DELETEqueries. But it's a bad idea to delete huge numbers of records at a time if the operation will be more than a few seconds. So deleting in small batches can be done like this:
DELETE FROM mytable WHERE id = ANY(ARRAY(SELECT id from mytable WHERE myfield = 10 LIMIT 10))
To use this code from ActiveRecord in Ruby and get the number of affected rows:
rows_affected_count = ActiveRecord::Base.connection.execute(query).cmd_tuples
You can do this in a loop to keep the queries moving through the system without slow queries choking things up. When
rows_affected_countis less than the size of the batch, you're done.