[mapserver-dev] Implement msPostgisLayerGetExtent

[Tamas Szekeres]

Hi Devs,

I've recently added an implementation for msPostgisLayerGetExtent in the
master branch based upon user request, but we entered into a debate with
Thomas that this feature should never have been implemented at all.

The key point against this feature is that it may cause performance issues
for the existing configurations (as said 'denial of service') when the
extent is not specified manually (in the mapfiles) and MapServer will now
turn to the driver specific implementation instead of returning MS_FAILURE
(in LayerDefaultGetExtent). Thomas also mentioned that this kind of
addition should be disabled by default unless voted forward by the PSC.

I did not think that we ever had an agreement not to implement
msPostgisLayerGetExtent at all to address potential performance issues by
the driver, as this has already been implemented for most of the drivers
without such issues manifested. It is already documented that the extent
should be configured manually  "to avoid the speed cost of having MapServer
compute the extents of the data" (http://mapserver.org/mapfile/layer.html).

Anyway, I would not be against a configuration option to specify that the
extent is 'not set' for a layer, but it should be out of scope of a driver
implementation. And I'm not really convinced about the usefulness of such
setting.

The complete discussion about this change can be found here:
https://github.com/mapserver/mapserver/pull/4825

Let me know if you prefer to keep this functionality or revert to the
original (not implemented) behaviour.

Best regards,

Tamas
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