Add Gitlab::Database::Subquery.self_join to delete_all with limit

`delete_all` doesn't support limit, so you'd need to subquery
that. And instead of subquerying with `where(id: query)`, it's better
to use an `INNER JOIN`. This method also works with MySQL, while
subquerying doesn't (without another layer of subquerying)

Reference:
https://stackoverflow.com/questions/17892762/mysql-this-version-of-mysql-doesnt-yet-support-limit-in-all-any-some-subqu/17892886#17892886

2 files changed, 64 insertions, 0 deletions

diff --git a/doc/development/README.md b/doc/development/README.md
index d8dbc993442..94e604d125d 100644
--- a/doc/development/README.md
+++ b/doc/development/README.md
@@ -94,6 +94,7 @@ description: 'Learn how to contribute to GitLab.'
 - [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)
 
 ## Testing guides
 
diff --git a/doc/development/database_helpers.md b/doc/development/database_helpers.md
new file mode 100644
index 00000000000..21e4e725de6
--- /dev/null
+++ b/doc/development/database_helpers.md
@@ -0,0 +1,63 @@
+# 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.