[QGIS-Developer] Feature count for DB provider.

[Jacky Volpes]

Hello Rémi,

The function you refer to
https://github.com/qgis/QGIS/blob/master/src/core/vector/qgsvectorlayerfeaturecounter.cpp#L76-L90
is only called when the feature count must be done for the symbols (for 
instance when a categorized symbology is used, then QGIS does not use 
estimated metadata, and the count for each symbol is then evaluated and 
displayed).

Normally there is no SELECT count(*) FROM or SELECT * FROM when 
estimated metadata is used.

Regards,

-- 
Jacky Volpes

Ingénieur SIG - Oslandia


Le 26/06/2022 à 20:20, Rémi Desgrange via QGIS-Developer a écrit :
>
> Thanks Regis.
>
> I tried with an existing project, and with no project. The information 
> of the layers is:
>
> service='mycomp_prod' sslmode=require key='gid' estimatedmetadata=true 
> srid=27572 type=Point checkPrimaryKeyUnicity='1' table="public"."mylayer"
>
> with service in `.pg_service.conf` :
>
> [mycomp_prod]
>
> user=rd
>
> sslmode=require
>
> host=gis.mycomp.fr
>
> port=5432
>
> dbname=mycomp
>
> (don’t blame me for the 27572 OK 😅)
>
> I think I’ll open an issue because it _/seems/_ to do a SELECT 
> count(*) FROM mylayer + SELECT * FROM mylayer until all features have 
> been fetched. In my example, I have a 11M Points layer so it takes a 
> while 😊.
>
> *From: *Régis Haubourg <regis.haubourg at gmail.com>
> *Date: *Saturday, 25 June 2022 at 12:54
> *To: *Rémi Desgrange <remi.desgrange at ielo.net>
> *Cc: *QGIS Developers List <qgis-developer at lists.osgeo.org>
> *Subject: *Re: [QGIS-Developer] Feature count for DB provider.
>
> 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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