Bug report #17031: getFeatures (by Fids) extremely slow on large PostgreSQL table WITH PK ( OK if NO PK) - QGIS Application - QGIS Issue Tracking

Bug report #17031

getFeatures (by Fids) extremely slow on large PostgreSQL table WITH PK ( OK if NO PK)

Added by Vincent Dionne over 7 years ago. Updated about 6 years ago.

Status:	Closed
Priority:	High
Assignee:	-
Category:	Data Provider/PostGIS
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

Description

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.

History

#1 Updated by Giovanni Manghi over 7 years ago

Status changed from Open to Feedback
Priority changed from Normal to High

Was ok on a previous LTR like 2.8 or 2.14?

#2 Updated by Vincent Dionne over 7 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' ...