[PGSQL] Optimize method PostgresMailboxMessageDAO#listDistinctUserFlags

[vttranlina]

The unnest function supported by PostgreSQL is the reason the query for user flags is heavy.

select distinct unnest(user_flags)
from message_mailbox
where mailbox_id = cast($1 as uuid)

Detail: https://github.com/vttranlina/vttranlina.github.io/blob/main/gatling_20241126/before/pg_stat_statements_before.csv

I suggest move "unnest" logic to james code application layer and verifying its impact.

[chibenwa]

Less convinced about this one: choose your evil...

Unsure that sending zillions of rows over the network be more efficient...