[IMP] mail: avoid costly queries on messages

The combined domain for moderated messages used to trigger a complex
database query with two OR conditions on the mail_message table.
Unfortunately PostgreSQL seems to be unable to optimize this with
indexes, or to detect that one of the conditions is void (included `AND
FALSE`). On large mail_message tables, this significantly slows down
calls to message_fetch, e.g. 6s instead of ~70ms, while returning the
same results.

As a workaround, move the logic for generating the moderation domain to
the server side, where it can be:
 - optimized out when the user is not a moderator
 - split into 2 distinct DB queries in order to avoid the pathological
   case

After this change, calls to message_fetch with and without messages
to moderate execute in respectively ~50ms and ~150ms, instead of 6s, in
a test database with 25 million messages.

The modified message_fetch signature is backwards-compatible.

+ modified mock server used for the tests, which unfortunately
duplicates most of the business logic.

0 files changed, 0 insertions, 0 deletions