Re: 9.4 -> 9.5 upgrade problem when both python2 and python3 present

Paul Jones <pbj@cmicdo.com> writes:
> pg_upgrade complains about not being able to find $libdir/plpython3
> when upgrading a 9.4 cluster that has both python2 and python3 used.

No, that's not what the error message says:

> Could not load library "$libdir/plpython3"
> FATAL:  Python major version mismatch in session
> DETAIL:  This session has previously used Python major version 2, and it is now attempting to use Python major
version3. 
> HINT:  Start a new session to use a different Python major version.

This is a restriction we put in place because libpython2 and libpython3
don't coexist nicely in the same address space.  Unfortunately, it makes
it problematic to restore a dump that contains references to both python2
and python3 functions.

It looks like pg_upgrade tries to load all libraries from functions in
any database in the old cluster into a single session in the new cluster,
which will fail in a scenario like this even if you keep python2 and
python3 functions rigorously separated into distinct databases.  I'm
not sure if we could weaken that test enough to work.

> I dropped the python2 database but still got the problem.

You must still have at least one database that contains references
to python2 (check pg_language to be sure).

            regards, tom lane