[mapserver-dev] Implement msPostgisLayerGetExtent

[Stephen Woodbridge]

Can someone provide a little background on why and where layer extents 
are used?

I have never used them and in fact never noticed that there was an 
extents at the layer level. I have always set the extents at the map 
level, so I guess by default the layers inherit that.

I presume that the layer extents are checked to see if the whole layer 
can be rejected if the image extents does not overlap with the layer 
extents. Is this correct?

Assuming that it is correct, are we changing this behavior?
ie: the layer inheriting from the map extents? (assuming that it did). I 
would not want to take a performance penalty or not having this and 
would not want to have to change all my mapfiles to add the extents to 
all the layers.

-0 based on not understanding why this is needed and where it gets used!

-Steve W



On 12/31/2013 10:58 AM, Tamas Szekeres wrote:
> 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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