Why doesn't sudo know where psql is?

Why doesn't sudo know where psql is?

The problem
The CKAN installation documentation shows you how to list the installed
PostgreSQL databases.
The command looks like this:
sudo -u postgres psql -l
When I try that in my shell, I get an error:
$ sudo -u postgres psql -l
sudo: psql: command not found
The workaround
Daniel2d2art on the CentOS forum worked around the problem by fully
qualifying the path to psql.
My psql lives in the directory /usr/pgsql-9.2/bin, so my workaround now
looks like this:
sudo -u postgres /usr/pgsql-9.2/bin/psql -l
When I try that in my shell, it works:
$ sudo -u postgres /usr/pgsql-9.2/bin/psql -l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access
privileges
--------------+----------+----------+-------------+-------------+-----------------------
postgis_test | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
=c/postgres +
| | | | |
postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
=c/postgres +
| | | | |
postgres=CTc/postgres
(4 rows)
How do I fix it properly?
I shouldn't have to fully qualify the path, right?
The postgres user already has psql in its path:
$ sudo -u postgres echo $PATH
/usr/pgsql-9.2/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin
How do I fix this properly?