Improve follow recommendations SQL query

[renchap]

A server admin reported that this query took 29 hours to run on their instance, using PostgreSQL 14 with up-to-date vacuum & statistics.

After some investigation, we found out that the plan was very wrong and resulted in 300B+ rows being generated in a temporary table.

Switching from a JOIN to NOT EXISTS fixes the issue and the query is now behaving as expected.

This has been tested on PostgreSQL 12 on a large server, and performs the same as with the JOIN on this server.

Fixes #25191

[ClearlyClaire]

follow_recommendations_v02.sql describes a materialized view, and updating it will require a database migration: see db/migrate/20210505174616_update_follow_recommendations_to_version_2.rb for an example.

[renchap]

I am trying to get this through a migration, but as this is a materialized view it needs to be dropped and then re-created if we go with a scenic migration.
As computing the view takes about 10 minutes on mastodon.social and it locks the table, we are not sure this is the correct way to go.

An alternative way of updating materialized views has been proposed in scenic, which involves creating the new view under a temporary name, then drop the old one and rename the new:
scenic-views/scenic#387

Unfortunately you can not easily replicate this behaviour from a migration, as it wont work with the version: parameter.

[Gargron]

Should we rename this view? Would that unstall this PR? I wouldn't mind having the FollowRecommendation name be available for a different model.

[renchap]

Yes we could rename it. But that would not solve the issue if one day we have another materialised view that needs to be updated.

The alternative I had in mind was to copy the way it is done in scenic-views/scenic#387 into a helper, and use this helper for the migration.

[renchap]

Closing if favor of #26545