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scan.go
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scan.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 (
"bytes"
"fmt"
"github.com/pkg/errors"
"golang.org/x/net/context"
"github.com/cockroachdb/cockroach/pkg/roachpb"
"github.com/cockroachdb/cockroach/pkg/sql/parser"
"github.com/cockroachdb/cockroach/pkg/sql/privilege"
"github.com/cockroachdb/cockroach/pkg/sql/sqlbase"
"github.com/cockroachdb/cockroach/pkg/util"
"github.com/cockroachdb/cockroach/pkg/util/tracing"
)
// A scanNode handles scanning over the key/value pairs for a table and
// reconstructing them into rows.
type scanNode struct {
p *planner
desc sqlbase.TableDescriptor
index *sqlbase.IndexDescriptor
// Set if an index was explicitly specified.
specifiedIndex *sqlbase.IndexDescriptor
// Set if the NO_INDEX_JOIN hint was given.
noIndexJoin bool
// The table columns, possibly including ones currently in schema changes.
cols []sqlbase.ColumnDescriptor
// There is a 1-1 correspondence between cols and resultColumns.
resultColumns sqlbase.ResultColumns
// Contains values for the current row. There is a 1-1 correspondence
// between resultColumns and values in row.
row parser.Datums
// For each column in resultColumns, indicates if the value is
// needed (used as an optimization when the upper layer doesn't need
// all values).
// TODO(radu/knz): currently the optimization always loads the
// entire row from KV and only skips unnecessary decodes to
// Datum. Investigate whether performance is to be gained (e.g. for
// tables with wide rows) by reading only certain columns from KV
// using point lookups instead of a single range lookup for the
// entire row.
valNeededForCol []bool
// Map used to get the index for columns in cols.
colIdxMap map[sqlbase.ColumnID]int
spans []roachpb.Span
isSecondaryIndex bool
reverse bool
ordering orderingInfo
explain explainMode
rowIndex int64 // the index of the current row
debugVals debugValues
// filter that can be evaluated using only this table/index; it contains
// parser.IndexedVar leaves generated using filterVars.
filter parser.TypedExpr
filterVars parser.IndexedVarHelper
scanInitialized bool
fetcher sqlbase.RowFetcher
// if non-zero, hardLimit indicates that the scanNode only needs to provide
// this many rows (after applying any filter). It is a "hard" guarantee that
// Next will only be called this many times.
hardLimit int64
// if non-zero, softLimit is an estimation that only this many rows (after
// applying any filter) might be needed. It is a (potentially optimistic)
// "hint". If hardLimit is set (non-zero), softLimit must be unset (zero).
softLimit int64
disableBatchLimits bool
scanVisibility scanVisibility
// This struct must be allocated on the heap and its location stay
// stable after construction because it implements
// IndexedVarContainer and the IndexedVar objects in sub-expressions
// will link to it by reference after checkRenderStar / analyzeExpr.
// Enforce this using NoCopy.
noCopy util.NoCopy
}
func (p *planner) Scan() *scanNode {
return &scanNode{p: p}
}
func (n *scanNode) Columns() sqlbase.ResultColumns {
return n.resultColumns
}
func (n *scanNode) Ordering() orderingInfo {
return n.ordering
}
func (n *scanNode) Values() parser.Datums {
return n.row
}
func (n *scanNode) MarkDebug(mode explainMode) {
if mode != explainDebug {
panic(fmt.Sprintf("unknown debug mode %d", mode))
}
n.explain = mode
}
func (n *scanNode) DebugValues() debugValues {
if n.explain != explainDebug {
panic(fmt.Sprintf("node not in debug mode (mode %d)", n.explain))
}
return n.debugVals
}
// disableBatchLimit disables the kvfetcher batch limits. Used for index-join,
// where we scan batches of unordered spans.
func (n *scanNode) disableBatchLimit() {
n.disableBatchLimits = true
n.hardLimit = 0
n.softLimit = 0
}
func (n *scanNode) Start(context.Context) error {
return n.fetcher.Init(&n.desc, n.colIdxMap, n.index, n.reverse, n.isSecondaryIndex, n.cols,
n.valNeededForCol, false /* returnRangeInfo */)
}
func (n *scanNode) Close(context.Context) {}
// initScan sets up the rowFetcher and starts a scan.
func (n *scanNode) initScan(ctx context.Context) error {
limitHint := n.limitHint()
if err := n.fetcher.StartScan(ctx, n.p.txn, n.spans, !n.disableBatchLimits, limitHint); err != nil {
return err
}
n.scanInitialized = true
return nil
}
func (n *scanNode) limitHint() int64 {
var limitHint int64
if n.hardLimit != 0 {
limitHint = n.hardLimit
if !isFilterTrue(n.filter) {
// The limit is hard, but it applies after the filter; read a multiple of
// the limit to avoid needing a second batch. The multiple should be an
// estimate for the selectivity of the filter, but we have no way of
// calculating that right now.
limitHint *= 2
}
} else {
// Like above, read a multiple of the limit when the limit is "soft".
limitHint = n.softLimit * 2
}
return limitHint
}
// debugNext is a helper function used by Next() when in explainDebug mode.
func (n *scanNode) debugNext(ctx context.Context) (bool, error) {
if n.hardLimit > 0 && n.rowIndex >= n.hardLimit {
return false, nil
}
// In debug mode, we output a set of debug values for each key.
n.debugVals.rowIdx = int(n.rowIndex)
var err error
var encRow sqlbase.EncDatumRow
n.debugVals.key, n.debugVals.value, encRow, err = n.fetcher.NextKeyDebug(ctx)
if err != nil || n.debugVals.key == "" {
return false, err
}
if encRow == nil {
n.row = nil
n.debugVals.output = debugValuePartial
return true, nil
}
datums := make(parser.Datums, len(encRow))
var da sqlbase.DatumAlloc
if err := sqlbase.EncDatumRowToDatums(datums, encRow, &da); err != nil {
return false, errors.Errorf("Could not decode row: %v", err)
}
n.row = datums
passesFilter, err := sqlbase.RunFilter(n.filter, &n.p.evalCtx)
if err != nil {
return false, err
}
if passesFilter {
n.debugVals.output = debugValueRow
} else {
n.debugVals.output = debugValueFiltered
}
n.rowIndex++
return true, nil
}
func (n *scanNode) Next(ctx context.Context) (bool, error) {
tracing.AnnotateTrace()
if !n.scanInitialized {
if err := n.initScan(ctx); err != nil {
return false, err
}
}
if n.explain == explainDebug {
return n.debugNext(ctx)
}
// We fetch one row at a time until we find one that passes the filter.
for n.hardLimit == 0 || n.rowIndex < n.hardLimit {
var err error
n.row, err = n.fetcher.NextRowDecoded(ctx)
if err != nil || n.row == nil {
return false, err
}
passesFilter, err := sqlbase.RunFilter(n.filter, &n.p.evalCtx)
if err != nil {
return false, err
}
if passesFilter {
n.rowIndex++
return true, nil
}
}
return false, nil
}
// Initializes a scanNode with a table descriptor.
func (n *scanNode) initTable(
p *planner,
desc *sqlbase.TableDescriptor,
indexHints *parser.IndexHints,
scanVisibility scanVisibility,
wantedColumns []parser.ColumnID,
) error {
n.desc = *desc
if !p.skipSelectPrivilegeChecks {
if err := p.CheckPrivilege(&n.desc, privilege.SELECT); err != nil {
return err
}
}
if indexHints != nil {
if err := n.lookupSpecifiedIndex(indexHints); err != nil {
return err
}
}
n.noIndexJoin = (indexHints != nil && indexHints.NoIndexJoin)
return n.initDescDefaults(scanVisibility, wantedColumns)
}
func (n *scanNode) lookupSpecifiedIndex(indexHints *parser.IndexHints) error {
if indexHints.Index != "" {
// Search index by name.
indexName := indexHints.Index.Normalize()
if indexName == parser.ReNormalizeName(n.desc.PrimaryIndex.Name) {
n.specifiedIndex = &n.desc.PrimaryIndex
} else {
for i := range n.desc.Indexes {
if indexName == parser.ReNormalizeName(n.desc.Indexes[i].Name) {
n.specifiedIndex = &n.desc.Indexes[i]
break
}
}
}
if n.specifiedIndex == nil {
return errors.Errorf("index \"%s\" not found", indexName)
}
} else if indexHints.IndexID != 0 {
// Search index by ID.
if n.desc.PrimaryIndex.ID == sqlbase.IndexID(indexHints.IndexID) {
n.specifiedIndex = &n.desc.PrimaryIndex
} else {
for i := range n.desc.Indexes {
if n.desc.Indexes[i].ID == sqlbase.IndexID(indexHints.IndexID) {
n.specifiedIndex = &n.desc.Indexes[i]
break
}
}
}
if n.specifiedIndex == nil {
return errors.Errorf("index %d not found", indexHints.IndexID)
}
}
return nil
}
// Either pick all columns or just those selected.
func filterColumns(
dst []sqlbase.ColumnDescriptor, wantedColumns []parser.ColumnID, src []sqlbase.ColumnDescriptor,
) ([]sqlbase.ColumnDescriptor, error) {
if wantedColumns == nil {
dst = append(dst, src...)
} else {
for _, wc := range wantedColumns {
found := false
for _, c := range src {
if c.ID == sqlbase.ColumnID(wc) {
dst = append(dst, c)
found = true
break
}
}
if !found {
return nil, errors.Errorf("column %d does not exist", wc)
}
}
dst = appendUnselectedColumns(dst, wantedColumns, src)
}
return dst, nil
}
// appendUnselectedColumns adds into dst all the column descriptors
// from src that are not already listed in wantedColumns. The added
// columns, if any, are marked "hidden".
func appendUnselectedColumns(
dst []sqlbase.ColumnDescriptor, wantedColumns []parser.ColumnID, src []sqlbase.ColumnDescriptor,
) []sqlbase.ColumnDescriptor {
for _, c := range src {
found := false
for _, wc := range wantedColumns {
if sqlbase.ColumnID(wc) == c.ID {
found = true
break
}
}
if !found {
col := c
col.Hidden = true
dst = append(dst, col)
}
}
return dst
}
// Initializes the column structures.
// An error may be returned only if wantedColumns is set.
func (n *scanNode) initDescDefaults(
scanVisibility scanVisibility, wantedColumns []parser.ColumnID,
) error {
n.scanVisibility = scanVisibility
n.index = &n.desc.PrimaryIndex
n.cols = make([]sqlbase.ColumnDescriptor, 0, len(n.desc.Columns)+len(n.desc.Mutations))
var err error
n.cols, err = filterColumns(n.cols, wantedColumns, n.desc.Columns)
if err != nil {
return err
}
switch scanVisibility {
case publicColumns:
// Mutations are invisible.
case publicAndNonPublicColumns:
for _, mutation := range n.desc.Mutations {
if c := mutation.GetColumn(); c != nil {
col := *c
// Even if the column is non-nullable it can be null in the
// middle of a schema change.
col.Nullable = true
n.cols = append(n.cols, col)
}
}
}
n.resultColumns = sqlbase.ResultColumnsFromColDescs(n.cols)
n.colIdxMap = make(map[sqlbase.ColumnID]int, len(n.cols))
for i, c := range n.cols {
n.colIdxMap[c.ID] = i
}
n.valNeededForCol = make([]bool, len(n.cols))
for i := range n.cols {
n.valNeededForCol[i] = true
}
n.row = make([]parser.Datum, len(n.cols))
n.filterVars = parser.MakeIndexedVarHelper(n, len(n.cols))
return nil
}
// initOrdering initializes the ordering info using the selected index. This
// must be called after index selection is performed.
func (n *scanNode) initOrdering(exactPrefix int) {
if n.index == nil {
return
}
n.ordering = n.computeOrdering(n.index, exactPrefix, n.reverse)
}
// computeOrdering calculates ordering information for table columns assuming that:
// - we scan a given index (potentially in reverse order), and
// - the first `exactPrefix` columns of the index each have an exact (single value) match
// (see orderingInfo).
func (n *scanNode) computeOrdering(
index *sqlbase.IndexDescriptor, exactPrefix int, reverse bool,
) orderingInfo {
var ordering orderingInfo
columnIDs, dirs := index.FullColumnIDs()
for i, colID := range columnIDs {
idx, ok := n.colIdxMap[colID]
if !ok {
panic(fmt.Sprintf("index refers to unknown column id %d", colID))
}
if i < exactPrefix {
ordering.addExactMatchColumn(idx)
} else {
dir := dirs[i]
if reverse {
dir = dir.Reverse()
}
ordering.addColumn(idx, dir)
}
}
// We included any implicit columns, so the results are unique.
ordering.unique = true
return ordering
}
func (n *scanNode) Spans(ctx context.Context) (reads, writes roachpb.Spans, err error) {
return n.spans, nil, nil
}
// scanNode implements parser.IndexedVarContainer.
var _ parser.IndexedVarContainer = &scanNode{}
func (n *scanNode) IndexedVarEval(idx int, ctx *parser.EvalContext) (parser.Datum, error) {
return n.row[idx].Eval(ctx)
}
func (n *scanNode) IndexedVarResolvedType(idx int) parser.Type {
return n.resultColumns[idx].Typ
}
func (n *scanNode) IndexedVarFormat(buf *bytes.Buffer, _ parser.FmtFlags, idx int) {
buf.WriteString(n.resultColumns[idx].Name)
}
// scanVisibility represents which table columns should be included in a scan.
type scanVisibility int
const (
publicColumns scanVisibility = 0
publicAndNonPublicColumns scanVisibility = 1
)