242 lines
7.2 KiB
Go
242 lines
7.2 KiB
Go
// Copyright 2023 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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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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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 sortexec
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import (
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"container/heap"
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"context"
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"slices"
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"sync/atomic"
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"github.com/pingcap/tidb/pkg/executor/internal/exec"
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plannercore "github.com/pingcap/tidb/pkg/planner/core"
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"github.com/pingcap/tidb/pkg/util/chunk"
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"github.com/pingcap/tidb/pkg/util/memory"
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)
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// TopNExec implements a Top-N algorithm and it is built from a SELECT statement with ORDER BY and LIMIT.
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// Instead of sorting all the rows fetched from the table, it keeps the Top-N elements only in a heap to reduce memory usage.
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type TopNExec struct {
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SortExec
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Limit *plannercore.PhysicalLimit
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totalLimit uint64
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chkHeap *topNChunkHeap
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}
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// topNChunkHeap implements heap.Interface.
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type topNChunkHeap struct {
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*TopNExec
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// rowChunks is the chunks to store row values.
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rowChunks *chunk.List
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// rowPointer store the chunk index and row index for each row.
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rowPtrs []chunk.RowPtr
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Idx int
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}
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// Less implement heap.Interface, but since we mantains a max heap,
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// this function returns true if row i is greater than row j.
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func (h *topNChunkHeap) Less(i, j int) bool {
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rowI := h.rowChunks.GetRow(h.rowPtrs[i])
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rowJ := h.rowChunks.GetRow(h.rowPtrs[j])
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return h.greaterRow(rowI, rowJ)
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}
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func (h *topNChunkHeap) greaterRow(rowI, rowJ chunk.Row) bool {
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for i, colIdx := range h.keyColumns {
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cmpFunc := h.keyCmpFuncs[i]
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cmp := cmpFunc(rowI, colIdx, rowJ, colIdx)
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if h.ByItems[i].Desc {
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cmp = -cmp
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}
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if cmp > 0 {
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return true
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} else if cmp < 0 {
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return false
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}
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}
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return false
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}
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func (h *topNChunkHeap) Len() int {
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return len(h.rowPtrs)
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}
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func (*topNChunkHeap) Push(any) {
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// Should never be called.
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}
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func (h *topNChunkHeap) Pop() any {
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h.rowPtrs = h.rowPtrs[:len(h.rowPtrs)-1]
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// We don't need the popped value, return nil to avoid memory allocation.
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return nil
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}
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func (h *topNChunkHeap) Swap(i, j int) {
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h.rowPtrs[i], h.rowPtrs[j] = h.rowPtrs[j], h.rowPtrs[i]
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}
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func (e *TopNExec) keyColumnsCompare(i, j chunk.RowPtr) int {
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rowI := e.chkHeap.rowChunks.GetRow(i)
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rowJ := e.chkHeap.rowChunks.GetRow(j)
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return e.compareRow(rowI, rowJ)
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}
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func (e *TopNExec) initPointers() {
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e.chkHeap.rowPtrs = make([]chunk.RowPtr, 0, e.chkHeap.rowChunks.Len())
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e.memTracker.Consume(int64(8 * e.chkHeap.rowChunks.Len()))
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for chkIdx := 0; chkIdx < e.chkHeap.rowChunks.NumChunks(); chkIdx++ {
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rowChk := e.chkHeap.rowChunks.GetChunk(chkIdx)
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for rowIdx := 0; rowIdx < rowChk.NumRows(); rowIdx++ {
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e.chkHeap.rowPtrs = append(e.chkHeap.rowPtrs, chunk.RowPtr{ChkIdx: uint32(chkIdx), RowIdx: uint32(rowIdx)})
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}
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}
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}
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// Open implements the Executor Open interface.
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func (e *TopNExec) Open(ctx context.Context) error {
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e.memTracker = memory.NewTracker(e.ID(), -1)
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e.memTracker.AttachTo(e.Ctx().GetSessionVars().StmtCtx.MemTracker)
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e.fetched = &atomic.Bool{}
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e.fetched.Store(false)
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e.chkHeap = &topNChunkHeap{TopNExec: e}
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e.chkHeap.Idx = 0
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return exec.Open(ctx, e.Children(0))
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}
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// Next implements the Executor Next interface.
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func (e *TopNExec) Next(ctx context.Context, req *chunk.Chunk) error {
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req.Reset()
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if e.fetched.CompareAndSwap(false, true) {
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e.totalLimit = e.Limit.Offset + e.Limit.Count
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e.chkHeap.Idx = int(e.Limit.Offset)
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err := e.loadChunksUntilTotalLimit(ctx)
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if err != nil {
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return err
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}
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err = e.executeTopN(ctx)
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if err != nil {
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return err
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}
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}
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if e.chkHeap.Idx >= len(e.chkHeap.rowPtrs) {
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return nil
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}
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if !req.IsFull() {
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numToAppend := min(len(e.chkHeap.rowPtrs)-e.chkHeap.Idx, req.RequiredRows()-req.NumRows())
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rows := make([]chunk.Row, numToAppend)
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for index := 0; index < numToAppend; index++ {
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rows[index] = e.chkHeap.rowChunks.GetRow(e.chkHeap.rowPtrs[e.chkHeap.Idx])
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e.chkHeap.Idx++
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}
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req.AppendRows(rows)
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}
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return nil
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}
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func (e *TopNExec) loadChunksUntilTotalLimit(ctx context.Context) error {
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e.chkHeap.rowChunks = chunk.NewList(exec.RetTypes(e), e.InitCap(), e.MaxChunkSize())
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e.chkHeap.rowChunks.GetMemTracker().AttachTo(e.memTracker)
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e.chkHeap.rowChunks.GetMemTracker().SetLabel(memory.LabelForRowChunks)
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for uint64(e.chkHeap.rowChunks.Len()) < e.totalLimit {
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srcChk := exec.TryNewCacheChunk(e.Children(0))
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// adjust required rows by total limit
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srcChk.SetRequiredRows(int(e.totalLimit-uint64(e.chkHeap.rowChunks.Len())), e.MaxChunkSize())
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err := exec.Next(ctx, e.Children(0), srcChk)
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if err != nil {
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return err
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}
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if srcChk.NumRows() == 0 {
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break
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}
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e.chkHeap.rowChunks.Add(srcChk)
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}
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e.initPointers()
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e.initCompareFuncs()
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e.buildKeyColumns()
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return nil
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}
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const topNCompactionFactor = 4
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func (e *TopNExec) executeTopN(ctx context.Context) error {
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heap.Init(e.chkHeap)
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for uint64(len(e.chkHeap.rowPtrs)) > e.totalLimit {
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// The number of rows we loaded may exceeds total limit, remove greatest rows by Pop.
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heap.Pop(e.chkHeap)
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}
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childRowChk := exec.TryNewCacheChunk(e.Children(0))
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for {
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err := exec.Next(ctx, e.Children(0), childRowChk)
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if err != nil {
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return err
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}
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if childRowChk.NumRows() == 0 {
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break
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}
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err = e.processChildChk(childRowChk)
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if err != nil {
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return err
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}
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if e.chkHeap.rowChunks.Len() > len(e.chkHeap.rowPtrs)*topNCompactionFactor {
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err = e.doCompaction(e.chkHeap)
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if err != nil {
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return err
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}
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}
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}
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slices.SortFunc(e.chkHeap.rowPtrs, e.keyColumnsCompare)
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return nil
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}
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func (e *TopNExec) processChildChk(childRowChk *chunk.Chunk) error {
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for i := 0; i < childRowChk.NumRows(); i++ {
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heapMaxPtr := e.chkHeap.rowPtrs[0]
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var heapMax, next chunk.Row
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heapMax = e.chkHeap.rowChunks.GetRow(heapMaxPtr)
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next = childRowChk.GetRow(i)
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if e.chkHeap.greaterRow(heapMax, next) {
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// Evict heap max, keep the next row.
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e.chkHeap.rowPtrs[0] = e.chkHeap.rowChunks.AppendRow(childRowChk.GetRow(i))
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heap.Fix(e.chkHeap, 0)
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}
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}
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return nil
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}
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// doCompaction rebuild the chunks and row pointers to release memory.
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// If we don't do compaction, in a extreme case like the child data is already ascending sorted
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// but we want descending top N, then we will keep all data in memory.
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// But if data is distributed randomly, this function will be called log(n) times.
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func (e *TopNExec) doCompaction(chkHeap *topNChunkHeap) error {
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newRowChunks := chunk.NewList(exec.RetTypes(e), e.InitCap(), e.MaxChunkSize())
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newRowPtrs := make([]chunk.RowPtr, 0, chkHeap.rowChunks.Len())
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for _, rowPtr := range chkHeap.rowPtrs {
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newRowPtr := newRowChunks.AppendRow(chkHeap.rowChunks.GetRow(rowPtr))
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newRowPtrs = append(newRowPtrs, newRowPtr)
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}
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newRowChunks.GetMemTracker().SetLabel(memory.LabelForRowChunks)
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e.memTracker.ReplaceChild(chkHeap.rowChunks.GetMemTracker(), newRowChunks.GetMemTracker())
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chkHeap.rowChunks = newRowChunks
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e.memTracker.Consume(int64(8 * (len(newRowPtrs) - len(chkHeap.rowPtrs))))
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chkHeap.rowPtrs = newRowPtrs
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return nil
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}
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