63 lines
1.4 KiB
Markdown
63 lines
1.4 KiB
Markdown
# 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.
|