[mapserver-users] querying WMS layer / WFS reprise

[Frank Warmerdam]

Gregor Mosheh wrote:
> I spent the last day experimenting with WFS, since that's what we *really*
> want to be sending instead of images, and came to the conclusion (that
> seems to have been confirmed by some others on the list) that WFS is
> incredibly slow (as in, several minutes) because of the GML encoding,
> which made it unsuitable for use in dynamic map layers.
> 
> WFS' slowness surprised me: should a feature list being sent to the client
> never contain more than several items? Or is this a "problem" with
> MapServer's implementation of a WFS server? Would another WFS server
> (GeoServer?) do it faster and be able to send more items in a reasonable
> period?

Gregor,

I don't think WFS queries should be inherently slow.  Encoding GML is not
a particularly expensive task.  But there are lots of issues to be careful
of with WFS.  For instance, it is easy to unexpectedly generate a very large
GML response set if you select alot of features or they are very complex.

I think you need to examine what you are doing more closely to see if:
  o the resultset is very large (dump response to file and inspect size)
  o the query being done by MapServer is expensive.  Some WFS geometric
    queries may be expensive to check.  MapServer is generally not terribly
    fast at attribute queries as far as I know.
  o The MapServer query architecture requires that each query result be
    fetched individually by id after a first pass to build a list of ids for
    features that satisfy the request.   This kind of read is very slow
    against some datasources (ie. some OGR drivers).

Lets try and dig into the performance a bit deeper to determine what the
issue is.

Best regards,
-- 
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