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cost.go
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cost.go
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// Copyright 2015 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 plan
import (
"math"
)
// Pre-defined cost factors.
const (
FullRangeCount = 10000
HalfRangeCount = 4000
MiddleRangeCount = 100
RowCost = 1.0
IndexCost = 1.1
SortCost = 2.0
FilterRate = 0.5
)
// CostEstimator estimates the cost of a plan.
// Enter implements Visitor Enter interface.
func estimate(p Plan) {
var child Plan
for _, c := range p.GetChildren() {
estimate(c)
child = c
}
switch v := p.(type) {
case *IndexScan:
indexScan(v)
case *Limit:
v.rowCount = child.RowCount()
v.startupCost = child.StartupCost()
v.totalCost = child.TotalCost()
case *SelectFields:
if child != nil {
v.startupCost = child.StartupCost()
v.rowCount = child.RowCount()
v.totalCost = child.TotalCost()
}
case *SelectLock:
v.startupCost = child.StartupCost()
v.rowCount = child.RowCount()
v.totalCost = child.TotalCost()
case *Sort:
// Sort plan must retrieve all the rows before returns the first row.
v.startupCost = child.TotalCost() + child.RowCount()*SortCost
if v.limit == 0 {
v.rowCount = child.RowCount()
} else {
v.rowCount = math.Min(child.RowCount(), v.limit)
}
v.totalCost = v.startupCost + v.rowCount*RowCost
case *TableScan:
tableScan(v)
}
}
func tableScan(v *TableScan) {
var rowCount float64 = FullRangeCount
for _, con := range v.AccessConditions {
rowCount *= guesstimateFilterRate(con)
}
v.startupCost = 0
if v.limit == 0 {
// limit is zero means no limit.
v.rowCount = rowCount
} else {
v.rowCount = math.Min(rowCount, v.limit)
}
v.totalCost = v.rowCount * RowCost
}
func indexScan(v *IndexScan) {
var rowCount float64 = FullRangeCount
for _, con := range v.AccessConditions {
rowCount *= guesstimateFilterRate(con)
}
v.startupCost = 0
if v.limit == 0 {
// limit is zero means no limit.
v.rowCount = rowCount
} else {
v.rowCount = math.Min(rowCount, v.limit)
}
v.totalCost = v.rowCount * IndexCost
}
// EstimateCost estimates the cost of the plan.
func EstimateCost(p Plan) float64 {
estimate(p)
return p.TotalCost()
}