Q: ERROR: could not access file “$libdir/plpython3”: No such file or directory
A: Use the ‘–prefiix’ arguments in the ‘.configure’ command line, it specifies where ‘make install’ copies files to plpython3u requires following files to be installed in the $GPHOME

├── lib
│   └── postgresql
│       └── plpython3.so
└── share
    └── postgresql
        └── extension
            ├── plpython3u--1.0.sql
            ├── plpython3u.control
            └── plpython3u--unpackaged--1.0.sql

Q: Error: could not load library “/usr/local/greenplum-db-6.23.0/lib/postgresql/plpython3.so”: libpython3.9.so.1.0: cannot open shared object file: No such file or directory (dfmgr.c:240)
A: please find where libpython3.9.so locate then

export LD_LIBRARY_PATH="path_to_libpython3.9.so:$LD_LIBRARY_PATH"
export PATH="path_to_python3.9/bin:$PATH"

Q: I use pip installed some packages like numpy but in plpython3u it raise this error

postgres=# CREATE OR REPLACE FUNCTION test_import()
RETURNS text AS $$
  import sys
  import numpy
  return str(numpy.__file__)
$$ language plpython3u;

select test_import();

ERROR:  ModuleNotFoundError: No module named 'numpy' (plpy_elog.c:121)
CONTEXT:  Traceback (most recent call last):
  PL/Python function "test_import", line 3, in <module>
    import numpy
PL/Python function "test_import"

A: You can use GUC to set custom location then ensure that you can import the package

python -m pip install --prefix=/home/gpadmin/my_python dill

$ psql -d testdb
testdb=# load 'plpython3';
testdb=# SET plpython3.python_path='/home/gpadmin/my_python';

Ensure that you configure plpython3.python_path before you create or call plpython3 functions in a session. If you set or change the parameter after plpython3u is initialized you receive the error:

ERROR: SET PYTHONPATH failed, the GUC value can only be changed before initializing the python interpreter.

To set a default value for the configuration parameter, use gpconfig instead:

gpconfig -c plpython3.python_path \
    -v "'/home/gpadmin/my_python'" \
    --skipvalidation
gpstop -u

Q: Can I use a different version of Python with PL/Python3 both python2 and python3?
A: Yes, plpython2 and plpython3 can co-exist, but they cannot be used in the same session.