[postgis-devel] TWKB_agg broken ?

[Sandro Santilli]

On Thu, Aug 07, 2014 at 01:09:54PM +0200, Nicklas Avén wrote:
> Ah, Actually delta values will probabaly save you some space.
> 
> 
> The coordinates is saved as signed varInt. 1 byte only reaches from -64 to 63.
> 1 bit is lost in the varInt format signalizing if there is any more bytes
> And since it is signed another byte is lost on the positive side.
> 
> 
> So all values outside -64 - 63 will need 2 bytes in the ST_AsTWKB-version but in the agg version many of them will only use 1 byte if the delta value to the point before per axis is between -64 and 63

Got it, but that's still a max of x2 size difference,
while we're talking about a x3 size difference here.

--strk;

> 
> 
> 2014-08-07 Sandro Santilli  wrote:
> 
> 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;
> >
> >

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