Most of the "stat" information that derby uses is automatically kept up
to date as part of underlying index and table maintenance. This info
includes count of rows in the table and data distribution of data in
indexes.
The one piece of info that is not kept up to date is average number of
duplicates for columns in an index. This stat is given a default and
then is updated whenever you create an index, run the discussed compress
option, and as a side effect of some of the alter table commands. I
don't remember what the default is, something like 10%.
/mikem
Kim Haase wrote:
Do you think it might also help to put the information into the Tuning
Guide under "Performance tips and tricks"?
http://db.apache.org/derby/docs/dev/tuning/tuning-single.html#ctunperf22457
Thanks,
Kim Haase
Matt Doran wrote:
[EMAIL PROTECTED] wrote:
Matt Doran <[EMAIL PROTECTED]> 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?
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.
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.
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.
Thanks. I had trouble understanding the behaviour ... so I though
that nobody would believe me unless I provided enough evidence.
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. 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.
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.
Regards,
Matt
