110 lines
2.7 KiB
Go
110 lines
2.7 KiB
Go
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// Copyright 2015 PingCAP, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package plan
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import (
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"math"
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)
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// Pre-defined cost factors.
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const (
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FullRangeCount = 10000
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HalfRangeCount = 4000
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MiddleRangeCount = 100
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RowCost = 1.0
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IndexCost = 2.0
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SortCost = 2.0
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FilterRate = 0.5
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)
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// CostEstimator estimates the cost of a plan.
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type costEstimator struct {
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}
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// Enter implements Visitor Enter interface.
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func (c *costEstimator) Enter(p Plan) (Plan, bool) {
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return p, false
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}
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// Leave implements Visitor Leave interface.
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func (c *costEstimator) Leave(p Plan) (Plan, bool) {
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switch v := p.(type) {
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case *IndexScan:
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c.indexScan(v)
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case *Limit:
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v.rowCount = v.Src().RowCount()
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v.startupCost = v.Src().StartupCost()
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v.totalCost = v.Src().TotalCost()
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case *SelectFields:
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if v.Src() != nil {
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v.startupCost = v.Src().StartupCost()
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v.rowCount = v.Src().RowCount()
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v.totalCost = v.Src().TotalCost()
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}
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case *SelectLock:
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v.startupCost = v.Src().StartupCost()
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v.rowCount = v.Src().RowCount()
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v.totalCost = v.Src().TotalCost()
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case *Sort:
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// Sort plan must retrieve all the rows before returns the first row.
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v.startupCost = v.Src().TotalCost() + v.Src().RowCount()*SortCost
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if v.limit == 0 {
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v.rowCount = v.Src().RowCount()
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} else {
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v.rowCount = math.Min(v.Src().RowCount(), v.limit)
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}
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v.totalCost = v.startupCost + v.rowCount*RowCost
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case *TableScan:
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c.tableScan(v)
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}
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return p, true
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}
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func (c *costEstimator) tableScan(v *TableScan) {
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var rowCount float64 = FullRangeCount
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for _, con := range v.AccessConditions {
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rowCount *= guesstimateFilterRate(con)
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}
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v.startupCost = 0
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if v.limit == 0 {
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// limit is zero means no limit.
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v.rowCount = rowCount
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} else {
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v.rowCount = math.Min(rowCount, v.limit)
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}
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v.totalCost = v.rowCount * RowCost
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}
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func (c *costEstimator) indexScan(v *IndexScan) {
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var rowCount float64 = FullRangeCount
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for _, con := range v.AccessConditions {
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rowCount *= guesstimateFilterRate(con)
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}
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v.startupCost = 0
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if v.limit == 0 {
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// limit is zero means no limit.
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v.rowCount = rowCount
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} else {
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v.rowCount = math.Min(rowCount, v.limit)
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}
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v.totalCost = v.rowCount * RowCost
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}
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// EstimateCost estimates the cost of the plan.
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func EstimateCost(p Plan) float64 {
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var estimator costEstimator
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p.Accept(&estimator)
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return p.TotalCost()
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}
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