Does ALTER TABLE lock table?

Does ALTER TABLE lock table?

Yes, it locks the table. From the docs on MySQL 8, The exception referred to earlier is that ALTER TABLE blocks reads (not just writes) at the point where it is ready to clear outdated table structures from the table and table definition caches. At this point, it must acquire an exclusive lock.

How do you unlock a table in postgresql?

PostgreSQL – how to unlock table record AFTER shutdown

1. Insert row in TABLE A.
2. Lock this record.
3. (At separate terminal) service postgresql-9.6 stop.
4. Wait a few moments.
5. (At separate terminal) service postgresql-9.6 start.
6. “try” to unlock the record by executing “COMMIT;” in the same terminal as #2.

When to avoid ” waiting for table metadata lock “?

If you no longer see “Waiting for table metadata lock” – good luck, it should finish soon (assuming that the table you’re changing is not very large, i.e. gigabytes or more). When your ALTER TABLE query is finished, remember to set interactive_timeout and wait_timeout values to what they previously were.

What happens if an ALTER TABLE statement is executed?

If you’re running a busy database, the chances are that a ALTER TABLE statement executed will not really work, you will end up with MySQL processlist full of “Waiting for table metadata lock” – including your original ALTER TABLE statement, MySQL processlimit will be reached, and your website/system will no longer work.

How to control the lock wait timeout for DDL?

So it’s probably not what you want. For blocking DDL statements you can control the timeout with ddl_lock_timeout. This gives the time to wait for DDL in seconds. Not that non-blocking DDL (such as alter table add column) will still wait for session 1 to commit/rollback.

Is there way to alter table without locking the table?

If it’s a big table, INSERT or UPDATE statements could be blocked for a looooong time. Is there a way to do a “hot alter”, like adding a column in such a way that the table is still updatable throughout the process?