Re: pg_locks doesn't check for interrupts?

On 11/18/2014 10:47 AM, Tom Lane wrote:
> Josh Berkus <josh@agliodbs.com> writes:
>> Since querying pg_locks can be intrusive due to needing to lock the lock
>> partitions, when I'm collecting data about locks I generally put a
>> statement_timeout on it.  However, I'm noticing that this
>> statement_timeout appears to be completely ignored; I've seen this query
>> run for up to 10 minutes* when the database is heavily loaded.  This it
>> seems likely to me that the functions under pg_locks aren't checking for
>> interrupts.  Anybody checked this already?
> 
> Whether they do or not, I don't think we allow CHECK_FOR_INTERRUPTS to
> trigger while holding an LWLock.  So this would not be a trivial thing
> to fix.

Hmm.  So the basic problem is that querying pg_locks itself can make an
already bad locking situation worse (I've seen it contribute to a total
lockup, which didn't resolve until I terminated the query against
pg_locks).  I don't see a clear way to make it less dangerous, so I was
hoping that at least making it time out made it safer to use.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com