[gdal-dev] Motion: Adopt RFC 29: OGR Set Ignored Fields

[Peter J Halls]

Frank,

Frank Warmerdam wrote:
> On Fri, Aug 6, 2010 at 10:48 AM, Tamas Szekeres <szekerest at gmail.com> wrote:
>> +1
>>
>> I assume that this feature will be utilized in the OGR sql query processor
>> and not directly in the ogr2ogr application.
> 
> Tamas,
> 
> ogr2ogr has a -select switch to identify desired fields
> to be copied to the output.  I imagine ogr2ogr could
> benefit from ignoring all the undesired attributes using
> this new api.
> 
> Currently there is no logic in the OGR SQL engine to
> ignore fields not needed in the result but I can see that
> adding this would be desirable.  It would require some
> work to walk all the expression trees (for the WHERE
> and each of the column expressions) to identify the
> fields that are used in the primary and joined tables.  I
> would potentially be willing to implement this optimization.

Please correct me if I am wrong, but is not the OGR SQL engine correct at 
present, ie conformant to the SQL standard, whilst this proposal would render 
OGR SQL non-conformant?

For example,

SELECT * FROM tablename WHERE condition

is the most common case and is the form currently delivered by OGR;

however,

SELECT column2,column3,column7,columnn FROM tablename WHERE condition

must be the second most common.  SQL provides for the specification of columns 
to return (* / all or a list) but does not, so far as I am aware, provide a 
construct by which one can specify columns to be omitted from the results 
(which, I suppose, would equate to * EXCEPT FOR).

Surely the goal is compliance with the standards?

Best wishes,

Peter

> 
> Re: the TestCapability(), I'd like to see this added too.
> I think this could be done without restarting the RFC
> process.
> 
> Best regards,

-- 
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Peter J Halls, GIS Advisor, University of York
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