Nested groups support uses queries along the lines of `path LIKE 'X/%'`.
For these queries to use an index on PostgreSQL we need to use either
the varchar_pattern_ops or text_pattern_ops operator class.
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/29554
Performance is improved in two steps:
1. On PostgreSQL an expression index is used for checking lower(email)
and lower(username).
2. The check to determine if we're searching for a username or Email is
moved to Ruby. Thanks to @haynes for suggesting and writing the
initial implementation of this.
Moving the check to Ruby makes this method an additional 1.5 times
faster compared to doing the check in the SQL query.
With performance being improved I've now also tweaked the amount of
iterations required by the User.by_login benchmark. This method now runs
between 900 and 1000 iterations per second.
