[postgis-devel] TWKB_agg broken ?

[Nicklas Avén]

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


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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