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259
vendor/github.com/cockroachdb/pebble/internal/rangekey/coalesce.go
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259
vendor/github.com/cockroachdb/pebble/internal/rangekey/coalesce.go
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// Copyright 2021 The LevelDB-Go and Pebble Authors. All rights reserved. Use
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// of this source code is governed by a BSD-style license that can be found in
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// the LICENSE file.
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package rangekey
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import (
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"bytes"
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"sort"
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"github.com/cockroachdb/pebble/internal/base"
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"github.com/cockroachdb/pebble/internal/invariants"
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"github.com/cockroachdb/pebble/internal/keyspan"
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)
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// InitUserIteration initializes the range key iterator stack for user
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// iteration. The resulting fragment iterator applies range key semantics,
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// defragments spans according to their user-observable state and removes all
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// Keys other than RangeKeySets describing the current state of range keys.
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//
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// The snapshot sequence number parameter determines which keys are visible. Any
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// keys not visible at the provided snapshot are ignored.
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func InitUserIteration(
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cmp base.Compare,
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snapshot uint64,
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miter *keyspan.MergingIter,
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diter *keyspan.DefragmentingIter,
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levelIters ...keyspan.FragmentIterator,
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) keyspan.FragmentIterator {
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miter.Init(cmp, userIterationTransform(snapshot), levelIters...)
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diter.Init(cmp, miter, userIterationDefragmenter(), keyspan.StaticDefragmentReducer)
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return diter
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}
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// userIterationTransform returns a keyspan.Transform for use with a
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// keyspan.MergingIter that transforms spans by resolving range keys at the
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// provided snapshot sequence number. Shadowing of keys is resolved (eg, removal
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// of unset keys, removal of keys overwritten by a set at the same suffix, etc)
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// and then non-RangeKeySet keys are removed. The resulting transformed spans
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// only contain RangeKeySets describing the state visible at the provided
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// sequence number.
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func userIterationTransform(snapshot uint64) keyspan.Transform {
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return func(cmp base.Compare, s keyspan.Span, dst *keyspan.Span) error {
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// Apply shadowing of keys.
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if err := Coalesce(cmp, s.Visible(snapshot), dst); err != nil {
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return err
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}
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// During user iteration over range keys, unsets and deletes don't
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// matter. Remove them. This step helps logical defragmentation during
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// iteration.
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keys := dst.Keys
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dst.Keys = dst.Keys[:0]
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for i := range keys {
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switch keys[i].Kind() {
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case base.InternalKeyKindRangeKeySet:
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dst.Keys = append(dst.Keys, keys[i])
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case base.InternalKeyKindRangeKeyUnset:
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// Skip.
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continue
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case base.InternalKeyKindRangeKeyDelete:
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// Skip.
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continue
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default:
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return base.CorruptionErrorf("pebble: unrecognized range key kind %s", keys[i].Kind())
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}
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}
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return nil
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}
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}
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// userIterationDefragmenter constructs a DefragmentMethod that configures a
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// DefragmentingIter to defragment spans of range keys if their user-visible
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// state is identical. This defragmenting method assumes the provided spans have
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// already been transformed through UserIterationTransform, so all RangeKeySets
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// are user-visible sets. This defragmenter checks for equality between set
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// suffixes and values (ignoring sequence numbers). It's intended for use during
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// user iteration, when the wrapped keyspan iterator is merging spans across all
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// levels of the LSM.
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//
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// The returned defragmenting method is stateful, and must not be used on
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// multiple DefragmentingIters concurrently.
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func userIterationDefragmenter() keyspan.DefragmentMethod {
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var bufA keysBySuffix
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var bufB keysBySuffix
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return func(cmp base.Compare, a, b keyspan.Span) bool {
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// UserIterationDefragmenter must only be used on spans that have
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// transformed by UserIterationTransform. The transform applies
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// shadowing and removes all keys besides the resulting Sets. Since
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// shadowing has been applied, each Set must set a unique suffix. If the
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// two spans are equivalent, they must have the same number of range key
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// sets.
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if len(a.Keys) != len(b.Keys) {
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return false
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}
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// The keys in both spans must be all sets. In order to be equivalent
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// during user iteration, they must set identical suffix-value mappings.
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// They do not need to have the same sequence number.
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//
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// Currently, the Keys are sorted by Trailer (as per the keyspan.Span
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// and keyspan.FragmentIterator interfaces). Collect all the keys in
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// both spans into buffers and sort them by suffix for comparison.
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bufA.cmp = cmp
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bufA.keys = append(bufA.keys[:0], a.Keys...)
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sort.Sort(bufA)
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bufB.cmp = cmp
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bufB.keys = append(bufB.keys[:0], b.Keys...)
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sort.Sort(bufB)
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for i := range bufA.keys {
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if bufA.keys[i].Kind() != base.InternalKeyKindRangeKeySet ||
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bufB.keys[i].Kind() != base.InternalKeyKindRangeKeySet {
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panic("pebble: unexpected non-RangeKeySet during defragmentation")
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}
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if cmp(bufA.keys[i].Suffix, bufB.keys[i].Suffix) != 0 {
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return false
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}
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if !bytes.Equal(bufA.keys[i].Value, bufB.keys[i].Value) {
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return false
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}
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}
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return true
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}
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}
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// Coalesce imposes range key semantics and coalesces range keys with the same
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// bounds. Coalesce drops any keys shadowed by more recent sets, unsets or
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// deletes. Coalesce modifies the provided span's Keys slice, reslicing the
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// slice to remove dropped keys.
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//
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// Coalescence has subtle behavior with respect to sequence numbers. Coalesce
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// depends on a keyspan.Span's Keys being sorted in sequence number descending
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// order. The first key has the largest sequence number. The returned coalesced
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// span includes only the largest sequence number. All other sequence numbers
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// are forgotten. When a compaction constructs output range keys from a
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// coalesced span, it produces at most one RANGEKEYSET, one RANGEKEYUNSET and
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// one RANGEKEYDEL. Each one of these keys adopt the largest sequence number.
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//
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// This has the potentially surprising effect of 'promoting' a key to a higher
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// sequence number. This is okay, because:
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// - There are no other overlapping keys within the coalesced span of
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// sequence numbers (otherwise they would be in the compaction, due to
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// the LSM invariant).
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// - Range key sequence numbers are never compared to point key sequence
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// numbers. Range keys and point keys have parallel existences.
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// - Compactions only coalesce within snapshot stripes.
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//
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// Additionally, internal range keys at the same sequence number have subtle
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// mechanics:
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// * RANGEKEYSETs shadow RANGEKEYUNSETs of the same suffix.
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// * RANGEKEYDELs only apply to keys at lower sequence numbers.
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// This is required for ingestion. Ingested sstables are assigned a single
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// sequence number for the file, at which all of the file's keys are visible.
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// The RANGEKEYSET, RANGEKEYUNSET and RANGEKEYDEL key kinds are ordered such
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// that among keys with equal sequence numbers (thus ordered by their kinds) the
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// keys do not affect one another. Ingested sstables are expected to be
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// consistent with respect to the set/unset suffixes: A given suffix should be
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// set or unset but not both.
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func Coalesce(cmp base.Compare, span keyspan.Span, dst *keyspan.Span) error {
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// TODO(jackson): Currently, Coalesce doesn't actually perform the sequence
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// number promotion described in the comment above.
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keysBySuffix := keysBySuffix{
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cmp: cmp,
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keys: dst.Keys[:0],
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}
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var deleted bool
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for i := 0; i < len(span.Keys) && !deleted; i++ {
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k := span.Keys[i]
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if invariants.Enabled && i > 0 && k.Trailer > span.Keys[i-1].Trailer {
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panic("pebble: invariant violation: span keys unordered")
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}
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// NB: Within a given sequence number, keys are ordered as:
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// RangeKeySet > RangeKeyUnset > RangeKeyDelete
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// This is significant, because this ensures that none of the range keys
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// sharing a sequence number shadow each other.
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switch k.Kind() {
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case base.InternalKeyKindRangeKeySet:
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n := len(keysBySuffix.keys)
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if keysBySuffix.get(n, k.Suffix) < n {
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// This suffix is already set or unset at a higher sequence
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// number. Skip.
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continue
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}
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keysBySuffix.keys = append(keysBySuffix.keys, k)
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sort.Sort(keysBySuffix)
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case base.InternalKeyKindRangeKeyUnset:
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n := len(keysBySuffix.keys)
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if keysBySuffix.get(n, k.Suffix) < n {
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// This suffix is already set or unset at a higher sequence
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// number. Skip.
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continue
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}
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keysBySuffix.keys = append(keysBySuffix.keys, k)
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sort.Sort(keysBySuffix)
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case base.InternalKeyKindRangeKeyDelete:
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// All remaining range keys in this span have been deleted by this
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// RangeKeyDelete. There's no need to continue looping, because all
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// the remaining keys are shadowed by this one. The for loop
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// condition will terminate when it sees the last key is a
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// range key deletion.
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keysBySuffix.keys = append(keysBySuffix.keys, k)
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deleted = true
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default:
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return base.CorruptionErrorf("pebble: unexpected range key kind %s", k.Kind())
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}
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}
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// Update the span with the (potentially reduced) keys slice, and re-sort it
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// by Trailer.
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*dst = keyspan.Span{
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Start: span.Start,
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End: span.End,
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Keys: keysBySuffix.keys,
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}
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keyspan.SortKeys(dst.Keys)
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return nil
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}
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// SortBySuffix sorts the provided keys by suffix.
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func SortBySuffix(cmp base.Compare, keys []keyspan.Key) {
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bySuffix := keysBySuffix{
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cmp: cmp,
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keys: keys,
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}
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sort.Sort(bySuffix)
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}
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type keysBySuffix struct {
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cmp base.Compare
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keys []keyspan.Key
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}
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// get searches for suffix among the first n keys in keys. If the suffix is
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// found, it returns the index of the item with the suffix. If the suffix is not
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// found, it returns n.
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func (s keysBySuffix) get(n int, suffix []byte) (i int) {
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// Binary search for the suffix to see if there's an existing key with the
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// suffix. Only binary search among the first n items. get is called while
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// appending new keys with suffixes that may sort before existing keys.
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// The n parameter indicates what portion of the keys slice is sorted and
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// may contain relevant keys.
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i = sort.Search(n, func(i int) bool {
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return s.cmp(s.keys[i].Suffix, suffix) >= 0
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})
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if i < n && s.cmp(s.keys[i].Suffix, suffix) == 0 {
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return i
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}
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return n
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}
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func (s keysBySuffix) Len() int { return len(s.keys) }
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func (s keysBySuffix) Less(i, j int) bool { return s.cmp(s.keys[i].Suffix, s.keys[j].Suffix) < 0 }
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func (s keysBySuffix) Swap(i, j int) { s.keys[i], s.keys[j] = s.keys[j], s.keys[i] }
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