Hi,
Mapserver does just that with GeoJSON. It streams data out and writes the
number of features last into JSON. I have checked the result with jsonlint and
geojsonlint and both validates it.
-Jukka Rahkonen-
Sampo Savolainen wrote:
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]<mailto:[email protected]>> wrote:
On Thu, Nov 26, 2015 at 4:58 AM, Jody Garnett
<[email protected]<mailto:[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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