[postgis-devel] Caching Double-based Boxes

[maplabs at light42.com]

I found it unexpected (and disconcerting) that the run times were 
always so much better immediately after a postgres restart. (3rd test 
run)   -Brian

On Tue, 29 Nov 2011 15:11:06 +0000, Mark Cave-Ayland 
<mark.cave-ayland at siriusit.co.uk> wrote:
On 29/11/11 14:14, Paul Ramsey wrote:
>
> > Well, two things (and notice to reader, we're discussing the doublebox
> > spike here):
> > - on the side of expected slowdown (systematic problem), we're pushing
> > twice as much data through the pipes (reading cached double instead of
> > cached float) to get the answer we want
>
> True, but I'd expect this effect to be minimal or perhaps a couple of 
> percent at most. 
>
> > - on the side of the unexpected slowdown (operator problem (Paul
> > error)), in order to do the comparisons between the cache keys
> > (doubles) and the index keys (floats) the cache keys now all have to
> > be converted to floats before the comparison so there's an extra step
> > in there that doesn't exist in the pure-float (cached float and float
> > indexes) implementation
>
> Now this might go some way to explaining some of it, although again 
> most of this is speculation without a profile. 
>
>
> ATB,
>
> Mark. 
>
> -- Mark Cave-Ayland - Senior Technical Architect
> PostgreSQL - PostGIS
> Sirius Corporation plc - control through freedom
> http://www.siriusit.co.uk
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>
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