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