WFS performance issue

[Yewondwossen Assefa]

Just some info on how the WFS GetFeature with <Filter> is done
internally in mapserver :
   the <Filter> part is transformed into mapserver class expressions and
then a normal spatial query (querybyrect or querybypoint) is performed.

If the <Filter> is soemhow complex (ex It contains several spatial
queries plus attribute queries), the <Filter> is broken into several
small pieces and then  queries are done for each piece and the results
are  merged to provide the final set of results.

Later,

Frank Warmerdam wrote:
>>MapServer seems to consecutively make full table scans for each
>>parameter. The only spatial restriction obviously being what has been
>>set in the MAP file extent parameter - as this kind of request does not
>>have a spatial component.
>
> ...
>
>>Never tired of a workaround we temporarily do it with a standard
>>MapServer request with DUMP=True and the request FILTER parameters as
>>[variables-] in the MAP file. This is far from being OGC WFS but it
>>works fine. For some reason this currently will only work with OGR, we
>>are still debugging thick oraclespatial.
>
>
> Arnulf,
>
> I'm not sure I understand all the issues here.  My understanding is that
> the MapServer WFS server is evaluating all the filter conditions by doing
> a pass through the complete dataset to produce a final list of matching
> ids, is that correct?
>
> Can anyone comment on whether the WFS filters are just turned into
> regular MapServer filters (as might have been set with the FILTER
> keyword on a layer)?  If so, how does the WFS know which filter format
> to set as it differs by data provider (OGR, shapefile, OracleSpatial)?
>
> In MapServer 4.5 (possibly retrofit to 4.4.1?) I implemented support
> in the MapServer OGR link to pass FILTER requests through OGR
> and to the underlying driver in some situations.  That means, in the
> case of OGR's OCI driver that the attribute query would be evaluated
> in one SELECT statement within Oracle which I think would resolve
> your performance problems.
>
> However, to enable this the FILTER needs to be prefixed by the
> WHERE keyword. Otherwise the filter is treated as being in the
> same syntax as the shapefile filters and is evaluated by MapServer
> with potentially terrible performance.
>
> To be honest, I am fuzzy on alot of how attribute queries are working
> in MapServer, and especially how WFS queries are handled so
> I may be out in left field.
>
> However, if the WFS code turns the WFS filter into a normal MapServer
> filter it may be easy to hack it so the filter can be passed through to
> OGR's low level driver.
>
> Best regards,
> --
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>


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Assefa Yewondwossen
Software Analyst

Email: assefa at dmsolutions.ca
http://www.dmsolutions.ca/

Phone: (613) 565-5056 (ext 14)
Fax:   (613) 565-0925
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