On Wed, Feb 25, 2009 at 12:38 AM, Lawrence, Ramon <ramon.lawrence@ubc.ca> wrote:
>> -----Original Message-----
>> From: Robert Haas
>> Sadly, there seem to be a number of cases in the Z7 database where the
>> optimization makes things significantly worse (specifically, queries
>> 2, 3, and 7, but especially query 3). Have you investigated what is
>> going on there? I had thought that we had sufficient safeguards in
>> place to prevent this optimization from kicking in in cases where it
>> doesn't help, but it seems not. There will certainly be real-world
>> databases that are more like Z7 than Z1.
>
> I agree that there should be no noticeable performance difference when
> the optimization is not used (single batch case or no skew). I think
> the patch achieves this. The optimization is not used in those cases,
> but we will review to see if it is the code that by-passes the
> optimization that is causing a difference.
Yeah we need to understand what's going on there.
> The query #3 timing difference is primarily due to a flaw in the
> experimental setup. For some reason, query #3 got executed before #4
> with the optimization on, and executed after #4 with the optimization
> off. This skewed the results for all runs (due to buffering issues),
> but is especially noticeable for Z7. Note how query #4 is always faster
> for the optimization on version even though the optimization is not
> actually used for those queries (because they were one batch). I expect
> that if you run query #3 on Z7 in isolation then the results should be
> basically identical.
>
> I have attached the SQL script that Joshua sent me. The raw data I have
> posted at: http://people.ok.ubc.ca/rlawrenc/test.output
I don't think we're really doing this the right way. EXPLAIN ANALYZE
has a measurable effect on the results, and we probably ought to stop
the database and drop the VM caches after each query. Are the Z1-Z7
datasets on line someplace? I might be able to rig up a script here.
...Robert