[postgis-devel] What's the difference between geom_accum andbuiltin postgresql array_append

[Obe, Regina]

Getting back to this topic.  Recall I said there was a situation where LWGEOM_accum worked where array_append didn't.  I can't replicate it so I suspect it was an earlier error in my indexing efforts that caused this issue.  I am also not seeing this LWGEOM_accum function used directly in any of the postgis code base.

So here is my half-assed suggestion since no one seems to know why we should maintain the LWGEOM_accum function instead of just using built-in PostgreSQL array_append function.

Simply get rid of LWGEOM_accum from our code base (or deprecate it) -- change

/**CREATE OR REPLACE FUNCTION st_geom_accum(geometry[], geometry)
  RETURNS geometry[] AS
'$libdir/liblwgeom.dll', 'LWGEOM_accum'
  LANGUAGE 'c' IMMUTABLE;
***/

To: 
CREATE OR REPLACE FUNCTION st_geom_accum(geometry[], geometry)
  RETURNS geometry[] AS
$$ SELECT array_append($1,$2);  $$
  LANGUAGE 'sql' IMMUTABLE;


And that's it.  In tests I've done doing above, I don't see a definitive speed difference and all aggs we have seem to have this function for collecting geometries.  

I would have suggested replacing our use in aggregates directly with array_append, but thought it would be better to keep this buffer for 3 reasons
1) We don't have a CREATE OR REPLACE AGGREGATE which makes changing aggs without a drop reload tricky. Changing this function will auto-magically correct the aggs. Granted 1.4 people should be doing a full reload anyway.

2) Don't know if anyone is relying on the st_geom_accum function so we probably should keep it around anyway.

3) In case we are wrong.  It won't be a major undertaking to correct the mistake.

Any thoughts, concerns?

Thanks,
Regina


-----Original Message-----
From: postgis-devel-bounces at postgis.refractions.net on behalf of Mark Cave-Ayland
Sent: Wed 8/20/2008 11:16 AM
To: PostGIS Development Discussion
Subject: Re: [postgis-devel] What's the difference between geom_accum andbuiltin postgresql array_append
 
Paul Ramsey wrote:
>> Yeah, that's one good argument. The reason the 8.3 changes hit us so hard
>> were because we were accessing the varlena structures directly rather than
>> using the relevant macros :(
> 
> Speaking of which, I see those damned macros sprinkled hither and yon,
> but I have no idea where they are documented. PgSQL source code only?
> It would be nice to know what they do.
> 
> P.

Yeah, it's generally a case of "read the source" :(  While it can be 
tricky to find the relevant header file, the macros are generally well 
documented when you do find them.


ATB,

Mark.

-- 
Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
http://www.siriusit.co.uk
T: +44 870 608 0063
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