Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11807
For now, RocksDB has limited support for using merge with wide columns: when a bunch of merge operands have to be applied to a wide-column base value, RocksDB currently passes only the value of the default column to the application's `MergeOperator`, which means there is no way to update any other columns during a merge. As a first step in making this more general, the patch adds a new API `FullMergeV3` to `MergeOperator`.
`FullMergeV3`'s interface enables applications to receive a plain, wide-column, or non-existent base value as merge input, and to produce a new plain value, a new wide-column value, or an existing operand as merge output. Note that there are no limitations on the column names and values if the merge result is a wide-column entity. Also, the interface is general in the sense that it makes it possible e.g. for a merge that takes a plain base value and some deltas to produce a wide-column entity as a result.
For backward compatibility, the default implementation of `FullMergeV3` falls back to `FullMergeV2` and implements the current logic where merge operands are applied to the default column of the base entity and any other columns are unchanged. (Note that with `FullMergeV3` in the `MergeOperator` interface, this behavior will become customizable.)
This patch just introduces the new API and the default backward compatible implementation. I plan to integrate `FullMergeV3` into the query and compaction logic in subsequent diffs.
Reviewed By: jaykorean
Differential Revision: D49117253
fbshipit-source-id: 109e016f25cd130fc504790818d927bae7fec6bd
