WritePrepared Txn: Support merge operator
Summary: CompactionIterator invoke MergeHelper::MergeUntil() to do partial merge between snapshot boundaries. Previously it only depend on sequence number to tell snapshot boundary, but we also need to make use of snapshot_checker to verify visibility of the merge operands to the snapshots. For example, say there is a snapshot with seq = 2 but only can see data with seq <= 1. There are three merges, each with seq = 1, 2, 3. A correct compaction output would be (1),(2+3). Without taking snapshot_checker into account when generating merge result, compaction will generate output (1+2),(3). By filtering uncommitted keys with read callback, the read path already take care of merges well and don't need additional updates. Closes https://github.com/facebook/rocksdb/pull/3475 Differential Revision: D6926087 Pulled By: yiwu-arbug fbshipit-source-id: 8f539d6f897cfe29b6dc27a8992f68c2a629d40a
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