Remove Gitlab::Database::Subquery.self_join helper

This change removes the db helper
Gitlab::Database::Subquery.self_join
since it does not need to be used anymore

doc/development/README.md

@@ -134,7 +134,6 @@ Complementary reads:
 - [Verifying database capabilities](verifying_database_capabilities.md)
 - [Database Debugging and Troubleshooting](database_debugging.md)
 - [Query Count Limits](query_count_limits.md)
-- [Database helper modules](database_helpers.md)
 - [Code comments](code_comments.md)
 - [Creating enums](creating_enums.md)
 

doc/development/database_helpers.md

-# Database helpers
-
-There are a number of useful helper modules defined in `/lib/gitlab/database/`.
-
-## Subquery
-
-In some cases it is not possible to perform an operation on a query.
-For example:
-
-```ruby
-Geo::EventLog.where('id < 100').limit(10).delete_all
-```
-
-Will give this error:
-
-> ActiveRecord::ActiveRecordError: delete_all doesn't support limit
-
-One solution would be to wrap it in another `where`:
-
-```ruby
-Geo::EventLog.where(id: Geo::EventLog.where('id < 100').limit(10)).delete_all
-```
-
-This works with PostgreSQL, but with MySQL it gives this error:
-
-> ActiveRecord::StatementInvalid: Mysql2::Error: This version of MySQL
-> doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
-
-Also, that query doesn't have very good performance. Using a
-`INNER JOIN` with itself is better.
-
-So instead of this query:
-
-```sql
-SELECT geo_event_log.*
-FROM geo_event_log
-WHERE geo_event_log.id IN
-    (SELECT geo_event_log.id
-     FROM geo_event_log
-     WHERE (id < 100)
-     LIMIT 10)
-```
-
-It's better to write:
-
-```sql
-SELECT geo_event_log.*
-FROM geo_event_log
-INNER JOIN
-  (SELECT geo_event_log.*
-   FROM geo_event_log
-   WHERE (id < 100)
-   LIMIT 10) t2 ON geo_event_log.id = t2.id
-```
-
-And this is where `Gitlab::Database::Subquery.self_join` can help
-you. So you can rewrite the above statement as:
-
-```ruby
-Gitlab::Database::Subquery.self_join(Geo::EventLog.where('id < 100').limit(10)).delete_all
-```
-
-And this also works with MySQL, so you don't need to worry about that.

ee/app/models/concerns/geo/eventable.rb

@@ -15,8 +15,8 @@ module Geo
           .where(Geo::EventLog.arel_table[:id].lteq(geo_event_log_id))
       end
 
-      def delete_with_limit(limit)
-        ::Gitlab::Database::Subquery.self_join(limit(limit)).delete_all
+      def delete_with_limit(maximum)
+        limit(maximum).delete_all
       end
     end
 

lib/gitlab/database/subquery.rb

-# frozen_string_literal: true
-
-module Gitlab
-  module Database
-    module Subquery
-      class << self
-        def self_join(relation)
-          t = relation.arel_table
-          t2 = relation.arel.as('t2')
-
-          relation.unscoped.joins(t.join(t2).on(t[:id].eq(t2[:id])).join_sources.first)
-        end
-      end
-    end
-  end
-end

spec/lib/gitlab/database/subquery_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe Gitlab::Database::Subquery do
-  describe '.self_join' do
-    set(:project) { create(:project) }
-
-    it 'allows you to delete_all rows with WHERE and LIMIT' do
-      events = create_list(:event, 8, project: project)
-
-      expect do
-        described_class.self_join(Event.where('id < ?', events[5]).recent.limit(2)).delete_all
-      end.to change { Event.count }.by(-2)
-    end
-  end
-end