[Qgis-user] What is postgresql client version and does it have anything to do with different postgresql versions

[Richard Duivenvoorde]

On 2/4/21 10:16 AM, Groene Bij wrote:
> Take this example: 
> I would like to see DB Manager as just a shell or GUI which will work with
> every postgresql and postgis version the same way, but in fact it isn't. I
> found DB manager to work differently from qgis3.10 to 3.12 and above.
> Something changed and as a result, simple select queries can take up 10 to a
> 100 times longer in qgis3.12 and up compared to qgis3.10
> (see also https://github.com/qgis/QGIS/issues/40643)
> 
> So, I am mostly looking for some kind of reassurance. Things like: qgis3.16
> will work fine with postgresql version a to d and postgis version f to l. Or
> qgis will work fine with any postgresql above version x and postgis above
> version y. I do realize these are difficult statements to make (how to test
> them?)

Hi Jeroen,

I think you are complicating things :-)

There are postgres/gis SERVER versions
And postgres/gis/ CLIENT LIBRARY version

In general: IF you have a SERVER version you should try to have the same CLIENT (=library) version.
BUT: only to be able to do the new things introduced in the SERVER version OR to have a better client version because they fixed bugs in that client(library) version.
In general: you can probably also connect to a new SERVER version with an older CLIENT (unless off course you want to do things that at the time of the CLIENT version was just not possible).

My guess: your issue above has anything to do with the pg client libraries used, or the combi server/client.
Chances are bigger that either QGIS is now running other SQL queries OR the logic (of loops for example) in QGIS has changed.

If you really want to know:
- enable logging in postgresql so you can see all queries the QGIS is sending 
- check if there are differences
- run the same queries from withing pgsql on the server and do timing there.

Else we try to find a simple solution ("just use the right version") for a rather complex performance issue, which can have it's cause on many (other) levels... :-)

Groetjes,

Richard Duivenvoorde