Bug report #17031
getFeatures (by Fids) extremely slow on large PostgreSQL table WITH PK ( OK if NO PK)
|Affected QGIS version:||2.18.3||Regression?:||Yes|
|Operating System:||Easy fix?:||No|
|Pull Request or Patch supplied:||No||Resolution:||not reproducable|
|Crashes QGIS or corrupts data:||No||Copied to github as #:||24930|
Hello, here is the case:
I have a PostGis Table containing 5 millions GPS point. I want to selected some of them and then call getFeatures in Python.
The table has a standard serial PK.
The call to loLayer.getFeatures(loRequest) is extremely slow. First I did a loRequest.setFilterFids(loLayer.selectedFeaturesIds()) to get the selection.
Removing the PK on the table solves the problem => super fast.
After some research, I found that the Postgre Data Provider use CTID in its WHERE clause (FAST) when no PK is found (disabling editing features, wich ,in my case is okay). And in all othe cases, the fids are cast into strings in the where clause... (SLOW - the index is not use i guess)
I understand it is related with the fix of #14262.
At last there is a huge performance regression when working with large tables. At some point, when no editing is needed ( ie select statement), the where clause should be working with a integer PK.
I hope I was clear enough describing the problem... thanks.
#2 Updated by Vincent Dionne about 4 years ago
Tested in 2.14 and it is OK
Here is the postgreSQL log showing the WHERE clause in 2.14 : (exec time : ~ 2 seconds)
DECLARE qgis_4 BINARY CURSOR FOR SELECT
FROM "scad"."20170322_ibus" WHERE "id" IN (3453850,3453851,5800408,3453852,5800409,5800410,5800411,1121748,1121749,1121750,1121751,1121 ...
in 2.18: (exec time : i don't know , I stopped it after 15 minutes)
DECLARE qgis_5 BINARY CURSOR FOR SELECT
FROM "scad"."20170322_ibus" WHERE ("id"::text='5174976' OR "id"::text='5064624' OR "id"::text='5174977' OR "id"::text='5064625' OR "id"::text='5174978' OR "id"::text='5174979' OR "id"::text='5174980' OR "id"::text='5174981' OR "id"::text='5174982' OR "id"::text='5174983' OR "id"::text='5174984' OR "id"::text='5174985' ...