Support append only table

[evenyag]

What type of enhancement is this?

API improvement, User experience

What does the enhancement do?

Now the storage engine overwrites duplicate entries, that share the same row key by default. We may also need to support append mode, which never overwrites duplicate entries. However, users might expect to store duplicate rows in the table.

It's helpful to support an append-only mode for the table.

Implementation challenges

We remove duplicate rows in several places.

• memtables iters
  • the time series memtable
  • the merge tree memtable
• the merge reader
  • during query
  • during compaction

We have to

• add an append_only option and disable dedup in memtables
• bypass the merge reader in query and compaction

A remaining problem is how to support DELETE as the merge reader requires each source support dedup. For simplicity, we can only support it while the table is not an append only table. We might need to persist this option in the manifest.

Implement History

• Adds a dedup option to the time series memtable feat: support append-only mode in time-series memtable #3540
• Adds the append mode option to region options feat: Implement append mode for a region #3558
• SeqScan removes the merge reader if the region is append only feat: Implement an unordered scanner for append mode #3598
• Adds the append mode option to table options feat: add append mode to table options #3624

[evenyag]

Since #3624 was merged, I'm going to close this issue. I'll create new issues for further enhancements.