remove_column should only be used in the up (or change) step of a migration if
it's a post-deployment migration. Otherwise there will be downtime due to the
ActiveRecord column cache, which we can avoid by using the IgnorableColumn
concern in combination with a post-deployment migration.
add_column_with_default is implemented in terms of update_column_in_batches, but
update_column_in_batches can be used independently. Neither of these should be
used on the specified large tables, because they will cause issues on large
instances like GitLab.com.
This also ignores the cop for all existing migrations, renaming
AddColumnWithDefaultToLargeTable where appropriate.
This moves tracking of the pushes since the last Git GC from PostgreSQL
to Redis. This reduces the number of writes on the "projects" table.
This in turn reduces the vacuuming overhead.
The lease used for incrementing the counter has been removed. This lease
was mostly put in place to prevent high database load but this isn't
needed anymore due to the counter now being stored in Redis.
Fixesgitlab-org/gitlab-ce#22125
