The intention of this feature is to give the ability to slip into a normal workload for non-urgent maintenance work. In essence, instead of lock waiters being in a Queue, DEFERRABLE causes the current lock statement to always be last.
Good idea; this was on my list of things to implement. I was going to call it WAIT PATIENTLY option.
It was discussed at last years pgCon as useful for replication tools adding/removing triggers. I've also seen more than one plpgsql loop using subtransactions and LOCK TABLE .. NOWAIT to achieve a similar effect. IMO, it's much cleaner built in.
Agreed, but your implementation is essentially just the same looping concept, which I don't much like.
If a lock is successfully obtained on one table, but not on all tables, it releases that lock and will retry to get them as a group in the future. Since inheritance acts as a group of tables (top + recursive cascade to children), this implementation is necessary even if only a single table is specified in the command.
I'd prefer to see this as a lock wait mode where it sits in the normal lock queue BUT other lock requestors are allowed to queue jump past it. That should be just a few lines changed in the lock conflict checker and some sleight of hand in the lock queue code.
That way we avoid the busy-wait loop and multiple DEFERRABLE lock waiters queue up normally.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
