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# 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.
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