Hi Martin
>- it may be interesting to measure the proportion of time necessary
>for field calculation / searching. I believe the the evaluation takes
>just a small amount of time compared to time required for fetching of
>features, therefore a fast engine might not necessarily speed things
>up significantly
That would be very interesting (probably it can be tested much better
now thanks to qgis_bench).
My only experience here was the WMS benchmark 2011 with many layers and
a lot of expressions (osm data, a lot of classification rules, many
layers). For requests with a lot of features in view, expression
evaluation showed up considerably in the profiler, both in QGIS server
and UMN mapserver. Yes, feature fetching wasn't considered (still
cachegrind) and I don't have the cachegrind dumps at hand. However, it
still shows that there are use-cases where evaluation is not neglectable.
Regards,
Marco
On 10.01.2012 14:03, Martin Dobias wrote:
On Tue, Jan 10, 2012 at 1:00 AM, Pirmin Kalberer<[email protected]> wrote:
Pros:
-Better performance
-Full Javascript language set included
-Possibility for writing custum functions
Cons:
-New language for expressions
-More fat (3.7MB for libv8.so)
Thanks for the benchmark.
I have several things to note:
- it may be interesting to measure the proportion of time necessary
for field calculation / searching. I believe the the evaluation takes
just a small amount of time compared to time required for fetching of
features, therefore a fast engine might not necessarily speed things
up significantly
- V8 may be aware of the fact that the benchmarked expressions are
constant - and evaluate them during compilation, resulting in much
shorter execution time. It would be good to include a benchmark that
uses variables (column references).
- the engine implements three value logic (true/false/unknown) like in
SQL, while JavaScript (AFAIK) uses usual two value (true/false) logic.
How would you add support for NULL values in operators?
- Python is another language which could be used for the expressions -
with similar advantages as JavaScript
At some point we may support several languages for expressions
(ideally with just one evaluation engine). Regarding JavaScript, as
Juergen noted, there is already JavaScript engine in Qt, so adding V8
dependency looks like a high cost for the exchange of faster
expression evaluation. My impression is that such a state of the art
engine makes sense for web apps with thousands of lines in JavaScript,
but not that much for simple expressions.
Regards
Martin
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Dr. Marco Hugentobler
Sourcepole - Linux& Open Source Solutions
Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
[email protected] http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee
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