The very quick exclusive lock that postgres needs for things that are normally thought of as "free" like `ALTER TABLE ADD COLUMN` without a default can still ca

The very quick exclusive lock that postgres needs for things that are normally thought of as "free" like `ALTER TABLE ADD COLUMN` without a default can still cause problems on tables that are under extremely heavy contention. If you have hundreds of clients running long transactions against a table, even this fast metadata-only lock can cause an outage due to head-of-line blocking. Our team has had good luck with this lock-polling approach for migrations against very busy tables [1]. The idea is that you combine `lock_timeout` and attempt to `LOCK TABLE IN ACCESS EXCLUSIVE MODE` in a loop prior to running the DDL. Note that this is only safe for migrations that are actually instantaneous once the lock is acquired. [1] http://www.databasesoup.com/2015/08/lock-polling-script-for-alter-table.html http://www.databasesoup.com/2015/08/lock-polling-script-for-...