First Row

By specifying 'merge-engine' = 'first-row', users can keep the first row of the same primary key. It differs from the deduplicate merge engine that in the first-row merge engine, it will generate insert only changelog.

1. You can not specify sequence.field.
2. Not accept DELETE and UPDATE_BEFORE message. You can config ignore-delete to ignore these two kinds records.
3. Visibility guarantee: Tables with First Row engine, the files with level 0 will only be visible after compaction. So by default, compaction is synchronous, and if asynchronous is turned on, there may be delays in the data.

This is of great help in replacing log deduplication in streaming computation.