This is designed to help PostgreSQL have better specificity and
locality in its indexes. Subsequent commits will adjust the code to
make sure that we use these indexes rather than the `realm_id`-less
versions.
We do not add a `realm_id` variation to the full-text index, since
it is a GIN index; multi-column GIN indexes are not terribly
performant, require the `btree_gin` extension for `int` types (which
requires superuser privileges on PostgreSQL 12 and earlier), and
cannot be consistently added concurrently on running instances.
After all indexes have been made, we also run `CREATE STATISTICS` in
order to give PostgreSQL the opportunity to realize that recipient and
sender are highly correlated with message realm, allowing it to
estimate that `(realm_id, recipient_id)` is likely as specific as
matching a given `recipient_id`, instead of as likely as matching
`realm_id` times matching a `recipient_id`. Finally, those statistics
must be filled by `ANALYZE zerver_message`, which is run last.
Alex Vandiver