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.haubo...@gmail.com>
*Date: *Saturday, 25 June 2022 at 12:54
*To: *Rémi Desgrange <remi.desgra...@ielo.net>
*Cc: *QGIS Developers List <qgis-developer@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@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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