tidb/executor/aggregate/agg_hash_partial_worker.go

// Copyright 2023 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 aggregate

import (
	"sync"
	"time"

	"github.com/pingcap/failpoint"
	"github.com/pingcap/tidb/expression"
	"github.com/pingcap/tidb/sessionctx"
	"github.com/pingcap/tidb/sessionctx/stmtctx"
	"github.com/pingcap/tidb/util/chunk"
	"github.com/twmb/murmur3"
)

// HashAggIntermData indicates the intermediate data of aggregation execution.
type HashAggIntermData struct {
	groupKeys        []string
	cursor           int
	partialResultMap AggPartialResultMapper
}

// HashAggPartialWorker indicates the partial workers of parallel hash agg execution,
// the number of the worker can be set by `tidb_hashagg_partial_concurrency`.
type HashAggPartialWorker struct {
	baseHashAggWorker

	inputCh           chan *chunk.Chunk
	outputChs         []chan *HashAggIntermData
	globalOutputCh    chan *AfFinalResult
	giveBackCh        chan<- *HashAggInput
	partialResultsMap AggPartialResultMapper
	groupByItems      []expression.Expression
	groupKey          [][]byte
	// chk stores the input data from child,
	// and is reused by childExec and partial worker.
	chk *chunk.Chunk
}

func (w *HashAggPartialWorker) getChildInput() bool {
	select {
	case <-w.finishCh:
		return false
	case chk, ok := <-w.inputCh:
		if !ok {
			return false
		}
		w.chk.SwapColumns(chk)
		w.giveBackCh <- &HashAggInput{
			chk:        chk,
			giveBackCh: w.inputCh,
		}
	}
	return true
}

func (w *HashAggPartialWorker) run(ctx sessionctx.Context, waitGroup *sync.WaitGroup, finalConcurrency int) {
	start := time.Now()
	needShuffle, sc := false, ctx.GetSessionVars().StmtCtx
	defer func() {
		if r := recover(); r != nil {
			recoveryHashAgg(w.globalOutputCh, r)
		}
		if needShuffle {
			w.shuffleIntermData(sc, finalConcurrency)
		}
		w.memTracker.Consume(-w.chk.MemoryUsage())
		if w.stats != nil {
			w.stats.WorkerTime += int64(time.Since(start))
		}
		waitGroup.Done()
	}()
	for {
		waitStart := time.Now()
		ok := w.getChildInput()
		if w.stats != nil {
			w.stats.WaitTime += int64(time.Since(waitStart))
		}
		if !ok {
			return
		}
		execStart := time.Now()
		if err := w.updatePartialResult(ctx, sc, w.chk, len(w.partialResultsMap)); err != nil {
			w.globalOutputCh <- &AfFinalResult{err: err}
			return
		}
		if w.stats != nil {
			w.stats.ExecTime += int64(time.Since(execStart))
			w.stats.TaskNum++
		}
		// The intermData can be promised to be not empty if reaching here,
		// so we set needShuffle to be true.
		needShuffle = true
	}
}

func (w *HashAggPartialWorker) updatePartialResult(ctx sessionctx.Context, sc *stmtctx.StatementContext, chk *chunk.Chunk, _ int) (err error) {
	memSize := getGroupKeyMemUsage(w.groupKey)
	w.groupKey, err = GetGroupKey(w.ctx, chk, w.groupKey, w.groupByItems)
	failpoint.Inject("ConsumeRandomPanic", nil)
	w.memTracker.Consume(getGroupKeyMemUsage(w.groupKey) - memSize)
	if err != nil {
		return err
	}

	partialResults := w.getPartialResult(sc, w.groupKey, w.partialResultsMap)
	numRows := chk.NumRows()
	rows := make([]chunk.Row, 1)
	allMemDelta := int64(0)
	for i := 0; i < numRows; i++ {
		for j, af := range w.aggFuncs {
			rows[0] = chk.GetRow(i)
			memDelta, err := af.UpdatePartialResult(ctx, rows, partialResults[i][j])
			if err != nil {
				return err
			}
			allMemDelta += memDelta
		}
	}
	w.memTracker.Consume(allMemDelta)
	return nil
}

// shuffleIntermData shuffles the intermediate data of partial workers to corresponded final workers.
// We only support parallel execution for single-machine, so process of encode and decode can be skipped.
func (w *HashAggPartialWorker) shuffleIntermData(_ *stmtctx.StatementContext, finalConcurrency int) {
	groupKeysSlice := make([][]string, finalConcurrency)
	for groupKey := range w.partialResultsMap {
		finalWorkerIdx := int(murmur3.Sum32([]byte(groupKey))) % finalConcurrency
		if groupKeysSlice[finalWorkerIdx] == nil {
			groupKeysSlice[finalWorkerIdx] = make([]string, 0, len(w.partialResultsMap)/finalConcurrency)
		}
		groupKeysSlice[finalWorkerIdx] = append(groupKeysSlice[finalWorkerIdx], groupKey)
	}

	for i := range groupKeysSlice {
		if groupKeysSlice[i] == nil {
			continue
		}
		w.outputChs[i] <- &HashAggIntermData{
			groupKeys:        groupKeysSlice[i],
			partialResultMap: w.partialResultsMap,
		}
	}
}