187b29938c
Summary: Since ForwardIterator is on a level below DBIter, the latter may call Next() on it (e.g. in order to skip deletion markers). Since this also updates `prev_key_`, it may prevent the Seek() optimization. For example, assume that there's only one SST file and it contains the following entries: 0101, 0201 (`ValueType::kTypeDeletion`, i.e. a tombstone record), 0201 (`kTypeValue`), 0202. Memtable is empty. `Seek(0102)` will result in `prev_key_` being set to `0201` instead of `0102`, since `DBIter::Seek()` will call `ForwardIterator::Next()` to skip record 0201. Therefore, when `Seek(0102)` is called again, `NeedToSeekImmutable()` will return true. This fix relies on `prefix_extractor_` to detect prefix changes. `prev_key_` is only set to `current_->key()` as long as they have the same prefix. I also made a small change to `NeedToSeekImmutable()` so it no longer returns true when the db is empty (i.e. there's nothing but a memtable). Test Plan: $ TEST_TMPDIR=/dev/shm/rocksdbtest ROCKSDB_TESTS=TailingIterator ./db_test Reviewers: sdong, igor, ljin Reviewed By: ljin Subscribers: leveldb Differential Revision: https://reviews.facebook.net/D23823
RocksDB: A Persistent Key-Value Store for Flash and RAM Storage
RocksDB is developed and maintained by Facebook Database Engineering Team. It is built on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)
This code is a library that forms the core building block for a fast key value server, especially suited for storing data on flash drives. It has a Log-Structured-Merge-Database (LSM) design with flexible tradeoffs between Write-Amplification-Factor (WAF), Read-Amplification-Factor (RAF) and Space-Amplification-Factor (SAF). It has multi-threaded compactions, making it specially suitable for storing multiple terabytes of data in a single database.
Start with example usage here: https://github.com/facebook/rocksdb/tree/master/examples
See the github wiki for more explanation.
The public interface is in include/. Callers should not include or
rely on the details of any other header files in this package. Those
internal APIs may be changed without warning.
Design discussions are conducted in https://www.facebook.com/groups/rocksdb.dev/