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1 : // Copyright 2019 The LevelDB-Go and Pebble Authors. All rights reserved. Use
2 : // of this source code is governed by a BSD-style license that can be found in
3 : // the LICENSE file.
4 :
5 : package base
6 :
7 : import (
8 : "fmt"
9 : "time"
10 : )
11 :
12 : // InternalIterator iterates over a DB's key/value pairs in key order. Unlike
13 : // the Iterator interface, the returned keys are InternalKeys composed of the
14 : // user-key, a sequence number and a key kind. In forward iteration, key/value
15 : // pairs for identical user-keys are returned in descending sequence order. In
16 : // reverse iteration, key/value pairs for identical user-keys are returned in
17 : // ascending sequence order.
18 : //
19 : // InternalIterators provide 5 absolute positioning methods and 2 relative
20 : // positioning methods. The absolute positioning methods are:
21 : //
22 : // - SeekGE
23 : // - SeekPrefixGE
24 : // - SeekLT
25 : // - First
26 : // - Last
27 : //
28 : // The relative positioning methods are:
29 : //
30 : // - Next
31 : // - Prev
32 : //
33 : // The relative positioning methods can be used in conjunction with any of the
34 : // absolute positioning methods with one exception: SeekPrefixGE does not
35 : // support reverse iteration via Prev. It is undefined to call relative
36 : // positioning methods without ever calling an absolute positioning method.
37 : //
38 : // InternalIterators can optionally implement a prefix iteration mode. This
39 : // mode is entered by calling SeekPrefixGE and exited by any other absolute
40 : // positioning method (SeekGE, SeekLT, First, Last). When in prefix iteration
41 : // mode, a call to Next will advance to the next key which has the same
42 : // "prefix" as the one supplied to SeekPrefixGE. Note that "prefix" in this
43 : // context is not a strict byte prefix, but defined by byte equality for the
44 : // result of the Comparer.Split method. An InternalIterator is not required to
45 : // support prefix iteration mode, and can implement SeekPrefixGE by forwarding
46 : // to SeekGE. When the iteration prefix is exhausted, it is not valid to call
47 : // Next on an internal iterator that's already returned (nil,nilv) or a key
48 : // beyond the prefix.
49 : //
50 : // Bounds, [lower, upper), can be set on iterators, either using the SetBounds()
51 : // function in the interface, or in implementation specific ways during iterator
52 : // creation. The forward positioning routines (SeekGE, First, and Next) only
53 : // check the upper bound. The reverse positioning routines (SeekLT, Last, and
54 : // Prev) only check the lower bound. It is up to the caller to ensure that the
55 : // forward positioning routines respect the lower bound and the reverse
56 : // positioning routines respect the upper bound (i.e. calling SeekGE instead of
57 : // First if there is a lower bound, and SeekLT instead of Last if there is an
58 : // upper bound). This imposition is done in order to elevate that enforcement to
59 : // the caller (generally pebble.Iterator or pebble.mergingIter) rather than
60 : // having it duplicated in every InternalIterator implementation.
61 : //
62 : // Additionally, the caller needs to ensure that SeekGE/SeekPrefixGE are not
63 : // called with a key > the upper bound, and SeekLT is not called with a key <
64 : // the lower bound. InternalIterator implementations are required to respect
65 : // the iterator bounds, never returning records outside of the bounds with one
66 : // exception: an iterator may generate synthetic RANGEDEL marker records. See
67 : // levelIter.syntheticBoundary for the sole existing example of this behavior.
68 : // Specifically, levelIter can return synthetic keys whose user key is equal to
69 : // the lower/upper bound.
70 : //
71 : // The bounds provided to an internal iterator must remain valid until a
72 : // subsequent call to SetBounds has returned. This requirement exists so that
73 : // iterator implementations may compare old and new bounds to apply low-level
74 : // optimizations. The pebble.Iterator satisfies this requirement by maintaining
75 : // two bound buffers and switching between them.
76 : //
77 : // An iterator must be closed after use, but it is not necessary to read an
78 : // iterator until exhaustion.
79 : //
80 : // An iterator is not goroutine-safe, but it is safe to use multiple iterators
81 : // concurrently, either in separate goroutines or switching between the
82 : // iterators in a single goroutine.
83 : //
84 : // It is also safe to use an iterator concurrently with modifying its
85 : // underlying DB, if that DB permits modification. However, the resultant
86 : // key/value pairs are not guaranteed to be a consistent snapshot of that DB
87 : // at a particular point in time.
88 : //
89 : // InternalIterators accumulate errors encountered during operation, exposing
90 : // them through the Error method. All of the absolute positioning methods
91 : // reset any accumulated error before positioning. Relative positioning
92 : // methods return without advancing if the iterator has accumulated an error.
93 : //
94 : // nilv == shorthand for LazyValue{}, which represents a nil value.
95 : type InternalIterator interface {
96 : // SeekGE moves the iterator to the first key/value pair whose key is greater
97 : // than or equal to the given key. Returns the key and value if the iterator
98 : // is pointing at a valid entry, and (nil, nilv) otherwise. Note that SeekGE
99 : // only checks the upper bound. It is up to the caller to ensure that key
100 : // is greater than or equal to the lower bound.
101 : SeekGE(key []byte, flags SeekGEFlags) (*InternalKey, LazyValue)
102 :
103 : // SeekPrefixGE moves the iterator to the first key/value pair whose key is
104 : // greater than or equal to the given key. Returns the key and value if the
105 : // iterator is pointing at a valid entry, and (nil, nilv) otherwise. Note that
106 : // SeekPrefixGE only checks the upper bound. It is up to the caller to ensure
107 : // that key is greater than or equal to the lower bound.
108 : //
109 : // The prefix argument is used by some InternalIterator implementations (e.g.
110 : // sstable.Reader) to avoid expensive operations. A user-defined Split
111 : // function must be supplied to the Comparer for the DB. The supplied prefix
112 : // will be the prefix of the given key returned by that Split function. If
113 : // the iterator is able to determine that no key with the prefix exists, it
114 : // can return (nil,nilv). Unlike SeekGE, this is not an indication that
115 : // iteration is exhausted.
116 : //
117 : // Note that the iterator may return keys not matching the prefix. It is up
118 : // to the caller to check if the prefix matches.
119 : //
120 : // Calling SeekPrefixGE places the receiver into prefix iteration mode. Once
121 : // in this mode, reverse iteration may not be supported and will return an
122 : // error. Note that pebble/Iterator.SeekPrefixGE has this same restriction on
123 : // not supporting reverse iteration in prefix iteration mode until a
124 : // different positioning routine (SeekGE, SeekLT, First or Last) switches the
125 : // iterator out of prefix iteration.
126 : SeekPrefixGE(prefix, key []byte, flags SeekGEFlags) (*InternalKey, LazyValue)
127 :
128 : // SeekLT moves the iterator to the last key/value pair whose key is less
129 : // than the given key. Returns the key and value if the iterator is pointing
130 : // at a valid entry, and (nil, nilv) otherwise. Note that SeekLT only checks
131 : // the lower bound. It is up to the caller to ensure that key is less than
132 : // the upper bound.
133 : SeekLT(key []byte, flags SeekLTFlags) (*InternalKey, LazyValue)
134 :
135 : // First moves the iterator the the first key/value pair. Returns the key and
136 : // value if the iterator is pointing at a valid entry, and (nil, nilv)
137 : // otherwise. Note that First only checks the upper bound. It is up to the
138 : // caller to ensure that First() is not called when there is a lower bound,
139 : // and instead call SeekGE(lower).
140 : First() (*InternalKey, LazyValue)
141 :
142 : // Last moves the iterator the the last key/value pair. Returns the key and
143 : // value if the iterator is pointing at a valid entry, and (nil, nilv)
144 : // otherwise. Note that Last only checks the lower bound. It is up to the
145 : // caller to ensure that Last() is not called when there is an upper bound,
146 : // and instead call SeekLT(upper).
147 : Last() (*InternalKey, LazyValue)
148 :
149 : // Next moves the iterator to the next key/value pair. Returns the key and
150 : // value if the iterator is pointing at a valid entry, and (nil, nilv)
151 : // otherwise. Note that Next only checks the upper bound. It is up to the
152 : // caller to ensure that key is greater than or equal to the lower bound.
153 : //
154 : // It is valid to call Next when the iterator is positioned before the first
155 : // key/value pair due to either a prior call to SeekLT or Prev which returned
156 : // (nil, nilv). It is not allowed to call Next when the previous call to SeekGE,
157 : // SeekPrefixGE or Next returned (nil, nilv).
158 : Next() (*InternalKey, LazyValue)
159 :
160 : // NextPrefix moves the iterator to the next key/value pair with a different
161 : // prefix than the key at the current iterator position. Returns the key and
162 : // value if the iterator is pointing at a valid entry, and (nil, nil)
163 : // otherwise. Note that NextPrefix only checks the upper bound. It is up to
164 : // the caller to ensure that key is greater than or equal to the lower
165 : // bound.
166 : //
167 : // NextPrefix is passed the immediate successor to the current prefix key. A
168 : // valid implementation of NextPrefix is to call SeekGE with succKey.
169 : //
170 : // It is not allowed to call NextPrefix when the previous call was a reverse
171 : // positioning operation or a call to a forward positioning method that
172 : // returned (nil, nilv). It is also not allowed to call NextPrefix when the
173 : // iterator is in prefix iteration mode.
174 : NextPrefix(succKey []byte) (*InternalKey, LazyValue)
175 :
176 : // Prev moves the iterator to the previous key/value pair. Returns the key
177 : // and value if the iterator is pointing at a valid entry, and (nil, nilv)
178 : // otherwise. Note that Prev only checks the lower bound. It is up to the
179 : // caller to ensure that key is less than the upper bound.
180 : //
181 : // It is valid to call Prev when the iterator is positioned after the last
182 : // key/value pair due to either a prior call to SeekGE or Next which returned
183 : // (nil, nilv). It is not allowed to call Prev when the previous call to SeekLT
184 : // or Prev returned (nil, nilv).
185 : Prev() (*InternalKey, LazyValue)
186 :
187 : // Error returns any accumulated error. It may not include errors returned
188 : // to the client when calling LazyValue.Value().
189 : Error() error
190 :
191 : // Close closes the iterator and returns any accumulated error. Exhausting
192 : // all the key/value pairs in a table is not considered to be an error.
193 : // It is valid to call Close multiple times. Other methods should not be
194 : // called after the iterator has been closed.
195 : Close() error
196 :
197 : // SetBounds sets the lower and upper bounds for the iterator. Note that the
198 : // result of Next and Prev will be undefined until the iterator has been
199 : // repositioned with SeekGE, SeekPrefixGE, SeekLT, First, or Last.
200 : //
201 : // The bounds provided must remain valid until a subsequent call to
202 : // SetBounds has returned. This requirement exists so that iterator
203 : // implementations may compare old and new bounds to apply low-level
204 : // optimizations.
205 : SetBounds(lower, upper []byte)
206 :
207 : fmt.Stringer
208 : }
209 :
210 : // SeekGEFlags holds flags that may configure the behavior of a forward seek.
211 : // Not all flags are relevant to all iterators.
212 : type SeekGEFlags uint8
213 :
214 : const (
215 : seekGEFlagTrySeekUsingNext uint8 = iota
216 : seekGEFlagRelativeSeek
217 : seekGEFlagBatchJustRefreshed
218 : )
219 :
220 : // SeekGEFlagsNone is the default value of SeekGEFlags, with all flags disabled.
221 : const SeekGEFlagsNone = SeekGEFlags(0)
222 :
223 : // TrySeekUsingNext indicates whether a performance optimization was enabled
224 : // by a caller, indicating the caller has not done any action to move this
225 : // iterator beyond the first key that would be found if this iterator were to
226 : // honestly do the intended seek. For example, say the caller did a
227 : // SeekGE(k1...), followed by SeekGE(k2...) where k1 <= k2, without any
228 : // intermediate positioning calls. The caller can safely specify true for this
229 : // parameter in the second call. As another example, say the caller did do one
230 : // call to Next between the two Seek calls, and k1 < k2. Again, the caller can
231 : // safely specify a true value for this parameter. Note that a false value is
232 : // always safe. The callee is free to ignore the true value if its
233 : // implementation does not permit this optimization.
234 : //
235 : // We make the caller do this determination since a string comparison of k1, k2
236 : // is not necessarily cheap, and there may be many iterators in the iterator
237 : // stack. Doing it once at the root of the iterator stack is cheaper.
238 : //
239 : // This optimization could also be applied to SeekLT (where it would be
240 : // trySeekUsingPrev). We currently only do it for SeekPrefixGE and SeekGE
241 : // because this is where this optimization helps the performance of CockroachDB.
242 : // The SeekLT cases in CockroachDB are typically accompanied with bounds that
243 : // change between seek calls, and is optimized inside certain iterator
244 : // implementations, like singleLevelIterator, without any extra parameter
245 : // passing (though the same amortization of string comparisons could be done to
246 : // improve that optimization, by making the root of the iterator stack do it).
247 1 : func (s SeekGEFlags) TrySeekUsingNext() bool { return (s & (1 << seekGEFlagTrySeekUsingNext)) != 0 }
248 :
249 : // RelativeSeek is set when in the course of a forward positioning operation, a
250 : // higher-level iterator seeks a lower-level iterator to a larger key than the
251 : // one at the current iterator position.
252 : //
253 : // Concretely, this occurs when the merging iterator observes a range deletion
254 : // covering the key at a level's current position, and the merging iterator
255 : // seeks the level to the range deletion's end key. During lazy-combined
256 : // iteration, this flag signals to the level iterator that the seek is NOT an
257 : // absolute-positioning operation from the perspective of the pebble.Iterator,
258 : // and the level iterator must look for range keys in tables between the current
259 : // iterator position and the new seeked position.
260 1 : func (s SeekGEFlags) RelativeSeek() bool { return (s & (1 << seekGEFlagRelativeSeek)) != 0 }
261 :
262 : // BatchJustRefreshed is set by Seek[Prefix]GE when an iterator's view of an
263 : // indexed batch was just refreshed. It serves as a signal to the batch iterator
264 : // to ignore the TrySeekUsingNext optimization, because the external knowledge
265 : // imparted by the TrySeekUsingNext flag does not apply to the batch iterator's
266 : // position. See (pebble.Iterator).batchJustRefreshed.
267 1 : func (s SeekGEFlags) BatchJustRefreshed() bool { return (s & (1 << seekGEFlagBatchJustRefreshed)) != 0 }
268 :
269 : // EnableTrySeekUsingNext returns the provided flags with the
270 : // try-seek-using-next optimization enabled. See TrySeekUsingNext for an
271 : // explanation of this optimization.
272 1 : func (s SeekGEFlags) EnableTrySeekUsingNext() SeekGEFlags {
273 1 : return s | (1 << seekGEFlagTrySeekUsingNext)
274 1 : }
275 :
276 : // DisableTrySeekUsingNext returns the provided flags with the
277 : // try-seek-using-next optimization disabled.
278 1 : func (s SeekGEFlags) DisableTrySeekUsingNext() SeekGEFlags {
279 1 : return s &^ (1 << seekGEFlagTrySeekUsingNext)
280 1 : }
281 :
282 : // EnableRelativeSeek returns the provided flags with the relative-seek flag
283 : // enabled. See RelativeSeek for an explanation of this flag's use.
284 1 : func (s SeekGEFlags) EnableRelativeSeek() SeekGEFlags {
285 1 : return s | (1 << seekGEFlagRelativeSeek)
286 1 : }
287 :
288 : // DisableRelativeSeek returns the provided flags with the relative-seek flag
289 : // disabled.
290 0 : func (s SeekGEFlags) DisableRelativeSeek() SeekGEFlags {
291 0 : return s &^ (1 << seekGEFlagRelativeSeek)
292 0 : }
293 :
294 : // EnableBatchJustRefreshed returns the provided flags with the
295 : // batch-just-refreshed bit set. See BatchJustRefreshed for an explanation of
296 : // this flag.
297 0 : func (s SeekGEFlags) EnableBatchJustRefreshed() SeekGEFlags {
298 0 : return s | (1 << seekGEFlagBatchJustRefreshed)
299 0 : }
300 :
301 : // DisableBatchJustRefreshed returns the provided flags with the
302 : // batch-just-refreshed bit unset.
303 0 : func (s SeekGEFlags) DisableBatchJustRefreshed() SeekGEFlags {
304 0 : return s &^ (1 << seekGEFlagBatchJustRefreshed)
305 0 : }
306 :
307 : // SeekLTFlags holds flags that may configure the behavior of a reverse seek.
308 : // Not all flags are relevant to all iterators.
309 : type SeekLTFlags uint8
310 :
311 : const (
312 : seekLTFlagRelativeSeek uint8 = iota
313 : )
314 :
315 : // SeekLTFlagsNone is the default value of SeekLTFlags, with all flags disabled.
316 : const SeekLTFlagsNone = SeekLTFlags(0)
317 :
318 : // RelativeSeek is set when in the course of a reverse positioning operation, a
319 : // higher-level iterator seeks a lower-level iterator to a smaller key than the
320 : // one at the current iterator position.
321 : //
322 : // Concretely, this occurs when the merging iterator observes a range deletion
323 : // covering the key at a level's current position, and the merging iterator
324 : // seeks the level to the range deletion's start key. During lazy-combined
325 : // iteration, this flag signals to the level iterator that the seek is NOT an
326 : // absolute-positioning operation from the perspective of the pebble.Iterator,
327 : // and the level iterator must look for range keys in tables between the current
328 : // iterator position and the new seeked position.
329 1 : func (s SeekLTFlags) RelativeSeek() bool { return s&(1<<seekLTFlagRelativeSeek) != 0 }
330 :
331 : // EnableRelativeSeek returns the provided flags with the relative-seek flag
332 : // enabled. See RelativeSeek for an explanation of this flag's use.
333 1 : func (s SeekLTFlags) EnableRelativeSeek() SeekLTFlags {
334 1 : return s | (1 << seekLTFlagRelativeSeek)
335 1 : }
336 :
337 : // DisableRelativeSeek returns the provided flags with the relative-seek flag
338 : // disabled.
339 0 : func (s SeekLTFlags) DisableRelativeSeek() SeekLTFlags {
340 0 : return s &^ (1 << seekLTFlagRelativeSeek)
341 0 : }
342 :
343 : // InternalIteratorStats contains miscellaneous stats produced by
344 : // InternalIterators that are part of the InternalIterator tree. Not every
345 : // field is relevant for an InternalIterator implementation. The field values
346 : // are aggregated as one goes up the InternalIterator tree.
347 : type InternalIteratorStats struct {
348 : // Bytes in the loaded blocks. If the block was compressed, this is the
349 : // compressed bytes. Currently, only the index blocks, data blocks
350 : // containing points, and filter blocks are included.
351 : BlockBytes uint64
352 : // Subset of BlockBytes that were in the block cache.
353 : BlockBytesInCache uint64
354 : // BlockReadDuration accumulates the duration spent fetching blocks
355 : // due to block cache misses.
356 : // TODO(sumeer): this currently excludes the time spent in Reader creation,
357 : // and in reading the rangedel and rangekey blocks. Fix that.
358 : BlockReadDuration time.Duration
359 : // The following can repeatedly count the same points if they are iterated
360 : // over multiple times. Additionally, they may count a point twice when
361 : // switching directions. The latter could be improved if needed.
362 :
363 : // Bytes in keys that were iterated over. Currently, only point keys are
364 : // included.
365 : KeyBytes uint64
366 : // Bytes in values that were iterated over. Currently, only point values are
367 : // included. For separated values, this is the size of the handle.
368 : ValueBytes uint64
369 : // The count of points iterated over.
370 : PointCount uint64
371 : // Points that were iterated over that were covered by range tombstones. It
372 : // can be useful for discovering instances of
373 : // https://github.com/cockroachdb/pebble/issues/1070.
374 : PointsCoveredByRangeTombstones uint64
375 :
376 : // Stats related to points in value blocks encountered during iteration.
377 : // These are useful to understand outliers, since typical user facing
378 : // iteration should tend to only look at the latest point, and hence have
379 : // the following stats close to 0.
380 : SeparatedPointValue struct {
381 : // Count is a count of points that were in value blocks. This is not a
382 : // subset of PointCount: PointCount is produced by mergingIter and if
383 : // positioned once, and successful in returning a point, will have a
384 : // PointCount of 1, regardless of how many sstables (and memtables etc.)
385 : // in the heap got positioned. The count here includes every sstable
386 : // iterator that got positioned in the heap.
387 : Count uint64
388 : // ValueBytes represent the total byte length of the values (in value
389 : // blocks) of the points corresponding to Count.
390 : ValueBytes uint64
391 : // ValueBytesFetched is the total byte length of the values (in value
392 : // blocks) that were retrieved.
393 : ValueBytesFetched uint64
394 : }
395 : }
396 :
397 : // Merge merges the stats in from into the given stats.
398 0 : func (s *InternalIteratorStats) Merge(from InternalIteratorStats) {
399 0 : s.BlockBytes += from.BlockBytes
400 0 : s.BlockBytesInCache += from.BlockBytesInCache
401 0 : s.BlockReadDuration += from.BlockReadDuration
402 0 : s.KeyBytes += from.KeyBytes
403 0 : s.ValueBytes += from.ValueBytes
404 0 : s.PointCount += from.PointCount
405 0 : s.PointsCoveredByRangeTombstones += from.PointsCoveredByRangeTombstones
406 0 : s.SeparatedPointValue.Count += from.SeparatedPointValue.Count
407 0 : s.SeparatedPointValue.ValueBytes += from.SeparatedPointValue.ValueBytes
408 0 : s.SeparatedPointValue.ValueBytesFetched += from.SeparatedPointValue.ValueBytesFetched
409 0 : }
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