@joernc_unibi I asked the developers about the database choice, because I read that line you quoted. I wanted their opinion if it worths migrating from PG to MariaDB since we have these problems with postgresql DB scripts.
I will study a possible migration to MariaDB, because it is time consuming try to fix things that are only broken on PG scripts. It is tiny problems like droping a collumn or an index that should exists and brokes the upgrade sequence. Even though it is tiny problems, we (the final user) dont have the impact of bypassing these drops.

Hello joernc_unibi ,

Observation: The site_group index error occours only the first time it is executed. The second time it wont happen.

Executing the patch_pagelinks_drop_pl_title.sql manually, I found the following outputs:

Adding another info, I dont think this task have something to do with T330382. I missread pl_namespace with tl_namespace.
Is this error will be fixed if you add if exist in the patch-pagelinks-drop-pl_title.sql code? This was the solution for T330382.

I want to add some information. The error occurred while upgrading using the web interface.
Today, we tried to test the upgrade again with a fresh database on version 1.39.4 again, but now using the run.php update command and the error we encountered is the same described in https://phabricator.wikimedia.org/T374042. I dont know if the error "index "pl_namespace" does not exist" will appears using run.php update command. I will apply the fix described in https://phabricator.wikimedia.org/T374042 and see if the issue of this task will repeat.