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projection.go
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projection.go
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// Copyright 2018 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,
// See the License for the specific language governing permissions and
// limitations under the License.
package executor
import (
"time"
"github.com/pingcap/tidb/expression"
"github.com/pingcap/tidb/sessionctx"
"github.com/pingcap/tidb/util/chunk"
"github.com/pkg/errors"
"golang.org/x/net/context"
)
// This file contains the implementation of the physical Projection Operator:
// https://en.wikipedia.org/wiki/Projection_(relational_algebra)
//
// NOTE:
// 1. The number of "projectionWorker" is controlled by the global session
// variable "tidb_projection_concurrency".
// 2. Unparallel version is used when one of the following situations occurs:
// a. "tidb_projection_concurrency" is set to 0.
// b. The estimated input size is smaller than "tidb_max_chunk_size".
// c. This projection can not be executed vectorially.
type projectionInput struct {
chk *chunk.Chunk
targetWorker *projectionWorker
}
type projectionOutput struct {
chk *chunk.Chunk
done chan error
}
// ProjectionExec implements the physical Projection Operator:
// https://en.wikipedia.org/wiki/Projection_(relational_algebra)
type ProjectionExec struct {
baseExecutor
evaluatorSuit *expression.EvaluatorSuit
calculateNoDelay bool
prepared bool
finishCh chan struct{}
outputCh chan *projectionOutput
fetcher projectionInputFetcher
numWorkers int64
workers []*projectionWorker
childResult *chunk.Chunk
}
// Open implements the Executor Open interface.
func (e *ProjectionExec) Open(ctx context.Context) error {
if err := e.baseExecutor.Open(ctx); err != nil {
return errors.Trace(err)
}
e.prepared = false
// For now a Projection can not be executed vectorially only because it
// contains "SetVar" or "GetVar" functions, in this scenario this
// Projection can not be executed parallelly.
if e.numWorkers > 0 && !e.evaluatorSuit.Vectorizable() {
e.numWorkers = 0
}
if e.isUnparallelExec() {
e.childResult = e.children[0].newFirstChunk()
}
return nil
}
// Next implements the Executor Next interface.
//
// Here we explain the execution flow of the parallel projection implementation.
// There are 3 main components:
// 1. "projectionInputFetcher": Fetch input "Chunk" from child.
// 2. "projectionWorker": Do the projection work.
// 3. "ProjectionExec.Next": Return result to parent.
//
// 1. "projectionInputFetcher" gets its input and output resources from its
// "inputCh" and "outputCh" channel, once the input and output resources are
// abtained, it fetches child's result into "input.chk" and:
// a. Dispatches this input to the worker specified in "input.targetWorker"
// b. Dispatches this output to the main thread: "ProjectionExec.Next"
// c. Dispatches this output to the worker specified in "input.targetWorker"
// It is finished and exited once:
// a. There is no more input from child.
// b. "ProjectionExec" close the "globalFinishCh"
//
// 2. "projectionWorker" gets its input and output resources from its
// "inputCh" and "outputCh" channel, once the input and output resources are
// abtained, it calculates the projection result use "input.chk" as the input
// and "output.chk" as the output, once the calculation is done, it:
// a. Sends "nil" or error to "output.done" to mark this input is finished.
// b. Returns the "input" resource to "projectionInputFetcher.inputCh"
// They are finished and exited once:
// a. "ProjectionExec" closes the "globalFinishCh"
//
// 3. "ProjectionExec.Next" gets its output resources from its "outputCh" channel.
// After receiving an output from "outputCh", it should wait to receive a "nil"
// or error from "output.done" channel. Once a "nil" or error is received:
// a. Returns this output to its parent
// b. Returns the "output" resource to "projectionInputFetcher.outputCh"
//
// +-----------+----------------------+--------------------------+
// | | | |
// | +--------+---------+ +--------+---------+ +--------+---------+
// | | projectionWorker | + projectionWorker | ... + projectionWorker |
// | +------------------+ +------------------+ +------------------+
// | ^ ^ ^ ^ ^ ^
// | | | | | | |
// | inputCh outputCh inputCh outputCh inputCh outputCh
// | ^ ^ ^ ^ ^ ^
// | | | | | | |
// | | |
// | | +----------------->outputCh
// | | | |
// | | | v
// | +-------+-------+--------+ +---------------------+
// | | projectionInputFetcher | | ProjectionExec.Next |
// | +------------------------+ +---------+-----------+
// | ^ ^ |
// | | | |
// | inputCh outputCh |
// | ^ ^ |
// | | | |
// +------------------------------+ +----------------------+
//
func (e *ProjectionExec) Next(ctx context.Context, chk *chunk.Chunk) error {
if e.runtimeStats != nil {
start := time.Now()
defer func() { e.runtimeStats.Record(time.Now().Sub(start), chk.NumRows()) }()
}
chk.GrowAndReset(e.maxChunkSize)
if e.isUnparallelExec() {
return errors.Trace(e.unParallelExecute(ctx, chk))
}
return errors.Trace(e.parallelExecute(ctx, chk))
}
func (e *ProjectionExec) isUnparallelExec() bool {
return e.numWorkers <= 0
}
func (e *ProjectionExec) unParallelExecute(ctx context.Context, chk *chunk.Chunk) error {
err := e.children[0].Next(ctx, e.childResult)
if err != nil {
return errors.Trace(err)
}
err = e.evaluatorSuit.Run(e.ctx, e.childResult, chk)
return errors.Trace(err)
}
func (e *ProjectionExec) parallelExecute(ctx context.Context, chk *chunk.Chunk) error {
if !e.prepared {
e.prepare(ctx)
e.prepared = true
}
output, ok := <-e.outputCh
if !ok {
return nil
}
err := <-output.done
if err != nil {
return errors.Trace(err)
}
chk.SwapColumns(output.chk)
e.fetcher.outputCh <- output
return nil
}
func (e *ProjectionExec) prepare(ctx context.Context) {
e.finishCh = make(chan struct{})
e.outputCh = make(chan *projectionOutput, e.numWorkers)
// Initialize projectionInputFetcher.
e.fetcher = projectionInputFetcher{
child: e.children[0],
globalFinishCh: e.finishCh,
globalOutputCh: e.outputCh,
inputCh: make(chan *projectionInput, e.numWorkers),
outputCh: make(chan *projectionOutput, e.numWorkers),
}
// Initialize projectionWorker.
e.workers = make([]*projectionWorker, 0, e.numWorkers)
for i := int64(0); i < e.numWorkers; i++ {
e.workers = append(e.workers, &projectionWorker{
sctx: e.ctx,
evaluatorSuit: e.evaluatorSuit,
globalFinishCh: e.finishCh,
inputGiveBackCh: e.fetcher.inputCh,
inputCh: make(chan *projectionInput, 1),
outputCh: make(chan *projectionOutput, 1),
})
e.fetcher.inputCh <- &projectionInput{
chk: e.children[0].newFirstChunk(),
targetWorker: e.workers[i],
}
e.fetcher.outputCh <- &projectionOutput{
chk: e.newFirstChunk(),
done: make(chan error, 1),
}
}
go e.fetcher.run(ctx)
for i := range e.workers {
go e.workers[i].run(ctx)
}
}
// Close implements the Executor Close interface.
func (e *ProjectionExec) Close() error {
if e.isUnparallelExec() {
e.childResult = nil
}
if e.outputCh != nil {
close(e.finishCh)
// Wait for "projectionInputFetcher" to finish and exit.
for range e.outputCh {
}
e.outputCh = nil
}
return errors.Trace(e.baseExecutor.Close())
}
type projectionInputFetcher struct {
child Executor
globalFinishCh <-chan struct{}
globalOutputCh chan<- *projectionOutput
inputCh chan *projectionInput
outputCh chan *projectionOutput
}
// run gets projectionInputFetcher's input and output resources from its
// "inputCh" and "outputCh" channel, once the input and output resources are
// abtained, it fetches child's result into "input.chk" and:
// a. Dispatches this input to the worker specified in "input.targetWorker"
// b. Dispatches this output to the main thread: "ProjectionExec.Next"
// c. Dispatches this output to the worker specified in "input.targetWorker"
//
// It is finished and exited once:
// a. There is no more input from child.
// b. "ProjectionExec" close the "globalFinishCh"
func (f *projectionInputFetcher) run(ctx context.Context) {
defer func() {
close(f.globalOutputCh)
}()
for {
input := readProjectionInput(f.inputCh, f.globalFinishCh)
if input == nil {
return
}
targetWorker := input.targetWorker
output := readProjectionOutput(f.outputCh, f.globalFinishCh)
if output == nil {
return
}
f.globalOutputCh <- output
err := f.child.Next(ctx, input.chk)
if err != nil || input.chk.NumRows() == 0 {
output.done <- errors.Trace(err)
return
}
targetWorker.inputCh <- input
targetWorker.outputCh <- output
}
}
type projectionWorker struct {
sctx sessionctx.Context
evaluatorSuit *expression.EvaluatorSuit
globalFinishCh <-chan struct{}
inputGiveBackCh chan<- *projectionInput
// channel "input" and "output" is :
// a. initialized by "ProjectionExec.prepare"
// b. written by "projectionInputFetcher.run"
// c. read by "projectionWorker.run"
inputCh chan *projectionInput
outputCh chan *projectionOutput
}
// run gets projectionWorker's input and output resources from its
// "inputCh" and "outputCh" channel, once the input and output resources are
// abtained, it calculate the projection result use "input.chk" as the input
// and "output.chk" as the output, once the calculation is done, it:
// a. Sends "nil" or error to "output.done" to mark this input is finished.
// b. Returns the "input" resource to "projectionInputFetcher.inputCh".
//
// It is finished and exited once:
// a. "ProjectionExec" closes the "globalFinishCh".
func (w *projectionWorker) run(ctx context.Context) {
for {
input := readProjectionInput(w.inputCh, w.globalFinishCh)
if input == nil {
return
}
output := readProjectionOutput(w.outputCh, w.globalFinishCh)
if output == nil {
return
}
err := w.evaluatorSuit.Run(w.sctx, input.chk, output.chk)
output.done <- errors.Trace(err)
if err != nil {
return
}
w.inputGiveBackCh <- input
}
}
func readProjectionInput(inputCh <-chan *projectionInput, finishCh <-chan struct{}) *projectionInput {
select {
case <-finishCh:
return nil
case input, ok := <-inputCh:
if !ok {
return nil
}
return input
}
}
func readProjectionOutput(outputCh <-chan *projectionOutput, finishCh <-chan struct{}) *projectionOutput {
select {
case <-finishCh:
return nil
case output, ok := <-outputCh:
if !ok {
return nil
}
return output
}
}