135 lines
3.5 KiB
Go
135 lines
3.5 KiB
Go
// 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 *TableScan:
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v.startupCost = 0
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if v.limit == 0 {
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v.rowCount = FullRangeCount
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} else {
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v.rowCount = math.Min(FullRangeCount, v.limit)
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}
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v.totalCost = v.rowCount * RowCost
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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 *Filter:
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v.startupCost = v.Src().StartupCost()
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v.rowCount = v.Src().RowCount() * FilterRate
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v.totalCost = v.Src().TotalCost()
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case *Sort:
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if v.Bypass {
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// Bypassed sort doesn't add extra cost.
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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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} else {
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// Sort plan must retrieves 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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}
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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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}
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return p, true
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}
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func (c *costEstimator) indexScan(v *IndexScan) {
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var rowCount float64
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if len(v.Ranges) == 1 && v.Ranges[0].LowVal[0] == nil && v.Ranges[0].HighVal[0] == MaxVal {
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// full range use default row count.
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rowCount = FullRangeCount
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} else {
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for _, v := range v.Ranges {
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// for condition like 'a = 0'.
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if v.IsPoint() {
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rowCount++
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continue
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}
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// For condition like 'a < 0'.
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if v.LowVal[0] == nil || v.LowVal[0] == MinNotNullVal {
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rowCount += HalfRangeCount
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}
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// For condition like 'a > 0'.
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if v.HighVal[0] == MaxVal {
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rowCount += HalfRangeCount
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}
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// For condition like 'a > 0 and a < 1'.
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rowCount += 100
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}
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// If the index has too many ranges, the row count may exceed the default row count.
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// Make sure the cost is lower than full range.
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if rowCount >= FullRangeCount {
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rowCount = FullRangeCount - 1
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}
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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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