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Henk Ernst Blok
 
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Default Re: Strange count(*) implementation? - 10-26-2004 , 10:04 AM





Hi Alvaro,

I used to do some research in extensibility of query optimizers to match
the extensibility of the operators. However, it's not really in the
focus of my research anymore so I can't spend much time on it,
unfortunately. I'll keep it in mind in case a student of the group where
I'm working is looking for an project.

Regards,


Henk Ernst.

Alvaro Herrera wrote:

Quote:
On Tue, Oct 26, 2004 at 01:56:41PM +0200, Henk Ernst Blok wrote:

Hi,



I assume(d) the more expensive statistics (e.g., value distribution
info) are updated only when outdated too much or on request (manual
vacuum). Usually, other/cheap statistics can easily be maintained
incrementally and thus reflect actual table state after each update. Of
course, the MVCC principle seems to make things a bit more complicated I
understand now.



It's not only MVCC; it's also the fact that aggregates are extensible.
So to the system they are just opaque functions and it doesn't know how
to optimize them.

Of course this can be done, e.g. by supplying an optimizing function with
each aggregate that would try to convert the step-by-step aggregate
execution plan into a completely different operation (say by looking at
an index to obtain a max() value), but as you see this is not a trivial
task, and more importantly, it hasn't been done. Which is to mean, if
you want to try and submit a patch, it could be improved in the future.
--
address: DB group, Computer Science, EEMCS Dept., University of Twente,
PO Box 217, 7500 AE, ENSCHEDE, THE NETHERLANDS
phone: ++31 (0)53 489 3754 (if no response: 3690)
email: h.e.blok (AT) utwente (DOT) nl
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