Some uses of ActiveRecord::QueryMethods#distinct pass field names but they
are incorrect for the current version of Rails.
ActiveRecord::QueryMethods#group provides the expected behavior and
benefits performance. See commit 6da24aad4cafdef8d8a2c92bac2002a5fc2fe9c8.
The old implementation had two queries:
1. The query constructed in Api::V1::FavouritesController#results
2. The query constructed in #cached_favourites, which is merged with 1.
Both of them are issued againt PostgreSQL. The combination of the two
queries caused the following problems:
- The small window between the two queries involves race conditions.
- Minor performance inefficiency.
Moreover, the construction of query 2, which involves merging with query
1 has a bug. Query 1 is finalized with paginate_by_id, but paginate_by_id
returns an array when min_id parameter is specified. The behavior prevents
from merging the query, and in the real world, ActiveRecord simply ignores
the merge (!), which results in querying the entire scan of statuses and
favourites table.
This change fixes these issues by simply letting query 1 get all the works
done.
DISTINCT clause removes duplicated records according to all the selected
attributes. In reality, it can remove duplicated records only looking at
statuses.id, but the clause confuses the query planner and yields
insufficient performance.
The behavior is also problematic if the scope produced by HashQueryService
is used to query columns without id (using pluck method, for example). The
scope is expected to contain unique statuses, but the uniquness will be
evaluated with some arbitrary columns other than id.
GROUP BY clause resolves those problem by explicitly specifying the
column to take into account for the record distinction.
A workaround for the problem of DISTINCT clause in
Api::V1::Timelines::TagController is no longer necessary and removed.
Akihiko Odaki