SELECT DISTINCT is bad for performance because it forces the use of
temporary tables [1]. Stop using it. As far as we could tell in our
testing, a simple s/SELECT DISTINCT/SELECT/ is enough in most places,
except two.

First, when listing a reader's projects. Readers are associated with
sections, not projects, and there's a many-to-one relation between
sections and projects, so we need the SELECT DISTINCT to avoid
repeating the project once for every section for a particular reader.

Second, when listing users according to a specific role. There's a
one-to-many relation between users and roles, and the latter are
stored as rows in the database. We need the SELECT DISTINCT to avoid
listing a user once for every role that they have.

Other than that, we seem to be able to get away with just not doing
DISTINCT.

[1] https://dba.stackexchange.com/questions/34674/mysql-performance-tuning-queries-stuck-on-copying-to-tmp-tables

Resolves: LibriVox#82