Maintain position in range deletions map
Summary: When deletion-collapsing mode is enabled (i.e., for DBIter/CompactionIterator), we maintain position in the tombstone maps across calls to ShouldDelete(). Since iterators often access keys sequentially (or reverse-sequentially), scanning forward/backward from the last position can be faster than binary-searching the map for every key. - When Next() is invoked on an iterator, we use kForwardTraversal to scan forwards, if needed, until arriving at the range deletion containing the next key. - Similarly for Prev(), we use kBackwardTraversal to scan backwards in the range deletion map. - When the iterator seeks, we use kBinarySearch for repositioning - After tombstones are added or before the first ShouldDelete() invocation, the current position is set to invalid, which forces kBinarySearch to be used. - Non-iterator users (i.e., Get()) use kFullScan, which has the same behavior as before---scan the whole map for every key passed to ShouldDelete(). Closes https://github.com/facebook/rocksdb/pull/1701 Differential Revision: D4350318 Pulled By: ajkr fbshipit-source-id: 5129b76
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