[postgis-devel] TWKB_agg broken ?

[Sandro Santilli]

On Thu, Aug 07, 2014 at 10:44:40AM +0200, Nicklas Avén wrote:
> 
> 
> 2014-08-07 Sandro Santilli  wrote:
> 
> On Wed, Aug 06, 2014 at 07:04:34PM +0200, Nicklas Avén wrote:
> >> On on., 2014-08-06 at 18:40 +0200, Sandro Santilli wrote:
> >> > Nicklas, I was looking at ST_AsTWKB_agg but I think it's broken.
> >> > Why would otherwise the output of ST_AsTWKB_agg be smaller than
> >> > the output of ST_AsTWKB ?
> >> 
> >> It is possible because the aggregate version uses delta values between
> >> the points which often gives smaller values to store. 
> >
> >Is there no separator between records in the output of the aggregator ?
> >How's it different from collecting the geometries and passing the
> >collection to ST_AsTWKB ?
> 
> The differnce is that by using ST_AsTWKBagg you won't loose the individual id:s.
> If you use ST_Collect it will just be a collection.

I requested no IDs, the queries were:

 ST_AsTWKB   (g,0)
 ST_AsTWKBagg(g,0)

The dataset I selected (10000 points) have this extent:

 BOX(-169.499567541294 -79.583924216684,-160.680307442788 -71.3369439435191)

I specified 0 as number of digits.  This means that any absolute value
also fits a single byte, right ?

So, if IDs are absent in both and delta values fit within one byte in both...
what else could make 10k points take 60k bytes with the non-aggregate
and  20k bytes (1/3) with the aggregate call ?

--strk;