[QGIS-Developer] Feature count for DB provider.

[Régis Haubourg]

Hi Rémy, There had been a lot of work for postGIS and oracle providers. A
count(*) might be really expensive for big tables, so there are options to
use db statistics instead of real count. I bet the loop you point out
occurs only in fallback situations but is not the main case.

One is here for instance by there are numerous PRs like this one
https://github.com/qgis/QGIS/pull/37619

In think @troopa81 has a clear view of the current state.
all the best
Régis

Le sam. 25 juin 2022 à 10:38, Rémi Desgrange via QGIS-Developer <
qgis-developer at lists.osgeo.org> a écrit :

> Hi,
>
>
>
> Just launched the new QGIS 3.24, and tests the SQL debugger, I just found
> out that the “feature count” does a while loop on all features in the layer
> to count.
>
>
>
> While it’s probably necessary for shapefiles or stuff like that, it
> appears suboptimal for databases access. I found this part in the code that
> looks like to be responsible for the feature counts,
> https://github.com/qgis/QGIS/blob/master/src/core/vector/qgsvectorlayerfeaturecounter.cpp#L76-L90
>
>
>
> Do you think I could make a special case for database-based provider
> (geopackage, postgres/oracle/mysql/etc…) that launch a “SELECT count(*)
> FROM table” instead of looping on all features.
>
>
>
> Thanks.
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