We dropped MySQL support and a lot of mysql specific code has been
removed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29608.

This comes in from the other direction and removes any `if postgresql?`
branches.

Using the sed script from
https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

None of this code can be reached any more, so it can all be removed

From pg_stat_wal_receiver_supported? to
postgresql_minimum_supported_version?

Also add test coverage

If there are any clients connected to the DB, PostgreSQL won't let you
drop the database. It's possible that Sidekiq, Unicorn, or some other
client will be hanging onto a connection, preventing the DROP DATABASE
from working. To workaround this problem, this method cancels all the
connections so that the db:reset command will work.

Note that there's still a slight possibility a client connects after its
connection is terminated. If this is an issue, we could solve it by
revoking CONNECT access, but for now it seems this works.

Closes https://gitlab.com/gitlab-org/gitlab-development-kit/issues/450

These changes are useful for CE as well. For example, the MR
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20720 can make
use of some of the backported methods.

Use data_source_exists? where possible instead of table_exists? in order to be Rails5 compatible

This removes the need for running a database query every time we want to
check the database version.

Closes #43355

This is most a backport of
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4987/diffs but includes
other columns that use column_exists? in a way that may cause unnecessary
schema loads.

This adds the keyword argument "return_ids" to
Gitlab::Database.bulk_insert. When set to `true` (and PostgreSQL is
used) this method will return an Array of the IDs of the inserted rows,
otherwise it will return an empty Array.

This changes various controllers to use the new EventCollection class
for retrieving events. This class uses a JOIN LATERAL query on
PostgreSQL to retrieve queries in a more efficient way, while falling
back to a simpler / less efficient query for MySQL.

The EventCollection class also includes a limit on the number of events
to display to prevent malicious users from cycling through all events,
as doing so could put a lot of pressure on the database.

JOIN LATERAL is only supported on PostgreSQL starting with version 9.3.0
and as such this optimisation is only used when using PostgreSQL 9.3 or
newer.

Remove superfluous lib: true, type: redis, service: true, models: true, services: true, no_db: true, api: true
Signed-off-by: NRémy Coutable <remy@rymai.me>

This adds an ID-less table containing one row per file, per merge request
diff. It has a column for each attribute on Gitlab::Git::Diff that is serialised
currently, with the advantage that we can easily query the attributes of this
new table.

It does not migrate existing data, so we have fallback code when the legacy
st_diffs column is present instead. For a merge request diff to be valid, it
should have at most one of:

* Rows in this new table, with the correct merge_request_diff_id.
* A non-NULL st_diffs column.

It may have neither, if the diff is empty.

This returns the ActiveRecord configuration for the current environment.

While CE doesn't use this very often, EE will use it in a few places for
the database load balancing code. I'm adding this to CE so we don't end
up with merge conflicts in this file.

This reverts commit e00fb2bdc2090e9cabeb1eb35a2672a882cc96e9.

# Conflicts:
#	.rubocop.yml
#	.rubocop_todo.yml
#	lib/gitlab/ci/config/entry/global.rb
#	lib/gitlab/ci/config/entry/jobs.rb
#	spec/lib/gitlab/ci/config/entry/factory_spec.rb
#	spec/lib/gitlab/ci/config/entry/global_spec.rb
#	spec/lib/gitlab/ci/config/entry/job_spec.rb
#	spec/lib/gitlab/ci/status/build/factory_spec.rb
#	spec/lib/gitlab/incoming_email_spec.rb

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

This allows you to set a custom host when calling
Gitlab::Database.create_connection_pool. This is necessary for load
balancing as in this case we want to inherit all settings except for the
hostname.

We don't need to connect when requesting the name of the database
adapter. This in turn should prevent us from requesting/leaking
connections just by asking whether we're using PostgreSQL or MySQL.

Signed-off-by: NRémy Coutable <remy@rymai.me>

Closes #12900