sort.go

// Copyright 2017 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 (
	"container/heap"
	"sort"

	"github.com/juju/errors"
	"github.com/pingcap/tidb/expression"
	"github.com/pingcap/tidb/plan"
	"github.com/pingcap/tidb/types"
	"github.com/pingcap/tidb/util/chunk"
	"github.com/pingcap/tidb/util/memory"
	"golang.org/x/net/context"
)

// SortExec represents sorting executor.
type SortExec struct {
	baseExecutor

	ByItems []*plan.ByItems
	Idx     int
	fetched bool
	schema  *expression.Schema

	keyExprs []expression.Expression
	keyTypes []*types.FieldType
	// keyColumns is the column index of the by items.
	keyColumns []int
	// keyCmpFuncs is used to compare each ByItem.
	keyCmpFuncs []chunk.CompareFunc
	// keyChunks is used to store ByItems values when not all ByItems are column.
	keyChunks *chunk.List
	// rowChunks is the chunks to store row values.
	rowChunks *chunk.List
	// rowPointer store the chunk index and row index for each row.
	rowPtrs []chunk.RowPtr

	memTracker *memory.Tracker
}

// Close implements the Executor Close interface.
func (e *SortExec) Close() error {
	e.memTracker.Detach()
	e.memTracker = nil
	return errors.Trace(e.children[0].Close())
}

// Open implements the Executor Open interface.
func (e *SortExec) Open(ctx context.Context) error {
	e.fetched = false
	e.Idx = 0

	// To avoid duplicated initialization for TopNExec.
	if e.memTracker == nil {
		e.memTracker = memory.NewTracker(e.id, e.ctx.GetSessionVars().MemQuotaSort)
		e.memTracker.AttachTo(e.ctx.GetSessionVars().StmtCtx.MemTracker)
	}
	return errors.Trace(e.children[0].Open(ctx))
}

// Next implements the Executor Next interface.
func (e *SortExec) Next(ctx context.Context, chk *chunk.Chunk) error {
	chk.Reset()
	if !e.fetched {
		err := e.fetchRowChunks(ctx)
		if err != nil {
			return errors.Trace(err)
		}
		e.initPointers()
		e.initCompareFuncs()
		allColumnExpr := e.buildKeyColumns()
		if allColumnExpr {
			sort.Slice(e.rowPtrs, e.keyColumnsLess)
		} else {
			e.buildKeyExprsAndTypes()
			err = e.buildKeyChunks()
			if err != nil {
				return errors.Trace(err)
			}
			sort.Slice(e.rowPtrs, e.keyChunksLess)
		}
		e.fetched = true
	}
	for chk.NumRows() < e.maxChunkSize {
		if e.Idx >= len(e.rowPtrs) {
			return nil
		}
		rowPtr := e.rowPtrs[e.Idx]
		chk.AppendRow(e.rowChunks.GetRow(rowPtr))
		e.Idx++
	}
	return nil
}

func (e *SortExec) fetchRowChunks(ctx context.Context) error {
	fields := e.retTypes()
	e.rowChunks = chunk.NewList(fields, e.maxChunkSize)
	e.rowChunks.GetMemTracker().AttachTo(e.memTracker)
	e.rowChunks.GetMemTracker().SetLabel("rowChunks")
	for {
		chk := chunk.NewChunk(fields)
		err := e.children[0].Next(ctx, chk)
		if err != nil {
			return errors.Trace(err)
		}
		rowCount := chk.NumRows()
		if rowCount == 0 {
			break
		}
		e.rowChunks.Add(chk)
	}
	return nil
}

func (e *SortExec) initPointers() {
	e.rowPtrs = make([]chunk.RowPtr, 0, e.rowChunks.Len())
	e.memTracker.Consume(int64(8 * e.rowChunks.Len()))
	for chkIdx := 0; chkIdx < e.rowChunks.NumChunks(); chkIdx++ {
		rowChk := e.rowChunks.GetChunk(chkIdx)
		for rowIdx := 0; rowIdx < rowChk.NumRows(); rowIdx++ {
			e.rowPtrs = append(e.rowPtrs, chunk.RowPtr{ChkIdx: uint32(chkIdx), RowIdx: uint32(rowIdx)})
		}
	}
}

func (e *SortExec) initCompareFuncs() {
	e.keyCmpFuncs = make([]chunk.CompareFunc, len(e.ByItems))
	for i := range e.ByItems {
		keyType := e.ByItems[i].Expr.GetType()
		e.keyCmpFuncs[i] = chunk.GetCompareFunc(keyType)
	}
}

func (e *SortExec) buildKeyColumns() (allColumnExpr bool) {
	e.keyColumns = make([]int, 0, len(e.ByItems))
	for _, by := range e.ByItems {
		if col, ok := by.Expr.(*expression.Column); ok {
			e.keyColumns = append(e.keyColumns, col.Index)
		} else {
			e.keyColumns = e.keyColumns[:0]
			for i := range e.ByItems {
				e.keyColumns = append(e.keyColumns, i)
			}
			return false
		}
	}
	return true
}

func (e *SortExec) buildKeyExprsAndTypes() {
	keyLen := len(e.ByItems)
	e.keyTypes = make([]*types.FieldType, keyLen)
	e.keyExprs = make([]expression.Expression, keyLen)
	for keyColIdx := range e.ByItems {
		e.keyExprs[keyColIdx] = e.ByItems[keyColIdx].Expr
		e.keyTypes[keyColIdx] = e.ByItems[keyColIdx].Expr.GetType()
	}
}

func (e *SortExec) buildKeyChunks() error {
	e.keyChunks = chunk.NewList(e.keyTypes, e.maxChunkSize)
	e.keyChunks.GetMemTracker().SetLabel("keyChunks")
	e.keyChunks.GetMemTracker().AttachTo(e.memTracker)

	for chkIdx := 0; chkIdx < e.rowChunks.NumChunks(); chkIdx++ {
		keyChk := chunk.NewChunk(e.keyTypes)
		childIter := chunk.NewIterator4Chunk(e.rowChunks.GetChunk(chkIdx))
		err := expression.VectorizedExecute(e.ctx, e.keyExprs, childIter, keyChk)
		if err != nil {
			return errors.Trace(err)
		}
		e.keyChunks.Add(keyChk)
	}
	return nil
}

func (e *SortExec) lessRow(rowI, rowJ chunk.Row) bool {
	for i, colIdx := range e.keyColumns {
		cmpFunc := e.keyCmpFuncs[i]
		cmp := cmpFunc(rowI, colIdx, rowJ, colIdx)
		if e.ByItems[i].Desc {
			cmp = -cmp
		}
		if cmp < 0 {
			return true
		} else if cmp > 0 {
			return false
		}
	}
	return false
}

// keyColumnsLess is the less function for key columns.
func (e *SortExec) keyColumnsLess(i, j int) bool {
	rowI := e.rowChunks.GetRow(e.rowPtrs[i])
	rowJ := e.rowChunks.GetRow(e.rowPtrs[j])
	return e.lessRow(rowI, rowJ)
}

// keyChunksLess is the less function for key chunk.
func (e *SortExec) keyChunksLess(i, j int) bool {
	keyRowI := e.keyChunks.GetRow(e.rowPtrs[i])
	keyRowJ := e.keyChunks.GetRow(e.rowPtrs[j])
	return e.lessRow(keyRowI, keyRowJ)
}

// TopNExec implements a Top-N algorithm and it is built from a SELECT statement with ORDER BY and LIMIT.
// Instead of sorting all the rows fetched from the table, it keeps the Top-N elements only in a heap to reduce memory usage.
type TopNExec struct {
	SortExec
	limit      *plan.PhysicalLimit
	totalLimit uint64

	chkHeap *topNChunkHeap
}

// topNChunkHeap implements heap.Interface.
type topNChunkHeap struct {
	*TopNExec
}

// Less implement heap.Interface, but since we mantains a max heap,
// this function returns true if row i is greater than row j.
func (h *topNChunkHeap) Less(i, j int) bool {
	if h.keyChunks != nil {
		return h.keyChunksGreater(i, j)
	}
	return h.keyColumnsGreater(i, j)
}

func (h *topNChunkHeap) keyChunksGreater(i, j int) bool {
	keyRowI := h.keyChunks.GetRow(h.rowPtrs[i])
	keyRowJ := h.keyChunks.GetRow(h.rowPtrs[j])
	return h.greaterRow(keyRowI, keyRowJ)
}

func (h *topNChunkHeap) keyColumnsGreater(i, j int) bool {
	rowI := h.rowChunks.GetRow(h.rowPtrs[i])
	rowJ := h.rowChunks.GetRow(h.rowPtrs[j])
	return h.greaterRow(rowI, rowJ)
}

func (h *topNChunkHeap) greaterRow(rowI, rowJ chunk.Row) bool {
	for i, colIdx := range h.keyColumns {
		cmpFunc := h.keyCmpFuncs[i]
		cmp := cmpFunc(rowI, colIdx, rowJ, colIdx)
		if h.ByItems[i].Desc {
			cmp = -cmp
		}
		if cmp > 0 {
			return true
		} else if cmp < 0 {
			return false
		}
	}
	return false
}

func (h *topNChunkHeap) Len() int {
	return len(h.rowPtrs)
}

func (h *topNChunkHeap) Push(x interface{}) {
	// Should never be called.
}

func (h *topNChunkHeap) Pop() interface{} {
	h.rowPtrs = h.rowPtrs[:len(h.rowPtrs)-1]
	// We don't need the popped value, return nil to avoid memory allocation.
	return nil
}

func (h *topNChunkHeap) Swap(i, j int) {
	h.rowPtrs[i], h.rowPtrs[j] = h.rowPtrs[j], h.rowPtrs[i]
}

// Open implements the Executor Open interface.
func (e *TopNExec) Open(ctx context.Context) error {
	e.memTracker = memory.NewTracker(e.id, e.ctx.GetSessionVars().MemQuotaTopn)
	e.memTracker.AttachTo(e.ctx.GetSessionVars().StmtCtx.MemTracker)
	return errors.Trace(e.SortExec.Open(ctx))
}

// Next implements the Executor Next interface.
func (e *TopNExec) Next(ctx context.Context, chk *chunk.Chunk) error {
	chk.Reset()
	if !e.fetched {
		e.totalLimit = e.limit.Offset + e.limit.Count
		e.Idx = int(e.limit.Offset)
		err := e.loadChunksUntilTotalLimit(ctx)
		if err != nil {
			return errors.Trace(err)
		}
		err = e.executeTopN(ctx)
		if err != nil {
			return errors.Trace(err)
		}
		e.fetched = true
	}
	if e.Idx >= len(e.rowPtrs) {
		return nil
	}
	for chk.NumRows() < e.maxChunkSize && e.Idx < len(e.rowPtrs) {
		row := e.rowChunks.GetRow(e.rowPtrs[e.Idx])
		chk.AppendRow(row)
		e.Idx++
	}
	return nil
}

func (e *TopNExec) loadChunksUntilTotalLimit(ctx context.Context) error {
	e.chkHeap = &topNChunkHeap{e}
	e.rowChunks = chunk.NewList(e.retTypes(), e.maxChunkSize)
	e.rowChunks.GetMemTracker().AttachTo(e.memTracker)
	e.rowChunks.GetMemTracker().SetLabel("rowChunks")
	for uint64(e.rowChunks.Len()) < e.totalLimit {
		srcChk := e.children[0].newChunk()
		err := e.children[0].Next(ctx, srcChk)
		if err != nil {
			return errors.Trace(err)
		}
		if srcChk.NumRows() == 0 {
			break
		}
		e.rowChunks.Add(srcChk)
	}
	e.initPointers()
	e.initCompareFuncs()
	allColumnExpr := e.buildKeyColumns()
	if !allColumnExpr {
		e.buildKeyExprsAndTypes()
		err := e.buildKeyChunks()
		if err != nil {
			return errors.Trace(err)
		}
	}
	return nil
}

const topNCompactionFactor = 4

func (e *TopNExec) executeTopN(ctx context.Context) error {
	heap.Init(e.chkHeap)
	for uint64(len(e.rowPtrs)) > e.totalLimit {
		// The number of rows we loaded may exceeds total limit, remove greatest rows by Pop.
		heap.Pop(e.chkHeap)
	}
	var childKeyChk *chunk.Chunk
	if e.keyChunks != nil {
		childKeyChk = chunk.NewChunk(e.keyTypes)
	}
	childRowChk := e.children[0].newChunk()
	for {
		err := e.children[0].Next(ctx, childRowChk)
		if err != nil {
			return errors.Trace(err)
		}
		if childRowChk.NumRows() == 0 {
			break
		}
		err = e.processChildChk(childRowChk, childKeyChk)
		if err != nil {
			return errors.Trace(err)
		}
		if e.rowChunks.Len() > len(e.rowPtrs)*topNCompactionFactor {
			err = e.doCompaction()
			if err != nil {
				return errors.Trace(err)
			}
		}
	}
	if e.keyChunks != nil {
		sort.Slice(e.rowPtrs, e.keyChunksLess)
	} else {
		sort.Slice(e.rowPtrs, e.keyColumnsLess)
	}
	return nil
}

func (e *TopNExec) processChildChk(childRowChk, childKeyChk *chunk.Chunk) error {
	if childKeyChk != nil {
		childKeyChk.Reset()
		err := expression.VectorizedExecute(e.ctx, e.keyExprs, chunk.NewIterator4Chunk(childRowChk), childKeyChk)
		if err != nil {
			return errors.Trace(err)
		}
	}
	for i := 0; i < childRowChk.NumRows(); i++ {
		heapMaxPtr := e.rowPtrs[0]
		var heapMax, next chunk.Row
		if childKeyChk != nil {
			heapMax = e.keyChunks.GetRow(heapMaxPtr)
			next = childKeyChk.GetRow(i)
		} else {
			heapMax = e.rowChunks.GetRow(heapMaxPtr)
			next = childRowChk.GetRow(i)
		}
		if e.chkHeap.greaterRow(heapMax, next) {
			// Evict heap max, keep the next row.
			e.rowPtrs[0] = e.rowChunks.AppendRow(childRowChk.GetRow(i))
			if childKeyChk != nil {
				e.keyChunks.AppendRow(childKeyChk.GetRow(i))
			}
			heap.Fix(e.chkHeap, 0)
		}
	}
	return nil
}

// doCompaction rebuild the chunks and row pointers to release memory.
// If we don't do compaction, in a extreme case like the child data is already ascending sorted
// but we want descending top N, then we will keep all data in memory.
// But if data is distributed randomly, this function will be called log(n) times.
func (e *TopNExec) doCompaction() error {
	newRowChunks := chunk.NewList(e.retTypes(), e.maxChunkSize)
	newRowPtrs := make([]chunk.RowPtr, 0, e.rowChunks.Len())
	for _, rowPtr := range e.rowPtrs {
		newRowPtr := newRowChunks.AppendRow(e.rowChunks.GetRow(rowPtr))
		newRowPtrs = append(newRowPtrs, newRowPtr)
	}
	newRowChunks.GetMemTracker().SetLabel("rowChunks")
	e.memTracker.ReplaceChild(e.rowChunks.GetMemTracker(), newRowChunks.GetMemTracker())
	e.rowChunks = newRowChunks

	if e.keyChunks != nil {
		newKeyChunks := chunk.NewList(e.keyTypes, e.maxChunkSize)
		for _, rowPtr := range e.rowPtrs {
			newKeyChunks.AppendRow(e.keyChunks.GetRow(rowPtr))
		}
		newKeyChunks.GetMemTracker().SetLabel("keyChunks")
		e.memTracker.ReplaceChild(e.keyChunks.GetMemTracker(), newKeyChunks.GetMemTracker())
		e.keyChunks = newKeyChunks
	}

	e.memTracker.Consume(int64(-8 * len(e.rowPtrs)))
	e.memTracker.Consume(int64(8 * len(newRowPtrs)))
	e.rowPtrs = newRowPtrs
	return nil
}