ceremonyclient/pebble/internal/keyspan/merging_iter.go

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2024-01-03 07:31:42 +00:00
// Copyright 2022 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
package keyspan
import (
"bytes"
"fmt"
"sort"
"github.com/cockroachdb/pebble/internal/base"
"github.com/cockroachdb/pebble/internal/invariants"
"github.com/cockroachdb/pebble/internal/manifest"
)
// TODO(jackson): Consider implementing an optimization to seek lower levels
// past higher levels' RANGEKEYDELs. This would be analaogous to the
// optimization pebble.mergingIter performs for RANGEDELs during point key
// seeks. It may not be worth it, because range keys are rare and cascading
// seeks would require introducing key comparisons to switchTo{Min,Max}Heap
// where there currently are none.
// TODO(jackson): There are several opportunities to use base.Equal in the
// MergingIter implementation, but will require a bit of plumbing to thread the
// Equal function.
// MergingIter merges spans across levels of the LSM, exposing an iterator over
// spans that yields sets of spans fragmented at unique user key boundaries.
//
// A MergingIter is initialized with an arbitrary number of child iterators over
// fragmented spans. Each child iterator exposes fragmented key spans, such that
// overlapping keys are surfaced in a single Span. Key spans from one child
// iterator may overlap key spans from another child iterator arbitrarily.
//
// The spans combined by MergingIter will return spans with keys sorted by
// trailer descending. If the MergingIter is configured with a Transformer, it's
// permitted to modify the ordering of the spans' keys returned by MergingIter.
//
// # Algorithm
//
// The merging iterator wraps child iterators, merging and fragmenting spans
// across levels. The high-level algorithm is:
//
// 1. Initialize the heap with bound keys from child iterators' spans.
// 2. Find the next [or previous] two unique user keys' from bounds.
// 3. Consider the span formed between the two unique user keys a candidate
// span.
// 4. Determine if any of the child iterators' spans overlap the candidate
// span.
// 4a. If any of the child iterator's current bounds are end keys
// (during forward iteration) or start keys (during reverse
// iteration), then all the spans with that bound overlap the
// candidate span.
// 4b. Apply the configured transform, which may remove keys.
// 4c. If no spans overlap, forget the smallest (forward iteration)
// or largest (reverse iteration) unique user key and advance
// the iterators to the next unique user key. Start again from 3.
//
// # Detailed algorithm
//
// Each level (i0, i1, ...) has a user-provided input FragmentIterator. The
// merging iterator steps through individual boundaries of the underlying
// spans separately. If the underlying FragmentIterator has fragments
// [a,b){#2,#1} [b,c){#1} the mergingIterLevel.{next,prev} step through:
//
// (a, start), (b, end), (b, start), (c, end)
//
// Note that (a, start) and (b, end) are observed ONCE each, despite two keys
// sharing those bounds. Also note that (b, end) and (b, start) are two distinct
// iterator positions of a mergingIterLevel.
//
// The merging iterator maintains a heap (min during forward iteration, max
// during reverse iteration) containing the boundKeys. Each boundKey is a
// 3-tuple holding the bound user key, whether the bound is a start or end key
// and the set of keys from that level that have that bound. The heap orders
// based on the boundKey's user key only.
//
// The merging iterator is responsible for merging spans across levels to
// determine which span is next, but it's also responsible for fragmenting
// overlapping spans. Consider the example:
//
// i0: b---d e-----h
// i1: a---c h-----k
// i2: a------------------------------p
//
// fragments: a-b-c-d-e-----h-----k----------p
//
// None of the individual child iterators contain a span with the exact bounds
// [c,d), but the merging iterator must produce a span [c,d). To accomplish
// this, the merging iterator visits every span between unique boundary user
// keys. In the above example, this is:
//
// [a,b), [b,c), [c,d), [d,e), [e, h), [h, k), [k, p)
//
// The merging iterator first initializes the heap to prepare for iteration.
// The description below discusses the mechanics of forward iteration after a
// call to First, but the mechanics are similar for reverse iteration and
// other positioning methods.
//
// During a call to First, the heap is initialized by seeking every
// mergingIterLevel to the first bound of the first fragment. In the above
// example, this seeks the child iterators to:
//
// i0: (b, boundKindFragmentStart, [ [b,d) ])
// i1: (a, boundKindFragmentStart, [ [a,c) ])
// i2: (a, boundKindFragmentStart, [ [a,p) ])
//
// After fixing up the heap, the root of the heap is a boundKey with the
// smallest user key ('a' in the example). Once the heap is setup for iteration
// in the appropriate direction and location, the merging iterator uses
// find{Next,Prev}FragmentSet to find the next/previous span bounds.
//
// During forward iteration, the root of the heap's user key is the start key
// key of next merged span. findNextFragmentSet sets m.start to this user
// key. The heap may contain other boundKeys with the same user key if another
// level has a fragment starting or ending at the same key, so the
// findNextFragmentSet method pulls from the heap until it finds the first key
// greater than m.start. This key is used as the end key.
//
// In the above example, this results in m.start = 'a', m.end = 'b' and child
// iterators in the following positions:
//
// i0: (b, boundKindFragmentStart, [ [b,d) ])
// i1: (c, boundKindFragmentEnd, [ [a,c) ])
// i2: (p, boundKindFragmentEnd, [ [a,p) ])
//
// With the user key bounds of the next merged span established,
// findNextFragmentSet must determine which, if any, fragments overlap the span.
// During forward iteration any child iterator that is now positioned at an end
// boundary has an overlapping span. (Justification: The child iterator's end
// boundary is ≥ m.end. The corresponding start boundary must be ≤ m.start since
// there were no other user keys between m.start and m.end. So the fragments
// associated with the iterator's current end boundary have start and end bounds
// such that start ≤ m.start < m.end ≤ end).
//
// findNextFragmentSet iterates over the levels, collecting keys from any child
// iterators positioned at end boundaries. In the above example, i1 and i2 are
// positioned at end boundaries, so findNextFragmentSet collects the keys of
// [a,c) and [a,p). These spans contain the merging iterator's [m.start, m.end)
// span, but they may also extend beyond the m.start and m.end. The merging
// iterator returns the keys with the merging iter's m.start and m.end bounds,
// preserving the underlying keys' sequence numbers, key kinds and values.
//
// A MergingIter is configured with a Transform that's applied to the span
// before surfacing it to the iterator user. A Transform may remove keys
// arbitrarily, but it may not modify the values themselves.
//
// It may be the case that findNextFragmentSet finds no levels positioned at end
// boundaries, or that there are no spans remaining after applying a transform,
// in which case the span [m.start, m.end) overlaps with nothing. In this case
// findNextFragmentSet loops, repeating the above process again until it finds a
// span that does contain keys.
//
// # Memory safety
//
// The FragmentIterator interface only guarantees stability of a Span and its
// associated slices until the next positioning method is called. Adjacent Spans
// may be contained in different sstables, requring the FragmentIterator
// implementation to close one sstable, releasing its memory, before opening the
// next. Most of the state used by the MergingIter is derived from spans at
// current child iterator positions only, ensuring state is stable. The one
// exception is the start bound during forward iteration and the end bound
// during reverse iteration.
//
// If the heap root originates from an end boundary when findNextFragmentSet
// begins, a Next on the heap root level may invalidate the end boundary. To
// accommodate this, find{Next,Prev}FragmentSet copy the initial boundary if the
// subsequent Next/Prev would move to the next span.
type MergingIter struct {
*MergingBuffers
// start and end hold the bounds for the span currently under the
// iterator position.
//
// Invariant: None of the levels' iterators contain spans with a bound
// between start and end. For all bounds b, b ≤ start || b ≥ end.
start, end []byte
// transformer defines a transformation to be applied to a span before it's
// yielded to the user. Transforming may filter individual keys contained
// within the span.
transformer Transformer
// span holds the iterator's current span. This span is used as the
// destination for transforms. Every tranformed span overwrites the
// previous.
span Span
err error
dir int8
// alloc preallocates mergingIterLevel and mergingIterItems for use by the
// merging iterator. As long as the merging iterator is used with
// manifest.NumLevels+3 and fewer fragment iterators, the merging iterator
// will not need to allocate upon initialization. The value NumLevels+3
// mirrors the preallocated levels in iterAlloc used for point iterators.
// Invariant: cap(levels) == cap(items)
alloc struct {
levels [manifest.NumLevels + 3]mergingIterLevel
items [manifest.NumLevels + 3]mergingIterItem
}
}
// MergingBuffers holds buffers used while merging keyspans.
type MergingBuffers struct {
// keys holds all of the keys across all levels that overlap the key span
// [start, end), sorted by Trailer descending. This slice is reconstituted
// in synthesizeKeys from each mergingIterLevel's keys every time the
// [start, end) bounds change.
//
// Each element points into a child iterator's memory, so the keys may not
// be directly modified.
keys keysBySeqNumKind
// levels holds levels allocated by MergingIter.init. The MergingIter will
// prefer use of its `manifest.NumLevels+3` array, so this slice will be
// longer if set.
levels []mergingIterLevel
// heap holds a slice for the merging iterator heap allocated by
// MergingIter.init. The MergingIter will prefer use of its
// `manifest.NumLevels+3` items array, so this slice will be longer if set.
heap mergingIterHeap
// buf is a buffer used to save [start, end) boundary keys.
buf []byte
}
// PrepareForReuse discards any excessively large buffers.
func (bufs *MergingBuffers) PrepareForReuse() {
if cap(bufs.buf) > bufferReuseMaxCapacity {
bufs.buf = nil
}
}
// MergingIter implements the FragmentIterator interface.
var _ FragmentIterator = (*MergingIter)(nil)
type mergingIterLevel struct {
iter FragmentIterator
// heapKey holds the current key at this level for use within the heap.
heapKey boundKey
}
func (l *mergingIterLevel) next() {
if l.heapKey.kind == boundKindFragmentStart {
l.heapKey = boundKey{
kind: boundKindFragmentEnd,
key: l.heapKey.span.End,
span: l.heapKey.span,
}
return
}
if s := l.iter.Next(); s == nil {
l.heapKey = boundKey{kind: boundKindInvalid}
} else {
l.heapKey = boundKey{
kind: boundKindFragmentStart,
key: s.Start,
span: s,
}
}
}
func (l *mergingIterLevel) prev() {
if l.heapKey.kind == boundKindFragmentEnd {
l.heapKey = boundKey{
kind: boundKindFragmentStart,
key: l.heapKey.span.Start,
span: l.heapKey.span,
}
return
}
if s := l.iter.Prev(); s == nil {
l.heapKey = boundKey{kind: boundKindInvalid}
} else {
l.heapKey = boundKey{
kind: boundKindFragmentEnd,
key: s.End,
span: s,
}
}
}
// Init initializes the merging iterator with the provided fragment iterators.
func (m *MergingIter) Init(
cmp base.Compare, transformer Transformer, bufs *MergingBuffers, iters ...FragmentIterator,
) {
*m = MergingIter{
MergingBuffers: bufs,
transformer: transformer,
}
m.heap.cmp = cmp
levels, items := m.levels, m.heap.items
// Invariant: cap(levels) >= cap(items)
// Invariant: cap(alloc.levels) == cap(alloc.items)
if len(iters) <= len(m.alloc.levels) {
// The slices allocated on the MergingIter struct are large enough.
m.levels = m.alloc.levels[:len(iters)]
m.heap.items = m.alloc.items[:0]
} else if len(iters) <= cap(levels) {
// The existing heap-allocated slices are large enough, so reuse them.
m.levels = levels[:len(iters)]
m.heap.items = items[:0]
} else {
// Heap allocate new slices.
m.levels = make([]mergingIterLevel, len(iters))
m.heap.items = make([]mergingIterItem, 0, len(iters))
}
for i := range m.levels {
m.levels[i] = mergingIterLevel{iter: iters[i]}
}
}
// AddLevel adds a new level to the bottom of the merging iterator. AddLevel
// must be called after Init and before any other method.
func (m *MergingIter) AddLevel(iter FragmentIterator) {
m.levels = append(m.levels, mergingIterLevel{iter: iter})
}
// SeekGE moves the iterator to the first span covering a key greater than
// or equal to the given key. This is equivalent to seeking to the first
// span with an end key greater than the given key.
func (m *MergingIter) SeekGE(key []byte) *Span {
m.invalidate() // clear state about current position
// SeekGE(k) seeks to the first span with an end key greater than the given
// key. The merged span M that we're searching for might straddle the seek
// `key`. In this case, the M.Start may be a key ≤ the seek key.
//
// Consider a SeekGE(dog) in the following example.
//
// i0: b---d e-----h
// i1: a---c h-----k
// i2: a------------------------------p
// merged: a-b-c-d-e-----h-----k----------p
//
// The merged span M containing 'dog' is [d,e). The 'd' of the merged span
// comes from i0's [b,d)'s end boundary. The [b,d) span does not cover any
// key >= dog, so we cannot find the span by positioning the child iterators
// using a SeekGE(dog).
//
// Instead, if we take all the child iterators' spans bounds:
// a b c d e h k p
// We want to partition them into keys ≤ `key` and keys > `key`.
// dog
// │
// a b c d│e h k p
// │
// The largest key on the left of the partition forms the merged span's
// start key, and the smallest key on the right of the partition forms the
// merged span's end key. Recharacterized:
//
// M.Start: the largest boundary ≤ k of any child span
// M.End: the smallest boundary > k of any child span
//
// The FragmentIterator interface doesn't implement seeking by all bounds,
// it implements seeking by containment. A SeekGE(k) will ensure we observe
// all start boundaries ≥ k and all end boundaries > k but does not ensure
// we observe end boundaries = k or any boundaries < k. A SeekLT(k) will
// ensure we observe all start boundaries < k and all end boundaries ≤ k but
// does not ensure we observe any start boundaries = k or any boundaries >
// k. This forces us to seek in one direction and step in the other.
//
// In a SeekGE, we want to end up oriented in the forward direction when
// complete, so we begin with searching for M.Start by SeekLT-ing every
// child iterator to `k`. For every child span found, we determine the
// largest bound ≤ `k` and use it to initialize our max heap. The resulting
// root of the max heap is a preliminary value for `M.Start`.
for i := range m.levels {
l := &m.levels[i]
s := l.iter.SeekLT(key)
if s == nil {
l.heapKey = boundKey{kind: boundKindInvalid}
} else if m.cmp(s.End, key) <= 0 {
l.heapKey = boundKey{
kind: boundKindFragmentEnd,
key: s.End,
span: s,
}
} else {
// s.End > key && s.Start < key
// We need to use this span's start bound, since that's the largest
// bound ≤ key.
l.heapKey = boundKey{
kind: boundKindFragmentStart,
key: s.Start,
span: s,
}
}
}
m.initMaxHeap()
if m.err != nil {
return nil
} else if len(m.heap.items) == 0 {
// There are no spans covering any key < `key`. There is no span that
// straddles the seek key. Reorient the heap into a min heap and return
// the first span we find in the forward direction.
m.switchToMinHeap()
return m.findNextFragmentSet()
}
// The heap root is now the largest boundary key b such that:
// 1. b < k
// 2. b = k, and b is an end boundary
// There's a third case that we will need to consider later, after we've
// switched to a min heap:
// 3. there exists a start boundary key b such that b = k.
// A start boundary key equal to k would not be surfaced when we seeked all
// the levels using SeekLT(k), since no key <k would be covered within a
// span within an inclusive `k` start boundary.
//
// Assume that the tightest boundary ≤ k is the current heap root (cases 1 &
// 2). After we switch to a min heap, we'll check for the third case and
// adjust the start boundary if necessary.
m.start = m.heap.items[0].boundKey.key
// Before switching the direction of the heap, save a copy of the start
// boundary if it's the end boundary of some child span. Next-ing the child
// iterator might switch files and invalidate the memory of the bound.
if m.heap.items[0].boundKey.kind == boundKindFragmentEnd {
m.buf = append(m.buf[:0], m.start...)
m.start = m.buf
}
// Switch to a min heap. This will move each level to the next bound in
// every level, and then establish a min heap. This allows us to obtain the
// smallest boundary key > `key`, which will serve as our candidate end
// bound.
m.switchToMinHeap()
if m.err != nil {
return nil
} else if len(m.heap.items) == 0 {
return nil
}
// Check for the case 3 described above. It's possible that when we switch
// heap directions, we discover a start boundary of some child span that is
// equal to the seek key `key`. In this case, we want this key to be our
// start boundary.
if m.heap.items[0].boundKey.kind == boundKindFragmentStart &&
m.cmp(m.heap.items[0].boundKey.key, key) == 0 {
// Call findNextFragmentSet, which will set m.start to the heap root and
// proceed forward.
return m.findNextFragmentSet()
}
m.end = m.heap.items[0].boundKey.key
if found, s := m.synthesizeKeys(+1); found && s != nil {
return s
}
return m.findNextFragmentSet()
}
// SeekLT moves the iterator to the last span covering a key less than the
// given key. This is equivalent to seeking to the last span with a start
// key less than the given key.
func (m *MergingIter) SeekLT(key []byte) *Span {
m.invalidate() // clear state about current position
// SeekLT(k) seeks to the last span with a start key less than the given
// key. The merged span M that we're searching for might straddle the seek
// `key`. In this case, the M.End may be a key ≥ the seek key.
//
// Consider a SeekLT(dog) in the following example.
//
// i0: b---d e-----h
// i1: a---c h-----k
// i2: a------------------------------p
// merged: a-b-c-d-e-----h-----k----------p
//
// The merged span M containing the largest key <'dog' is [d,e). The 'e' of
// the merged span comes from i0's [e,h)'s start boundary. The [e,h) span
// does not cover any key < dog, so we cannot find the span by positioning
// the child iterators using a SeekLT(dog).
//
// Instead, if we take all the child iterators' spans bounds:
// a b c d e h k p
// We want to partition them into keys < `key` and keys ≥ `key`.
// dog
// │
// a b c d│e h k p
// │
// The largest key on the left of the partition forms the merged span's
// start key, and the smallest key on the right of the partition forms the
// merged span's end key. Recharacterized:
//
// M.Start: the largest boundary < k of any child span
// M.End: the smallest boundary ≥ k of any child span
//
// The FragmentIterator interface doesn't implement seeking by all bounds,
// it implements seeking by containment. A SeekGE(k) will ensure we observe
// all start boundaries ≥ k and all end boundaries > k but does not ensure
// we observe end boundaries = k or any boundaries < k. A SeekLT(k) will
// ensure we observe all start boundaries < k and all end boundaries ≤ k but
// does not ensure we observe any start boundaries = k or any boundaries >
// k. This forces us to seek in one direction and step in the other.
//
// In a SeekLT, we want to end up oriented in the backward direction when
// complete, so we begin with searching for M.End by SeekGE-ing every
// child iterator to `k`. For every child span found, we determine the
// smallest bound ≥ `k` and use it to initialize our min heap. The resulting
// root of the min heap is a preliminary value for `M.End`.
for i := range m.levels {
l := &m.levels[i]
s := l.iter.SeekGE(key)
if s == nil {
l.heapKey = boundKey{kind: boundKindInvalid}
} else if m.cmp(s.Start, key) >= 0 {
l.heapKey = boundKey{
kind: boundKindFragmentStart,
key: s.Start,
span: s,
}
} else {
// s.Start < key
// We need to use this span's end bound, since that's the smallest
// bound > key.
l.heapKey = boundKey{
kind: boundKindFragmentEnd,
key: s.End,
span: s,
}
}
}
m.initMinHeap()
if m.err != nil {
return nil
} else if len(m.heap.items) == 0 {
// There are no spans covering any key ≥ `key`. There is no span that
// straddles the seek key. Reorient the heap into a max heap and return
// the first span we find in the reverse direction.
m.switchToMaxHeap()
return m.findPrevFragmentSet()
}
// The heap root is now the smallest boundary key b such that:
// 1. b > k
// 2. b = k, and b is a start boundary
// There's a third case that we will need to consider later, after we've
// switched to a max heap:
// 3. there exists an end boundary key b such that b = k.
// An end boundary key equal to k would not be surfaced when we seeked all
// the levels using SeekGE(k), since k would not be contained within the
// exclusive end boundary.
//
// Assume that the tightest boundary ≥ k is the current heap root (cases 1 &
// 2). After we switch to a max heap, we'll check for the third case and
// adjust the end boundary if necessary.
m.end = m.heap.items[0].boundKey.key
// Before switching the direction of the heap, save a copy of the end
// boundary if it's the start boundary of some child span. Prev-ing the
// child iterator might switch files and invalidate the memory of the bound.
if m.heap.items[0].boundKey.kind == boundKindFragmentStart {
m.buf = append(m.buf[:0], m.end...)
m.end = m.buf
}
// Switch to a max heap. This will move each level to the previous bound in
// every level, and then establish a max heap. This allows us to obtain the
// largest boundary key < `key`, which will serve as our candidate start
// bound.
m.switchToMaxHeap()
if m.err != nil {
return nil
} else if len(m.heap.items) == 0 {
return nil
}
// Check for the case 3 described above. It's possible that when we switch
// heap directions, we discover an end boundary of some child span that is
// equal to the seek key `key`. In this case, we want this key to be our end
// boundary.
if m.heap.items[0].boundKey.kind == boundKindFragmentEnd &&
m.cmp(m.heap.items[0].boundKey.key, key) == 0 {
// Call findPrevFragmentSet, which will set m.end to the heap root and
// proceed backwards.
return m.findPrevFragmentSet()
}
m.start = m.heap.items[0].boundKey.key
if found, s := m.synthesizeKeys(-1); found && s != nil {
return s
}
return m.findPrevFragmentSet()
}
// First seeks the iterator to the first span.
func (m *MergingIter) First() *Span {
m.invalidate() // clear state about current position
for i := range m.levels {
if s := m.levels[i].iter.First(); s == nil {
m.levels[i].heapKey = boundKey{kind: boundKindInvalid}
} else {
m.levels[i].heapKey = boundKey{
kind: boundKindFragmentStart,
key: s.Start,
span: s,
}
}
}
m.initMinHeap()
return m.findNextFragmentSet()
}
// Last seeks the iterator to the last span.
func (m *MergingIter) Last() *Span {
m.invalidate() // clear state about current position
for i := range m.levels {
if s := m.levels[i].iter.Last(); s == nil {
m.levels[i].heapKey = boundKey{kind: boundKindInvalid}
} else {
m.levels[i].heapKey = boundKey{
kind: boundKindFragmentEnd,
key: s.End,
span: s,
}
}
}
m.initMaxHeap()
return m.findPrevFragmentSet()
}
// Next advances the iterator to the next span.
func (m *MergingIter) Next() *Span {
if m.err != nil {
return nil
}
if m.dir == +1 && (m.end == nil || m.start == nil) {
return nil
}
if m.dir != +1 {
m.switchToMinHeap()
}
return m.findNextFragmentSet()
}
// Prev advances the iterator to the previous span.
func (m *MergingIter) Prev() *Span {
if m.err != nil {
return nil
}
if m.dir == -1 && (m.end == nil || m.start == nil) {
return nil
}
if m.dir != -1 {
m.switchToMaxHeap()
}
return m.findPrevFragmentSet()
}
// Error returns any accumulated error.
func (m *MergingIter) Error() error {
if m.heap.len() == 0 || m.err != nil {
return m.err
}
return m.levels[m.heap.items[0].index].iter.Error()
}
// Close closes the iterator, releasing all acquired resources.
func (m *MergingIter) Close() error {
for i := range m.levels {
if err := m.levels[i].iter.Close(); err != nil && m.err == nil {
m.err = err
}
}
m.levels = nil
m.heap.items = m.heap.items[:0]
return m.err
}
// String implements fmt.Stringer.
func (m *MergingIter) String() string {
return "merging-keyspan"
}
func (m *MergingIter) initMinHeap() {
m.dir = +1
m.heap.reverse = false
m.initHeap()
}
func (m *MergingIter) initMaxHeap() {
m.dir = -1
m.heap.reverse = true
m.initHeap()
}
func (m *MergingIter) initHeap() {
m.heap.items = m.heap.items[:0]
for i := range m.levels {
if l := &m.levels[i]; l.heapKey.kind != boundKindInvalid {
m.heap.items = append(m.heap.items, mergingIterItem{
index: i,
boundKey: &l.heapKey,
})
} else {
m.err = firstError(m.err, l.iter.Error())
if m.err != nil {
return
}
}
}
m.heap.init()
}
func (m *MergingIter) switchToMinHeap() {
// switchToMinHeap reorients the heap for forward iteration, without moving
// the current MergingIter position.
// The iterator is currently positioned at the span [m.start, m.end),
// oriented in the reverse direction, so each level's iterator is positioned
// to the largest key ≤ m.start. To reorient in the forward direction, we
// must advance each level's iterator to the smallest key ≥ m.end. Consider
// this three-level example.
//
// i0: b---d e-----h
// i1: a---c h-----k
// i2: a------------------------------p
//
// merged: a-b-c-d-e-----h-----k----------p
//
// If currently positioned at the merged span [c,d), then the level
// iterators' heap keys are:
//
// i0: (b, [b, d)) i1: (c, [a,c)) i2: (a, [a,p))
//
// Reversing the heap should not move the merging iterator and should not
// change the current [m.start, m.end) bounds. It should only prepare for
// forward iteration by updating the child iterators' heap keys to:
//
// i0: (d, [b, d)) i1: (h, [h,k)) i2: (p, [a,p))
//
// In every level the first key ≥ m.end is the next in the iterator.
// Justification: Suppose not and a level iterator's next key was some key k
// such that k < m.end. The max-heap invariant dictates that the current
// iterator position is the largest entry with a user key ≥ m.start. This
// means k > m.start. We started with the assumption that k < m.end, so
// m.start < k < m.end. But then k is between our current span bounds,
// and reverse iteration would have constructed the current interval to be
// [k, m.end) not [m.start, m.end).
if invariants.Enabled {
for i := range m.levels {
l := &m.levels[i]
if l.heapKey.kind != boundKindInvalid && m.cmp(l.heapKey.key, m.start) > 0 {
panic("pebble: invariant violation: max-heap key > m.start")
}
}
}
for i := range m.levels {
m.levels[i].next()
}
m.initMinHeap()
}
func (m *MergingIter) switchToMaxHeap() {
// switchToMaxHeap reorients the heap for reverse iteration, without moving
// the current MergingIter position.
// The iterator is currently positioned at the span [m.start, m.end),
// oriented in the forward direction. Each level's iterator is positioned at
// the smallest bound ≥ m.end. To reorient in the reverse direction, we must
// move each level's iterator to the largest key ≤ m.start. Consider this
// three-level example.
//
// i0: b---d e-----h
// i1: a---c h-----k
// i2: a------------------------------p
//
// merged: a-b-c-d-e-----h-----k----------p
//
// If currently positioned at the merged span [c,d), then the level
// iterators' heap keys are:
//
// i0: (d, [b, d)) i1: (h, [h,k)) i2: (p, [a,p))
//
// Reversing the heap should not move the merging iterator and should not
// change the current [m.start, m.end) bounds. It should only prepare for
// reverse iteration by updating the child iterators' heap keys to:
//
// i0: (b, [b, d)) i1: (c, [a,c)) i2: (a, [a,p))
//
// In every level the largest key ≤ m.start is the prev in the iterator.
// Justification: Suppose not and a level iterator's prev key was some key k
// such that k > m.start. The min-heap invariant dictates that the current
// iterator position is the smallest entry with a user key ≥ m.end. This
// means k < m.end, otherwise the iterator would be positioned at k. We
// started with the assumption that k > m.start, so m.start < k < m.end. But
// then k is between our current span bounds, and reverse iteration
// would have constructed the current interval to be [m.start, k) not
// [m.start, m.end).
if invariants.Enabled {
for i := range m.levels {
l := &m.levels[i]
if l.heapKey.kind != boundKindInvalid && m.cmp(l.heapKey.key, m.end) < 0 {
panic("pebble: invariant violation: min-heap key < m.end")
}
}
}
for i := range m.levels {
m.levels[i].prev()
}
m.initMaxHeap()
}
func (m *MergingIter) cmp(a, b []byte) int {
return m.heap.cmp(a, b)
}
func (m *MergingIter) findNextFragmentSet() *Span {
// Each iteration of this loop considers a new merged span between unique
// user keys. An iteration may find that there exists no overlap for a given
// span, (eg, if the spans [a,b), [d, e) exist within level iterators, the
// below loop will still consider [b,d) before continuing to [d, e)). It
// returns when it finds a span that is covered by at least one key.
for m.heap.len() > 0 && m.err == nil {
// Initialize the next span's start bound. SeekGE and First prepare the
// heap without advancing. Next leaves the heap in a state such that the
// root is the smallest bound key equal to the returned span's end key,
// so the heap is already positioned at the next merged span's start key.
// NB: m.heapRoot() might be either an end boundary OR a start boundary
// of a level's span. Both end and start boundaries may still be a start
// key of a span in the set of fragmented spans returned by MergingIter.
// Consider the scenario:
// a----------l #1
// b-----------m #2
//
// The merged, fully-fragmented spans that MergingIter exposes to the caller
// have bounds:
// a-b #1
// b--------l #1
// b--------l #2
// l-m #2
//
// When advancing to l-m#2, we must set m.start to 'l', which originated
// from [a,l)#1's end boundary.
m.start = m.heap.items[0].boundKey.key
// Before calling nextEntry, consider whether it might invalidate our
// start boundary. If the start boundary key originated from an end
// boundary, then we need to copy the start key before advancing the
// underlying iterator to the next Span.
if m.heap.items[0].boundKey.kind == boundKindFragmentEnd {
m.buf = append(m.buf[:0], m.start...)
m.start = m.buf
}
// There may be many entries all with the same user key. Spans in other
// levels may also start or end at this same user key. For eg:
// L1: [a, c) [c, d)
// L2: [c, e)
// If we're positioned at L1's end(c) end boundary, we want to advance
// to the first bound > c.
m.nextEntry()
for len(m.heap.items) > 0 && m.err == nil && m.cmp(m.heapRoot(), m.start) == 0 {
m.nextEntry()
}
if len(m.heap.items) == 0 || m.err != nil {
break
}
// The current entry at the top of the heap is the first key > m.start.
// It must become the end bound for the span we will return to the user.
// In the above example, the root of the heap is L1's end(d).
m.end = m.heap.items[0].boundKey.key
// Each level within m.levels may have a span that overlaps the
// fragmented key span [m.start, m.end). Update m.keys to point to them
// and sort them by kind, sequence number. There may not be any keys
// defined over [m.start, m.end) if we're between the end of one span
// and the start of the next, OR if the configured transform filters any
// keys out. We allow empty spans that were emitted by child iterators, but
// we elide empty spans created by the mergingIter itself that don't overlap
// with any child iterator returned spans (i.e. empty spans that bridge two
// distinct child-iterator-defined spans).
if found, s := m.synthesizeKeys(+1); found && s != nil {
return s
}
}
// Exhausted.
m.clear()
return nil
}
func (m *MergingIter) findPrevFragmentSet() *Span {
// Each iteration of this loop considers a new merged span between unique
// user keys. An iteration may find that there exists no overlap for a given
// span, (eg, if the spans [a,b), [d, e) exist within level iterators, the
// below loop will still consider [b,d) before continuing to [a, b)). It
// returns when it finds a span that is covered by at least one key.
for m.heap.len() > 0 && m.err == nil {
// Initialize the next span's end bound. SeekLT and Last prepare the
// heap without advancing. Prev leaves the heap in a state such that the
// root is the largest bound key equal to the returned span's start key,
// so the heap is already positioned at the next merged span's end key.
// NB: m.heapRoot() might be either an end boundary OR a start boundary
// of a level's span. Both end and start boundaries may still be a start
// key of a span returned by MergingIter. Consider the scenario:
// a----------l #2
// b-----------m #1
//
// The merged, fully-fragmented spans that MergingIter exposes to the caller
// have bounds:
// a-b #2
// b--------l #2
// b--------l #1
// l-m #1
//
// When Preving to a-b#2, we must set m.end to 'b', which originated
// from [b,m)#1's start boundary.
m.end = m.heap.items[0].boundKey.key
// Before calling prevEntry, consider whether it might invalidate our
// end boundary. If the end boundary key originated from a start
// boundary, then we need to copy the end key before advancing the
// underlying iterator to the previous Span.
if m.heap.items[0].boundKey.kind == boundKindFragmentStart {
m.buf = append(m.buf[:0], m.end...)
m.end = m.buf
}
// There may be many entries all with the same user key. Spans in other
// levels may also start or end at this same user key. For eg:
// L1: [a, c) [c, d)
// L2: [c, e)
// If we're positioned at L1's start(c) start boundary, we want to prev
// to move to the first bound < c.
m.prevEntry()
for len(m.heap.items) > 0 && m.err == nil && m.cmp(m.heapRoot(), m.end) == 0 {
m.prevEntry()
}
if len(m.heap.items) == 0 || m.err != nil {
break
}
// The current entry at the top of the heap is the first key < m.end.
// It must become the start bound for the span we will return to the
// user. In the above example, the root of the heap is L1's start(a).
m.start = m.heap.items[0].boundKey.key
// Each level within m.levels may have a set of keys that overlap the
// fragmented key span [m.start, m.end). Update m.keys to point to them
// and sort them by kind, sequence number. There may not be any keys
// spanning [m.start, m.end) if we're between the end of one span and
// the start of the next, OR if the configured transform filters any
// keys out. We allow empty spans that were emitted by child iterators, but
// we elide empty spans created by the mergingIter itself that don't overlap
// with any child iterator returned spans (i.e. empty spans that bridge two
// distinct child-iterator-defined spans).
if found, s := m.synthesizeKeys(-1); found && s != nil {
return s
}
}
// Exhausted.
m.clear()
return nil
}
func (m *MergingIter) heapRoot() []byte {
return m.heap.items[0].boundKey.key
}
// synthesizeKeys is called by find{Next,Prev}FragmentSet to populate and
// sort the set of keys overlapping [m.start, m.end).
//
// During forward iteration, if the current heap item is a fragment end,
// then the fragment's start must be ≤ m.start and the fragment overlaps the
// current iterator position of [m.start, m.end).
//
// During reverse iteration, if the current heap item is a fragment start,
// then the fragment's end must be ≥ m.end and the fragment overlaps the
// current iteration position of [m.start, m.end).
//
// The boolean return value, `found`, is true if the returned span overlaps
// with a span returned by a child iterator.
func (m *MergingIter) synthesizeKeys(dir int8) (bool, *Span) {
if invariants.Enabled {
if m.cmp(m.start, m.end) >= 0 {
panic(fmt.Sprintf("pebble: invariant violation: span start ≥ end: %s >= %s", m.start, m.end))
}
}
m.keys = m.keys[:0]
found := false
for i := range m.levels {
if dir == +1 && m.levels[i].heapKey.kind == boundKindFragmentEnd ||
dir == -1 && m.levels[i].heapKey.kind == boundKindFragmentStart {
m.keys = append(m.keys, m.levels[i].heapKey.span.Keys...)
found = true
}
}
// TODO(jackson): We should be able to remove this sort and instead
// guarantee that we'll return keys in the order of the levels they're from.
// With careful iterator construction, this would guarantee that they're
// sorted by trailer descending for the range key iteration use case.
sort.Sort(&m.keys)
// Apply the configured transform. See VisibleTransform.
m.span = Span{
Start: m.start,
End: m.end,
Keys: m.keys,
KeysOrder: ByTrailerDesc,
}
// NB: m.heap.cmp is a base.Compare, whereas m.cmp is a method on
// MergingIter.
if err := m.transformer.Transform(m.heap.cmp, m.span, &m.span); err != nil {
m.err = err
return false, nil
}
return found, &m.span
}
func (m *MergingIter) invalidate() {
m.err = nil
}
func (m *MergingIter) clear() {
for fi := range m.keys {
m.keys[fi] = Key{}
}
m.keys = m.keys[:0]
}
// nextEntry steps to the next entry.
func (m *MergingIter) nextEntry() {
l := &m.levels[m.heap.items[0].index]
l.next()
if !l.heapKey.valid() {
// l.iter is exhausted.
m.err = l.iter.Error()
if m.err == nil {
m.heap.pop()
}
return
}
if m.heap.len() > 1 {
m.heap.fix(0)
}
}
// prevEntry steps to the previous entry.
func (m *MergingIter) prevEntry() {
l := &m.levels[m.heap.items[0].index]
l.prev()
if !l.heapKey.valid() {
// l.iter is exhausted.
m.err = l.iter.Error()
if m.err == nil {
m.heap.pop()
}
return
}
if m.heap.len() > 1 {
m.heap.fix(0)
}
}
// DebugString returns a string representing the current internal state of the
// merging iterator and its heap for debugging purposes.
func (m *MergingIter) DebugString() string {
var buf bytes.Buffer
fmt.Fprintf(&buf, "Current bounds: [%q, %q)\n", m.start, m.end)
for i := range m.levels {
fmt.Fprintf(&buf, "%d: heap key %s\n", i, m.levels[i].heapKey)
}
return buf.String()
}
type mergingIterItem struct {
// boundKey points to the corresponding mergingIterLevel's `iterKey`.
*boundKey
// index is the index of this level within the MergingIter's levels field.
index int
}
// mergingIterHeap is copied from mergingIterHeap defined in the root pebble
// package for use with point keys.
type mergingIterHeap struct {
cmp base.Compare
reverse bool
items []mergingIterItem
}
func (h *mergingIterHeap) len() int {
return len(h.items)
}
func (h *mergingIterHeap) less(i, j int) bool {
// This key comparison only uses the user key and not the boundKind. Bound
// kind doesn't matter because when stepping over a user key,
// findNextFragmentSet and findPrevFragmentSet skip past all heap items with
// that user key, and makes no assumptions on ordering. All other heap
// examinations only consider the user key.
ik, jk := h.items[i].key, h.items[j].key
c := h.cmp(ik, jk)
if h.reverse {
return c > 0
}
return c < 0
}
func (h *mergingIterHeap) swap(i, j int) {
h.items[i], h.items[j] = h.items[j], h.items[i]
}
// init, fix, up and down are copied from the go stdlib.
func (h *mergingIterHeap) init() {
// heapify
n := h.len()
for i := n/2 - 1; i >= 0; i-- {
h.down(i, n)
}
}
func (h *mergingIterHeap) fix(i int) {
if !h.down(i, h.len()) {
h.up(i)
}
}
func (h *mergingIterHeap) pop() *mergingIterItem {
n := h.len() - 1
h.swap(0, n)
h.down(0, n)
item := &h.items[n]
h.items = h.items[:n]
return item
}
func (h *mergingIterHeap) up(j int) {
for {
i := (j - 1) / 2 // parent
if i == j || !h.less(j, i) {
break
}
h.swap(i, j)
j = i
}
}
func (h *mergingIterHeap) down(i0, n int) bool {
i := i0
for {
j1 := 2*i + 1
if j1 >= n || j1 < 0 { // j1 < 0 after int overflow
break
}
j := j1 // left child
if j2 := j1 + 1; j2 < n && h.less(j2, j1) {
j = j2 // = 2*i + 2 // right child
}
if !h.less(j, i) {
break
}
h.swap(i, j)
i = j
}
return i > i0
}
type boundKind int8
const (
boundKindInvalid boundKind = iota
boundKindFragmentStart
boundKindFragmentEnd
)
type boundKey struct {
kind boundKind
key []byte
// span holds the span the bound key comes from.
//
// If kind is boundKindFragmentStart, then key is span.Start. If kind is
// boundKindFragmentEnd, then key is span.End.
span *Span
}
func (k boundKey) valid() bool {
return k.kind != boundKindInvalid
}
func (k boundKey) String() string {
var buf bytes.Buffer
switch k.kind {
case boundKindInvalid:
fmt.Fprint(&buf, "invalid")
case boundKindFragmentStart:
fmt.Fprint(&buf, "fragment-start")
case boundKindFragmentEnd:
fmt.Fprint(&buf, "fragment-end ")
default:
fmt.Fprintf(&buf, "unknown-kind(%d)", k.kind)
}
fmt.Fprintf(&buf, " %s [", k.key)
fmt.Fprintf(&buf, "%s", k.span)
fmt.Fprint(&buf, "]")
return buf.String()
}