[gdal-dev] RFC 29: OGR Set Desired Fields

[Jez Walters]

Martin,


I may be misunderstanding how the API works (I'm not actually using it myself), but if someone wants to parse a given layer of the OS MasterMap vector data that is divided into chunks they can't use fixed field numbers, because the field order varies between chunks.

For example, suppose I want to only extract the 'physicalPresence' field from the 'TopographicArea' layer. Using your current proposed API I would have to sometimes specify to extract field 10 and sometimes to extract field 11, depending on which chunk I'm reading.

This CAN be accommodated of course (although it's rather fiddly), but the problem disappears if field names are used, as field names don't change between chunks.


Regards,


Jez


-----Original Message-----
From: Martin Dobias [mailto:wonder.sk at gmail.com]
Sent: Wednesday 21 July 2010 13:24
To: Jez Walters
Cc: gdal-dev at lists.osgeo.org
Subject: Re: [gdal-dev] RFC 29: OGR Set Desired Fields

On Wed, Jul 21, 2010 at 1:36 PM, Jez Walters <Jez.Walters at ipl.com> wrote:
> Although I take your point about the interface not affecting ogr2ogr, surely the potential speed boost is worth extending to this utility too - or would that be a separate ticket?  I for one would welcome improving the performance of ogr2ogr using the approach you have suggested - when I launch the current trunk development version of ogr2ogr from VBScript it gets stuck in an infinite loop trying to process a 5x5km vector data chunk.

ogr2ogr could probably use the API when "-select" option is used - to ignore fields which are not being requested. Any other performance improvements are unrelated to this RFC.


> This issue aside, won't the fact that field names are constant but field numbers are not in the OS MasterMap vector data present problems for those that wish to programmatically parse the OS GML using your interface?

AFAIK once a layer gets open, the indexes of fields for this layer stay fixed.

Regards
Martin


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