliminatePhysicalProjection(p PhysicalPlan) PhysicalPlan {
oldSchema := p.Schema()
newRoot := doPhysicalProjectionElimination(p)
newCols := newRoot.Schema().Columns
for i, oldCol := range oldSchema.Columns {
oldCol.Index = newCols[i].Index
newRoot.Schema().Columns[i] = oldCol
}
return newRoot
}
type projectionEliminater struct {
}
// optimize implements the logicalOptRule interface.
func (pe *projectionEliminater) optimize(lp LogicalPlan) (LogicalPlan, error) {
root := pe.eliminate(lp, make(map[string]*expression.Column), false)
return root, nil
}
// eliminate eliminates the redundant projection in a logical plan.
func (pe *projectionEliminater) eliminate(p LogicalPlan, replace map[string]*expression.Column, canEliminate bool) LogicalPlan {
proj, isProj := p.(*LogicalProjection)
childFlag := canEliminate
if _, isUnion := p.(*LogicalUnionAll); isUnion {
childFlag = false
} else if _, isAgg := p.(*LogicalAggregation); isAgg || isProj {
childFlag = true
}
for i, child := range p.Children() {
p.Children()[i] = pe.eliminate(child, replace, childFlag)
}
switch x := p.(type) {
case *LogicalJoin:
x.schema = buildLogicalJoinSchema(x.JoinType, x)
case *LogicalApply:
x.schema = buildLogicalJoinSchema(x.JoinType, x)
default:
for _, dst := range p.Schema().Columns {
resolveColumnAndReplace(dst, replace)
}
}
p.replaceExprColumns(replace)
if !(isProj && canEliminate && canProjectionBeEliminatedLoose(proj)) {
return p
}
exprs := proj.Exprs
for i, col := range proj.Schema().Columns {
replace[string(col.HashCode(nil))] = exprs[i].(*expression.Column)
}
return p.Children()[0]
}
func (p *LogicalJoin) replaceExprColumns(replace map[string]*expression.Column) {
for _, equalExpr := range p.EqualConditions {
resolveExprAndReplace(equalExpr, replace)
}
for _, leftExpr := range p.LeftConditions {
resolveExprAndReplace(leftExpr, replace)
}
for _, rightExpr := range p.RightConditions {
resolveExprAndReplace(rightExpr, replace)
}
for _, otherExpr := range p.OtherConditions {
resolveExprAndReplace(otherExpr, replace)
}
}
func (p *LogicalProjection) replaceExprColumns(replace map[string]*expression.Column) {
for _, expr := range p.Exprs {
resolveExprAndReplace(expr, replace)
}
}
func (la *LogicalAggregation) replaceExprColumns(replace map[string]*expression.Column) {
for _, agg := range la.AggFuncs {
for _, aggExpr := range agg.Args {
resolveExprAndReplace(aggExpr, replace)
}
}
for _, gbyItem := range la.GroupByItems {
resolveExprAndReplace(gbyItem, replace)
}
la.collectGroupByColumns()
}
func (p *LogicalSelection) replaceExprColumns(replace map[string]*expression.Column) {
for _, expr := range p.Conditions {
resolveExprAndReplace(expr, replace)
}
}
func (la *LogicalApply) replaceExprColumns(replace map[string]*expression.Column) {
la.LogicalJoin.replaceExprColumns(replace)
for _, coCol := range la.corCols {
dst := replace[string(coCol.Column.HashCode(nil))]
if dst != nil {
coCol.Column = *dst
}
}
}
func (ls *LogicalSort) replaceExprColumns(replace map[string]*expression.Column) {
for _, byItem := range ls.ByItems {
resolveExprAndReplace(byItem.Expr, replace)
}
}
func (lt *LogicalTopN) replaceExprColumns(replace map[string]*expression.Column) {
for _, byItem := range lt.ByItems {
resolveExprAndReplace(byItem.Expr, replace)
}
}