Steve Crawford reported in September 2011 at
http://archives.postgresql.org/pgsql-admin/2011-09/msg00101.php
on "a packaging/library issue that seems likely to bite anyone trying to do a parallel install of 9.0 and 9.1 from PGDG
CentOSrepo. Don't know if if affects any other packages."
I seem to have the same issue on RHEL 5. After installing the 9.1 package in parallel with the existing 9.0 version, I
executed
sudo -u postgres /usr/pgsql-9.1/bin/pg_ctl start -w -D /var/lib/pgsql/9.1/data -l /var/lib/pgsql/9.1/start.log
and got
symbol lookup error: /usr/pgsql-9.1/bin/pg_ctl: undefined symbol: PQping
As Crawford reported, 9.1 was launched.
I applied his correction by renaming /etc/ld.so.conf.d/postgresql-9.0.1.conf, then executing ldconfig, then restoring
thename of postgresql-9.0.1.conf, then stopping and starting postgres 9.1. This time the PQping error message did not
appear.