I have an enormous MySQL (InnoDB) database with millions of rows in the sessions table that were created by an unrelated, malfunctioning crawler running on the same server as ours. Unfortunately, I have to fix the mess now.
If I try to truncate table sessions; it seems to take an inordinately long time (upwards of 30 minutes). I don't care about the data; I just want to have the table wiped out as quickly as possible. Is there a quicker way, or will I have to just stick it out overnight?
The quickest way is to use DROP TABLE to drop the table completely and recreate it using the same definition. If you have no foreign key constraints on the table then you should do that.
If you're using MySQL version greater than 5.0.3, this will happen automatically with a TRUNCATE. You might get some useful information out of the manual as well, it describes how a TRUNCATE works with FK constraints. http://dev.mysql.com/doc/refman/5.0/en/truncate-table.html
EDIT: TRUNCATE is not the same as a drop or a DELETE FROM. For those that are confused about the differences, please check the manual link above. TRUNCATE will act the same as a drop if it can (if there are no FK's), otherwise it acts like a DELETE FROM with no where clause.
EDIT: If you have a large table, your MariaDB/MySQL is running with a binlog_format as ROW and you execute a DELETE without a predicate/WHERE clause, you are going to have issues to keep up the replication or even, to keep your Galera nodes running without hitting a flow control state. Also, binary logs can get your disk full. Be careful.
The best way I have found of doing this with MySQL is:
DELETE from table_name LIMIT 1000;
Or 10,000 (depending on how fast it happens).
Put that in a loop until all the rows are deleted.
Please do try this as it will actually work. It will take some time, but it will work.
Have you tried to use "drop"? I've used it on tables over 20GB and it always completes in seconds.
Truncate is fast, usually on the order of seconds or less. If it took 30 minutes, you probably had a case of some foreign keys referencing the table you were truncating. There may also be locking issues involved.
Truncate is effectively as efficient as one can empty a table, but you may have to remove the foreign key references unless you want those tables scrubbed as well.
We had these issues. We no longer use the database as a session store with Rails 2.x and the cookie store. However, dropping the table is a decent solution. You may want to consider stopping the mysql service, temporarily disable logging, start things up in safe mode and then do your drop/create. When done, turn on your logging again.
I'm not sure why it's taking so long. But perhaps try a rename, and recreate a blank table. Then you can drop the "extra" table without worrying how long it takes.