Why does ActiveRecord provide .delete_all and not .truncate?

Question

It seems strange that I have to manually execute SQL to use the TRUNCATE command. Is there something bad about it that DHH is protecting me against?

Answer 1

Using TRUNCATE on some databases does not run triggers. Using DELETE for each row will still run triggers. TRUNCATE also cannot be rolled back, so if you did a .destroy_all in a transaction, it would erase all the data even if you tried to rollback.

So, yes, you are being protected against the effects of truncate.

Answer 2

Assuming you're using MySQL or Postgre and not SQlite3 (which doesn't support TRUNCATE), you could add the following method to your model.

def self.truncate
  self.connection_pool.with_connection { |c| c.truncate(table_name) }
end

Note that this would not invoke ActiveRecord callbacks. There are better ways to do this that don't tie your code to a specific DB implementation, but even this is better than writing the SQL yourself.

Answer 3

There are many use cases for TRUNCATE, and in my opinion, the answers given here are very insufficient. Using Postgres as an example:

• TRUNCATE is definitely transaction safe in some RDBMS, as already noted by jsears. It can be rolled back, at least in Postgres. Isn't Rails supposed to be db-agnostic and allow for such things?
• TRUNCATE will also execute BEFORE TRUNCATE or AFTER TRUNCATE triggers in Postgres. If you are expecting a DELETE trigger to fire on TRUNCATE that's your design fault!
• TRUNCATE performs far quicker than DELETE especially for large datasets because it's a single short DDL statement.
• In Postgres' MVCC architecture, TRUNCATE removes all index and table bloat.

For these reasons it's crazy to me that Rails does not support TRUNCATE. No, I don't think the reasons given are good ones.