[GRASS5] Re: v.external FID problem: PostGIS/OGR

[Radim Blazek]

Frank Warmerdam wrote:
> On 5/18/05, Markus Neteler <neteler at itc.it> wrote:
> 
>>Hi,
>>
>>I tried to connect to a PostGIS map which I created last year, but:
>>
>>v.external   dsn="PG:host=myserver user=postgres dbname=spearfish" layer=streams output=pgstreams
>>ERROR 1: ERROR:  Attribute "fid" not found
>>
>>DBMI-OGR driver error:
>>Cannot select:
>>select FID from streams where FID = -1
>>
>>Building topology ...
>>WARNING: Random read is not supported by OGR for this layer, cannot build
>>         support.
> 
> 
> Markus, 
> 
> The above warning message is interesting.  I assume it is the grass
> code that generates this?   I wonder how it is determining if OGR
> supports random read.  It is quite possible that the TestCapabilities
> is not returning the correct result if that is being used. 

Yes, it is using OGR_L_TestCapability, but to work properly the FID 
should be persistent which probably is not true for PostGIS as mentioned 
in the link below.

>>Here I found a reference to OGC_FID
>>
>> http://grass.itc.it/pipermail/grass5/2004-May/014395.html
>> http://www.gdal.org/ogr/drv_pg.html
>> "If there is an "ogc_fid" field, it will be used to set the feature
>>  id of the features"
>>
>>Obviously there is no ogc_fid column
>>spearfish=# \d streams;
>>           Table "public.streams"
>> Column |         Type          | Modifiers
>>--------+-----------------------+-----------
>> cat    | integer               |
>> label  | character varying(10) |
>>
>>but once v.external was functional (maybe dynamic creation of ogc_fid?).
>>
>>Does anyone have insights if OGR changed?
> 
> I was going to write a big speel about _rowid_ and ogc_fid, but now
> realize I am mixing up PostGIS support with SQLite which I was also
> working on.  OGR might require the OGC_FID to do random reads
> (fid based reads), and if the v.support stuff requires that you may well
> be out of luck.

Would it be possible to support also OID, which is created by default, 
so many people use it.

Radim

> I don't think this aspect of OGR changed recently though.
> 
> Best regards,