[postgis-devel] automatic BVOL on spatial columns

[Markus Schaber]

Hi, Strk,

On Tue, 11 Jan 2005 08:57:11 +0100
strk at refractions.net wrote:

> Forgot to mention that I've implemented a trigger function for
> the purpose of 'manually' mantaining the bbox cache.
> The trigger use is based on the assumpion that you need a bbox cache
> for specific tables and not in intermediate geometries (the ones
> used for further manipulation).
> 
> To activate the trigger you sould issue the following command:
> 
> 	CREATE TRIGGER keep_cache
> 		BEFORE INSERT OR UPDATE
> 		ON mytable FOR EACH ROW
> 		EXECUTE PROCEDURE cache_bbox('field');
> 
> When dumping/reloading the trigger will take the effect and still
> ensure bbox cache for the relevant tables (the ones you'll use
> for fast spatial queries).

As far as I tested it, pg_dump first issues the COPY command, and the
CREATE TRIGGER afterwards. This makes sense because a dump usually is a
snapshot that already contains the effects of all triggers and rules. So
there is no need (and it can have bad side effects) to call the triggers
again when reloading the dump.

So when you pg_dump your database, and then reload the dump, all bbox
caches are lost because the canonical rep does not preserve them.

As this does not affect the default compiles that use the autocache, I
do not see a big problem here, but it should be clearly documented so
people who chose the manual approach know about the consequences.

> I haven't gone further with this, because Paul argued it would
> be a too complex path for users, but it is still available as
> it is if you set AUTOCACHE_BBOX to 0 in Makefile.config.
> 
> Maybe the whole bbox caching strategies should have their own
> chapter in the manual ...

Yes, I think this should be clearly documented.

> > Nooo! Not another extension ;)

This was not my intention, sorry for frightening you :-)

Thanks for your good job,
Markus

-- 
markus schaber | dipl. informatiker
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