[postgis-users] question on gist performance

[Mark Cave-Ayland]

On Fri, 2007-09-07 at 14:17 +0200, Stefan Zweig wrote:
> Gregory (and others),
> 
> some questions occurred after reading your posts: 
> 
> 1. Is it true that each index match is revalidated by looking at the corresponding data row?
> 2. Even it is true, both queries (based on the_geom or bbox, respecively) should then perform the *same* revalidation on the *same* (TOASTed) row data, resulting in identical query times?
> 3. Could anybody clarify where and when index and row data is stored in the file system? How do I know what files contains what indices and row data?
> 4. The bounding box cache should be inherent in the index, so hasBBOX(the_geom) should not play any role? (Btw, the result of hasBBOX(the_geom) is always true in our case, I even tried SELECT addbbox(the_geom) from mytable - without effect).
> 
> Thank you.

Yes. Number 1 is true because we have a RECHECK clause on the operator
class - it is this that provides the SRID checking since the SRID is
stored in the heap and not in the index. This might make sense since if
you're storing country outlines then the the geometries are likely to be
quite large...


ATB,

Mark.

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ILande - Open Source Consultancy
http://www.ilande.co.uk