We have hundreds if not thousands of customers using our product and
not many of them have seen this pathological performance problem. So
maybe the optimizer does a good enough job in 90% of cases. We just
happened to hit the an extremely bad case.
Matt Doran <firstname.lastname@example.org> writes:
I had not idea
that derby didn't keep any stats up-to-date without performing that
operation explicitly. Ideally it would keep this up-to-date itself.
The sys.sysstatistics didn't have any rows in it until I ran the
compress table operation.
Agreed, but strangely I cannot recall many users actually requesting
this. Maybe people just suffer silently?
So maybe it's just not something that people notice often. Or they
just think "oh it's an embedded java database, it probably doesn't
perform that well. Let's just upgrade to a real database". That's
what we did, and it's what other people probably do.
The beauty of the embedded DB is that it is self-maintaining. I
suspect that if it maintained the statistics by itself and therefore
there were performance benefits ... it will improve people's perception
that it performs well.
Thanks. I had trouble understanding the behaviour ... so I though that
nobody would believe me unless I provided enough evidence.
Anyway, thank you for what I would call an exemplary
bug-report/question! Even though you use Hibernate you took the time
to identfy the actual SQL causing the problem, identified a minimal
repro and provided query plans.
I'm not sure. No-one is every going to read a whole manual. But I
had read the ApacheCon performance presentations, and I don't remember
them ever mention this. I think those presentations would be one of
the first places people start when they have performance problems. I
know you can't change these retrospectively ... but maybe making this
clear in the wiki would be a start.
It really needs to be made more prominent in the documentation.
i.e. once your database is loaded with representative data, perform
the compress op for optimal performance.
Agreed. Any thoughts on where it would be good to mention it? If
you want, you can file a Jira issue about this.
I'm not sure if this is the appropriate page, but it was the only thing
that looked relevant to performance ( http://wiki.apache.org/db-derby/PerformanceDiagnosisTips).
It doesn't mention the stats/compress.
Maybe some of the tips in those presentations should be the distilled
into some performance tips wiki pages ... and also make it clear that
stats need to be updated.