On Tue, Jun 24, 2014 at 4:25 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Patrick Krecker <patrick@judicata.com> writes: > Hello everyone -- We received a strange error today on our production write > master. During a routine maintenance script, we got the following error:
> "ERROR: cache lookup failed for aggregate 5953992"
> I think I found the culprit. We have a cron script that (among other > things) recreates the aggregate function array_cat_aggregate()once every > minute.
Um. Why's it do that?
Well, it's admittedly a hack. The cron script uses the function just after creating it. It was an easy way of guaranteeing that the database will have the function when it's necessary. However, I suppose that assumption is wrong because you have confirmed that aggregate functions are not part of the snapshot provided by beginning a transaction.
> My guess is that a) transactions do not guarantee a snapshot of > custom functions and b) we got unlucky and the aggregate was deleted during > the execution of the UPDATE.
Yeah, there's no guarantee that dropping the aggregate wouldn't leave a window for this type of failure.
9.4 might be a little better about this because it doesn't use SnapshotNow for catalog fetches anymore, but I think you'd still be at some risk.
> Here is the query that produced the error:
It'd be more interesting to see what the cron script was doing to the aggregate definition.
FWIW the SQL is
DROP AGGREGATE IF EXISTS array_cat_aggregate(anyarray);
CREATE AGGREGATE array_cat_aggregate(anyarray) (
SFUNC = array_cat,
STYPE = anyarray,
INITCOND = '{}'
);
Followed by the other statement given in my previous email. But, I think you've thoroughly answered by question. Thanks!
