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From Matt Doran <matt.do...@papercut.com>
Subject Re: FW: Advice on *very* badly performing query
Date Tue, 04 Dec 2007 23:09:43 GMT
Dyre.Tjeldvoll@Sun.COM wrote:
> Matt Doran <matt.doran@papercut.com> 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.


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