Summary:
Something I am working on is going to expand usage of `BlockBasedTableBuilder::Rep::last_key`, but the existing code contract for `IndexBuilder::AddIndexEntry` makes that difficult because it modifies its `last_key` parameter to be the separator value recorded in the index, often something between the two boundary keys.
This change primarily changes the contract of that function and related functions to separate function inputs and outputs, without sacrificing efficiency. For efficiency, a reusable scratch string buffer is provided by the caller, which the callee can use (or not) in returning a result Slice. That should yield a performance improvement as we are reusing a buffer for keys rather than copying into a new one each time in the FindShort* functions, without any additional string copies or conditional branches.
Additional improvements in PartitionedIndexBuilder specifically:
* Reduce string copies by eliminating `sub_index_last_key_` and instead tracking the key for the next partition in a placeholder Entry.
* Simplify code and improve code quality by changing `sub_index_builder_` to unique_ptr.
* Eliminate unnecessary NewFlushBlockPolicy call/object.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12867
Test Plan: existing tests, crash test. Will validate performance along with the change this is setting up.
Reviewed By: anand1976
Differential Revision: D59793119
Pulled By: pdillinger
fbshipit-source-id: 556da75cf13b967511f84702b2713d152f536a07
f456a7213f