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support projection elimination (pingcap#1740)
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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. | ||
|
|
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| package plan | ||
|
|
||
| import ( | ||
| "github.com/pingcap/tidb/expression" | ||
| ) | ||
|
|
||
| // EliminateProjection eliminates projection operator to avoid the cost of memory copy in the iterator of projection. | ||
| func EliminateProjection(p LogicalPlan) LogicalPlan { | ||
| switch plan := p.(type) { | ||
| case *Projection: | ||
| if !projectionCanBeEliminated(plan) { | ||
| break | ||
| } | ||
| child := p.GetChildByIndex(0).(LogicalPlan) | ||
| // pointer of schema in PROJECTION operator may be referenced by parent operator, | ||
| // and attributes of child operator may be used later, so here we shallow copy child's schema | ||
| // to the schema of PROJECTION, and reset the child's schema as the schema of PROJECTION. | ||
| for i, col := range plan.GetSchema() { | ||
| plan.GetSchema()[i] = shallowCopyColumn(col, child.GetSchema()[i]) | ||
| } | ||
| child.SetSchema(plan.GetSchema()) | ||
| RemovePlan(p) | ||
| p = EliminateProjection(child) | ||
| case *DataSource: | ||
| // predicates may be pushed down when build physical plan, and the schema of Selection operator is | ||
| // always the same as the child operator, so here we copy the schema of Selection to DataSource. | ||
| if sel, ok := plan.GetParentByIndex(0).(*Selection); ok { | ||
| plan.SetSchema(sel.GetSchema()) | ||
| for i, cond := range sel.Conditions { | ||
| sel.Conditions[i], _ = retrieveColumnsInExpression(cond, plan.GetSchema()) | ||
| } | ||
| } | ||
| } | ||
| if len(p.GetChildren()) == 1 { | ||
| child := p.GetChildByIndex(0) | ||
| p.ReplaceChild(child, EliminateProjection(child.(LogicalPlan))) | ||
| } else { | ||
| children := make([]Plan, 0, len(p.GetChildren())) | ||
| for _, child := range p.GetChildren() { | ||
| children = append(children, EliminateProjection(child.(LogicalPlan))) | ||
| } | ||
| p.SetChildren(children...) | ||
| } | ||
| return p | ||
| } | ||
|
|
||
| func shallowCopyColumn(colDest, colSrc *expression.Column) *expression.Column { | ||
| colDest.Correlated = colSrc.Correlated | ||
| colDest.FromID = colSrc.FromID | ||
| colDest.Position = colSrc.Position | ||
| colDest.ID = colSrc.ID | ||
| colDest.IsAggOrSubq = colSrc.IsAggOrSubq | ||
| colDest.RetType = colSrc.RetType | ||
|
|
||
| return colDest | ||
| } | ||
|
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| // projectionCanBeEliminated checks if a PROJECTION operator can be eliminated. | ||
| // PROJECTION operator can be eliminated when meet the following conditions at the same time: | ||
| // 1. fields of PROJECTION are all columns | ||
| // 2. fields of PROJECTION are just the same as the schema of the child operator (including order, amount, etc.). | ||
| // expressions like following cases can not be eliminated: | ||
| // "SELECT b, a from t", | ||
| // or "SELECT c AS a, c AS b FROM t WHERE d = 1", | ||
| // or "select t1.a, t2.b, t1.b, t2.a from t1, t2 where t1.a < 0 and t2.b > 0". | ||
| func projectionCanBeEliminated(p *Projection) bool { | ||
| child := p.GetChildByIndex(0).(LogicalPlan) | ||
| if len(p.GetSchema()) != len(child.GetSchema()) { | ||
| return false | ||
| } | ||
| for i, expr := range p.Exprs { | ||
| col, ok := expr.(*expression.Column) | ||
| if !ok || col.Correlated { | ||
| return false | ||
| } | ||
| if col.FromID != child.GetSchema()[i].FromID || col.Position != child.GetSchema()[i].Position { | ||
| return false | ||
| } | ||
| } | ||
| return true | ||
| } |
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