Summary:
This PR added some optimizations for the per key handling for SST file for the user-defined timestamps in Memtable only feature. CPU profiling shows this part is a big culprit for regression. This optimization saves some string construction/destruction/appending/copying. vector operations like reserve/emplace_back.
When iterating keys in a block, we need to copy some shared bytes from previous key, put it together with the non shared bytes and find a right location to pad the min timestamp. Previously, we create a tmp local string buffer to first construct the key from its pieces, and then copying this local string's content into `IterKey`'s buffer. To avoid having this local string and to avoid this extra copy. Instead of piecing together the key in a local string first, we just track all the pieces that make this key in a reused Slice array. And then copy the pieces in order into `IterKey`'s buffer. Since the previous key should be kept intact while we are copying some shared bytes from it, we added a secondary buffer in `IterKey` and alternate between primary buffer and secondary buffer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13031
Test Plan: Existing tests.
Reviewed By: ltamasi
Differential Revision: D63416531
Pulled By: jowlyzhang
fbshipit-source-id: 9819b0e02301a2dbc90621b2fe4f651bc912113c
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