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sort.go
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// Copyright 2015 The Cockroach Authors.
//
// 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.
//
// Author: Peter Mattis (peter@cockroachlabs.com)
package sql
import (
"container/heap"
"fmt"
"strconv"
"strings"
"github.com/cockroachdb/cockroach/roachpb"
"github.com/cockroachdb/cockroach/sql/parser"
"github.com/cockroachdb/cockroach/util/encoding"
"github.com/cockroachdb/cockroach/util/log"
)
// orderBy constructs a sortNode based on the ORDER BY clause.
//
// In the general case (SELECT/UNION/VALUES), we can sort by a column index or a
// column name.
//
// However, for a SELECT, we can also sort by the pre-alias column name (SELECT
// a AS b ORDER BY b) as well as expressions (SELECT a, b, ORDER BY a+b). In
// this case, construction of the sortNode might adjust the number of render
// targets in the selectNode if any ordering expressions are specified.
//
// TODO(dan): SQL also allows sorting a VALUES or UNION by an expression.
// Support this. It will reduce some of the special casing below, but requires a
// generalization of how to add derived columns to a SelectStatement.
func (p *planner) orderBy(orderBy parser.OrderBy, n planNode) (*sortNode, *roachpb.Error) {
if orderBy == nil {
return nil, nil
}
// We grab a copy of columns here because we might add new render targets
// below. This is the set of columns requested by the query.
columns := n.Columns()
numOriginalCols := len(columns)
if s, ok := n.(*selectNode); ok {
numOriginalCols = s.numOriginalCols
}
var ordering columnOrdering
for _, o := range orderBy {
index := -1
// Normalize the expression which has the side-effect of evaluating
// constant expressions and unwrapping expressions like "((a))" to "a".
expr := o.Expr
for {
if paren, ok := expr.(*parser.ParenExpr); ok {
expr = paren.Expr
} else {
break
}
}
// expr, err := p.parser.NormalizeExpr(p.evalCtx, o.Expr.(parser.TypedExpr))
// if err != nil {
// return nil, roachpb.NewError(err)
// }
if qname, ok := expr.(*parser.QualifiedName); ok {
if len(qname.Indirect) == 0 {
// Look for an output column that matches the qualified name. This
// handles cases like:
//
// SELECT a AS b FROM t ORDER BY b
target := NormalizeName(string(qname.Base))
for j, col := range columns {
if NormalizeName(col.Name) == target {
index = j
break
}
}
}
if s, ok := n.(*selectNode); ok && index == -1 {
// No output column matched the qualified name, so look for an existing
// render target that matches the column name. This handles cases like:
//
// SELECT a AS b FROM t ORDER BY a
if err := qname.NormalizeColumnName(); err != nil {
return nil, roachpb.NewError(err)
}
if qname.Table() == "" || equalName(s.table.alias, qname.Table()) {
qnameCol := NormalizeName(qname.Column())
for j, r := range s.render {
if qval, ok := r.(*qvalue); ok {
if NormalizeName(qval.colRef.get().Name) == qnameCol {
index = j
break
}
}
}
}
}
}
if index == -1 {
// The order by expression matched neither an output column nor an
// existing render target.
if col, err := colIndex(numOriginalCols, expr); err != nil {
return nil, roachpb.NewError(err)
} else if col >= 0 {
index = col
} else if s, ok := n.(*selectNode); ok {
// TODO(dan): Once we support VALUES (1), (2) ORDER BY 3*4, this type
// check goes away.
// Add a new render expression to use for ordering. This handles cases
// were the expression is either not a qualified name or is a qualified
// name that is otherwise not referenced by the query:
//
// SELECT a FROM t ORDER by b
// SELECT a, b FROM t ORDER by a+b
if err := s.addRender(parser.SelectExpr{Expr: expr}, parser.DummyInt); err != nil {
return nil, err
}
index = len(s.columns) - 1
} else {
return nil, roachpb.NewErrorf("column %s does not exist", expr)
}
}
direction := encoding.Ascending
if o.Direction == parser.Descending {
direction = encoding.Descending
}
ordering = append(ordering, columnOrderInfo{index, direction})
}
return &sortNode{columns: columns, ordering: ordering}, nil
}
// colIndex takes an expression that refers to a column using an integer, verifies it refers to a
// valid render target and returns the corresponding column index. For example:
// SELECT a from T ORDER by 1
// Here "1" refers to the first render target "a". The returned index is 0.
func colIndex(numOriginalCols int, expr parser.Expr) (int, error) {
typedExpr, err := parser.TypeConstants(expr)
if err != nil {
return 0, err
}
switch i := typedExpr.(type) {
case *parser.DInt:
index := int(*i)
if numCols := numOriginalCols; index < 1 || index > numCols {
return -1, fmt.Errorf("invalid column index: %d not in range [1, %d]", index, numCols)
}
return index - 1, nil
case parser.Datum:
return -1, fmt.Errorf("non-integer constant column index: %s", typedExpr)
default:
// expr doesn't look like a col index (i.e. not a constant).
return -1, nil
}
}
type sortNode struct {
plan planNode
columns []ResultColumn
ordering columnOrdering
pErr *roachpb.Error
needSort bool
sortStrategy sortingStrategy
valueIter valueIterator
explain explainMode
debugVals debugValues
}
func (n *sortNode) Columns() []ResultColumn {
return n.columns
}
func (n *sortNode) Ordering() orderingInfo {
if n == nil {
return orderingInfo{}
}
return orderingInfo{exactMatchCols: nil, ordering: n.ordering}
}
func (n *sortNode) Values() parser.DTuple {
// If an ordering expression was used the number of columns in each row might
// differ from the number of columns requested, so trim the result.
return n.valueIter.Values()[:len(n.columns)]
}
func (n *sortNode) MarkDebug(mode explainMode) {
if mode != explainDebug {
panic(fmt.Sprintf("unknown debug mode %d", mode))
}
n.explain = mode
n.plan.MarkDebug(mode)
}
func (n *sortNode) DebugValues() debugValues {
if n.explain != explainDebug {
panic(fmt.Sprintf("node not in debug mode (mode %d)", n.explain))
}
return n.debugVals
}
func (n *sortNode) PErr() *roachpb.Error {
return n.pErr
}
func (n *sortNode) ExplainPlan(_ bool) (name, description string, children []planNode) {
if n.needSort {
name = "sort"
} else {
name = "nosort"
}
columns := n.plan.Columns()
strs := make([]string, len(n.ordering))
for i, o := range n.ordering {
prefix := '+'
if o.direction == encoding.Descending {
prefix = '-'
}
strs[i] = fmt.Sprintf("%c%s", prefix, columns[o.colIdx].Name)
}
description = strings.Join(strs, ",")
switch ss := n.sortStrategy.(type) {
case *iterativeSortStrategy:
description = fmt.Sprintf("%s (iterative)", description)
case *sortTopKStrategy:
description = fmt.Sprintf("%s (top %d)", description, ss.topK)
}
return name, description, []planNode{n.plan}
}
func (n *sortNode) SetLimitHint(numRows int64, soft bool) {
if !n.needSort {
// The limit is only useful to the wrapped node if we don't need to sort.
n.plan.SetLimitHint(numRows, soft)
} else {
v := &valuesNode{ordering: n.ordering}
if soft {
n.sortStrategy = newIterativeSortStrategy(v)
} else {
n.sortStrategy = newSortTopKStrategy(v, numRows)
}
}
}
// wrap the supplied planNode with the sortNode if sorting is required.
func (n *sortNode) wrap(plan planNode) planNode {
if n != nil {
// Check to see if the requested ordering is compatible with the existing
// ordering.
existingOrdering := plan.Ordering()
if log.V(2) {
log.Infof("Sort: existing=%d desired=%d", existingOrdering, n.ordering)
}
match := computeOrderingMatch(n.ordering, existingOrdering, false)
if match < len(n.ordering) {
n.plan = plan
n.needSort = true
return n
}
if len(n.columns) < len(plan.Columns()) {
// No sorting required, but we have to strip off the extra render
// expressions we added.
n.plan = plan
return n
}
}
if log.V(2) {
log.Infof("Sort: no sorting required")
}
return plan
}
func (n *sortNode) Start() *roachpb.Error {
return n.plan.Start()
}
func (n *sortNode) Next() bool {
if n.pErr != nil {
return false
}
for n.needSort {
if v, ok := n.plan.(*valuesNode); ok {
// The plan we wrap is already a values node. Just sort it.
v.ordering = n.ordering
n.sortStrategy = newSortAllStrategy(v)
n.sortStrategy.Finish()
n.needSort = false
break
} else if n.sortStrategy == nil {
v := &valuesNode{ordering: n.ordering}
n.sortStrategy = newSortAllStrategy(v)
}
// TODO(andrei): If we're scanning an index with a prefix matching an
// ordering prefix, we should only accumulate values for equal fields
// in this prefix, then sort the accumulated chunk and output.
if !n.plan.Next() {
n.pErr = n.plan.PErr()
if n.pErr != nil {
return false
}
n.sortStrategy.Finish()
n.valueIter = n.sortStrategy
n.needSort = false
break
}
if n.explain == explainDebug {
n.debugVals = n.plan.DebugValues()
if n.debugVals.output != debugValueRow {
// Pass through non-row debug values.
return true
}
}
values := n.plan.Values()
n.sortStrategy.Add(values)
if n.explain == explainDebug {
// Emit a "buffered" row.
n.debugVals.output = debugValueBuffered
return true
}
}
if n.valueIter == nil {
n.valueIter = n.plan
}
if !n.valueIter.Next() {
if n.valueIter == n.plan {
n.pErr = n.plan.PErr()
}
return false
}
if n.explain == explainDebug {
n.debugVals = n.valueIter.DebugValues()
}
return true
}
// valueIterator provides iterative access to a value source's values and
// debug values. It is a subset of the planNode interface, so all methods
// should conform to the comments expressed in the planNode definition.
type valueIterator interface {
Next() bool
Values() parser.DTuple
DebugValues() debugValues
}
type sortingStrategy interface {
valueIterator
// Add adds a single value to the sortingStrategy. It guarantees that
// if it decided to store the provided value, that it will make a deep
// copy of it.
Add(parser.DTuple)
// Finish terminates the sorting strategy, allowing for postprocessing
// after all values have been provided to the strategy. The method should
// not be called more than once, and should only be called after all Add
// calls have occurred.
Finish()
}
// sortAllStrategy reads in all values into the wrapped valuesNode and
// uses sort.Sort to sort all values in-place. It has a worst-case time
// complexity of O(n*log(n)) and a worst-case space complexity of O(n).
//
// The strategy is intended to be used when all values need to be sorted.
type sortAllStrategy struct {
vNode *valuesNode
}
func newSortAllStrategy(vNode *valuesNode) sortingStrategy {
return &sortAllStrategy{
vNode: vNode,
}
}
func (ss *sortAllStrategy) Add(values parser.DTuple) {
valuesCopy := make(parser.DTuple, len(values))
copy(valuesCopy, values)
ss.vNode.rows = append(ss.vNode.rows, valuesCopy)
}
func (ss *sortAllStrategy) Finish() {
ss.vNode.SortAll()
}
func (ss *sortAllStrategy) Next() bool {
return ss.vNode.Next()
}
func (ss *sortAllStrategy) Values() parser.DTuple {
return ss.vNode.Values()
}
func (ss *sortAllStrategy) DebugValues() debugValues {
return ss.vNode.DebugValues()
}
// iterativeSortStrategy reads in all values into the wrapped valuesNode
// and turns the underlying slice into a min-heap. It then pops a value
// off of the heap for each call to Next, meaning that it only needs to
// sort the number of values needed, instead of the entire slice. If the
// underlying value source provides n rows and the strategy produce only
// k rows, it has a worst-case time complexity of O(n + k*log(n)) and a
// worst-case space complexity of O(n).
//
// The strategy is intended to be used when an unknown number of values
// need to be sorted, but that most likely not all values need to be sorted.
type iterativeSortStrategy struct {
vNode *valuesNode
lastVal parser.DTuple
nextRowIdx int
}
func newIterativeSortStrategy(vNode *valuesNode) sortingStrategy {
return &iterativeSortStrategy{
vNode: vNode,
}
}
func (ss *iterativeSortStrategy) Add(values parser.DTuple) {
valuesCopy := make(parser.DTuple, len(values))
copy(valuesCopy, values)
ss.vNode.rows = append(ss.vNode.rows, valuesCopy)
}
func (ss *iterativeSortStrategy) Finish() {
ss.vNode.InitMinHeap()
}
func (ss *iterativeSortStrategy) Next() bool {
if ss.vNode.Len() == 0 {
return false
}
ss.lastVal = ss.vNode.PopValues()
ss.nextRowIdx++
return true
}
func (ss *iterativeSortStrategy) Values() parser.DTuple {
return ss.lastVal
}
func (ss *iterativeSortStrategy) DebugValues() debugValues {
return debugValues{
rowIdx: ss.nextRowIdx - 1,
key: strconv.Itoa(ss.nextRowIdx - 1),
value: ss.lastVal.String(),
output: debugValueRow,
}
}
// sortTopKStrategy creates a max-heap in its wrapped valuesNode and keeps
// this heap populated with only the top k values seen. It accomplishes this
// by comparing new values (before the deep copy) with the top of the heap.
// If the new value is less than the current top, the top will be replaced
// and the heap will be fixed. If not, the new value is dropped. When finished,
// all values in the heap are popped, sorting the values correctly in-place.
// It has a worst-case time complexity of O(n*log(k)) and a worst-case space
// complexity of O(k).
//
// The strategy is intended to be used when exactly k values need to be sorted,
// where k is known before sorting begins.
//
// TODO(nvanbenschoten) There are better algorithms that can achieve a sorted
// top k in a worst-case time complexity of O(n + k*log(k)) while maintaining
// a worst-case space complexity of O(k). For instance, the top k can be found
// in linear time, and then this can be sorted in linearithmic time.
type sortTopKStrategy struct {
vNode *valuesNode
topK int64
}
func newSortTopKStrategy(vNode *valuesNode, topK int64) sortingStrategy {
ss := &sortTopKStrategy{
vNode: vNode,
topK: topK,
}
ss.vNode.InitMaxHeap()
return ss
}
func (ss *sortTopKStrategy) Add(values parser.DTuple) {
switch {
case int64(ss.vNode.Len()) < ss.topK:
// The first k values all go into the max-heap.
valuesCopy := make(parser.DTuple, len(values))
copy(valuesCopy, values)
ss.vNode.PushValues(valuesCopy)
case ss.vNode.ValuesLess(values, ss.vNode.rows[0]):
// Once the heap is full, only replace the top
// value if a new value is less than it. If so
// replace and fix the heap.
valuesCopy := make(parser.DTuple, len(values))
copy(valuesCopy, values)
ss.vNode.rows[0] = valuesCopy
heap.Fix(ss.vNode, 0)
}
}
func (ss *sortTopKStrategy) Finish() {
// Pop all values in the heap, resulting in the inverted ordering
// being sorted in reverse. Therefore, the slice is ordered correctly
// in-place.
origLen := ss.vNode.Len()
for ss.vNode.Len() > 0 {
heap.Pop(ss.vNode)
}
ss.vNode.rows = ss.vNode.rows[:origLen]
}
func (ss *sortTopKStrategy) Next() bool {
return ss.vNode.Next()
}
func (ss *sortTopKStrategy) Values() parser.DTuple {
return ss.vNode.Values()
}
func (ss *sortTopKStrategy) DebugValues() debugValues {
return ss.vNode.DebugValues()
}
// TODO(pmattis): If the result set is large, we might need to perform the
// sort on disk. There is no point in doing this while we're buffering the
// entire result set in memory. If/when we start streaming results back to
// the client we should revisit.
//
// type onDiskSortStrategy struct{}