[Qgis-developer] Expressions - performances of spatial operators

[Matthias Kuhn]

Hi Michaël,

There are a couple of possibilities to improve the performance of
operations in two ways (which both help here AFAICS)

 * Give QgsExpression::prepare() more power
 * Compile functions (where possible)

The send one depends to some degree on the first one.

Let me know if you have sponsors, I've got some experimental code
somewhere and I am very interested in working on this project.

FYI, a first test - only the prepare part - some months ago improved the
2.5D rendering by ~30%.

Regards
Matthias

On 07/05/2016 03:11 PM, kimaidou wrote:
> Hi all,
> 
> I am a bit curious about performances concerning the use of Spatial
> expressions in QGIS, for example to select features like this
> 
>  intersects(
>     $geometry,
>     geom_from_wkt( 'MultiPolygon (((649308.28000000002793968
> 6978002.05999999959021807, 649295.32999999995809048
> 6977994.54999999981373549, 649235.92000000004190952
> 6978013.50999999977648258, 649182.53000000002793968
> 6978077.32000000029802322, 649212.34999999997671694
> 6978159.53000000026077032,  649308.28000000002793968
> 6978002.05999999959021807)))')
> )
> 
> NB: I have simplified the WKT representation here
> 
> I am running this expression inside QGIS 2.14, for a layer containing
> approx. 100 000 features ( buildings).
> The WKT polygon represents a very small area compared to the whole
> layer: 500 features only are selected.
> 
> It seems to me no spatial index is used, because it takes some time to
> run ~ 3 minutes.
> 
> I have checked the option to run expressions server side, and all the
> layers are PostGIS layers.
> 
> What could be done to improve this kind of expressions, which can be
> used elsewhere in QGIS ( to toggle labels for example ) ?
> 
> Regards
> Michaël
> 
> 
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