Hi,
I've finally got my head around Andrea's objections and you are absolutely
right. Refactoring the number of features returned as a lazily populated
number, similar to the total count, seems a sane way to achieve this. But
I'm a bit worried that if the feature count is added to the JSON output,
then the performance benefits of this would be lost also for that format. I
did some rudimentary tests in the environment I'm working with (oracle
backed feature, features derived from views) and doing the extra select
count() adds ~50% to the response time of the queries.
Do you have any idea what is the use case for having the count in the GML /
JSON response? In GML, having the count as an attribute in the root element
kills any possibility of fully streaming the results. Only a streaming
parser could take advantage of the count before the full response is read.
For GML, there is a guarantee that the consumer will receive the root
element before the features. But I don't think GeoJSON defines any order in
which fields in JSON objects need to be presented in, so there would be no
guarantee for the consumer that they would receive the number of features
before actually streaming the features themselves. And when you have the
features, you have the count.
Sampo
On Thu, Nov 26, 2015 at 9:21 AM, Andrea Aime <[email protected]>
wrote:
> On Thu, Nov 26, 2015 at 4:58 AM, Jody Garnett <[email protected]>
> wrote:
>
>> I am catching up on email, and kind of wish we had an example to point
>> to.
>>
>> Is it worth writing up a proposal page so you can clearly describe
>> "places you mentioned" where counting is still required?
>>
>> My take on Andrea's response is that the GML format is forcing us to
>> count features for WFS 2.0 GetFeature. I do not think he was rejecting your
>> approach, just pointing out a limitation of the specification.
>>
>
> Yes. It means whatever implementation we do has either lazy count, similar
> to number matched, or we have a way to reach to the output
> format and check its specific needs (something that would be useful for
> the other need we'd might have, checking if the output
> format has axis order assumptions... e.g., don't think anybody is happy
> with us producing lat/lon ordered data in anything
> but GML).
>
>
>>
>> Can I confirm that this feature counting is specific to GML output? Like
>> if you were outputting geojson you would not need to count features?
>>
>
> Yes and no, we have a sequence of pull requests that introduced counts in
> GeoJson output format, so someone definitely
> needs it (to the point they contributed code for it). Generally speaking,
> counting is useful for anyone in need of doing paging.
> But I guess it could be made optional with a new config flag, of course,
> only for the formats where it can be made optional.
>
> Cheers
> Andrea
>
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