All-shard queries

[levkk]

Is your feature request related to a problem? Please describe. I would like to be able to query multiple shards at the same time and get a single result set back; also known as a cross-shard / all-shard query.

Describe the solution you'd like I think a simple solution around issuing the same Q or P, B, and E to multiple shards (or all shards), and merging D packets. Verification in T packets should be done to ensure schema is identical on all shards.

Describe alternatives you've considered Postgres FDW can do this, but that requires running a Postgres server with shards as remote servers. Sometimes FDW doesn't push down predicates, causing slow queries that timeout.

Additional context Some queries will not work (hence why FDW was created), e.g. when GROUP BY spans multiple shards, COUNT(*) will be incorrect. Overall, this feature should be used carefully in specific use cases, e.g. simple fetches of rows will work well, but ordering across shards will be incorrect.