tidb paging 源码
tidb paging 代码
文件路径：/util/paging/paging.go
// Copyright 2021 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,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package paging
import "math"
// A paging request may be separated into multi requests if there are more data than a page.
// The paging size grows from min to max. See https://github.com/pingcap/tidb/issues/36328
// e.g. a paging request scans over range (r1, r200), it requires 128 rows in the first batch,
// if it's not drained, then the paging size grows, the new range is calculated like (r100, r200), then send a request again.
// Compare with the common unary request, paging request allows early access of data, it offers a streaming-like way processing data.
const (
MinPagingSize uint64 = 128
maxPagingSizeShift = 7
pagingSizeGrow = 2
MaxPagingSize = 50000
pagingGrowingSum = ((2 << maxPagingSizeShift) - 1) * MinPagingSize
Threshold uint64 = 960
)
// GrowPagingSize grows the paging size and ensures it does not exceed MaxPagingSize
func GrowPagingSize(size uint64, max uint64) uint64 {
if max < MaxPagingSize {
// Defensive programing, for example, call with max = 0.
// max should never less than MaxPagingSize.
// Otherwise the session variable maybe wrong, or the distsql request does not obey the session variable setting.
max = MaxPagingSize
}
size <<= 1
if size > max {
return max
}
return size
}
// CalculateSeekCnt calculates the seek count from expect count
func CalculateSeekCnt(expectCnt uint64) float64 {
if expectCnt == 0 {
return 0
}
if expectCnt > pagingGrowingSum {
// if the expectCnt is larger than pagingGrowingSum, calculate the seekCnt for the excess.
return float64(8 + (expectCnt-pagingGrowingSum+MaxPagingSize-1)/MaxPagingSize)
}
if expectCnt > MinPagingSize {
// if the expectCnt is less than pagingGrowingSum,
// calculate the seekCnt(number of terms) from the sum of a geometric progression.
// expectCnt = minPagingSize * (pagingSizeGrow ^ seekCnt - 1) / (pagingSizeGrow - 1)
// simplify (pagingSizeGrow ^ seekCnt - 1) to pagingSizeGrow ^ seekCnt, we can infer that
// seekCnt = log((pagingSizeGrow - 1) * expectCnt / minPagingSize) / log(pagingSizeGrow)
return 1 + float64(int(math.Log(float64((pagingSizeGrow-1)*expectCnt)/float64(MinPagingSize))/math.Log(float64(pagingSizeGrow))))
}
return 1
}
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