How to fix extension “pg_buffercache” already exists and relation “pg_buffercache” does not exist?

Question

PostgreSQL

When I try to create a pg_buffercahe extension using the following command:
➤ psql://[email protected]:5432/postgres
CREATE EXTENSION pg_buffercache;
... it returns an error:
ERROR: extension "pg_buffercache" already exists
If I try to extract information from pg_buffercahe using the following command
➤ psql://[email protected]:5432/postgres
select count(*) from pg_buffercache where not isdirty;
... I get an error:
ERROR: relation "pg_buffercache" does not exist
The following packages are installed on my system:
yum list postgresql95*
Installed Packages
postgresql95.x86_64 9.5.10-1PGDG.rhel7
postgresql95-contrib.x86_64 9.5.10-1PGDG.rhel7
postgresql95-libs.x86_64 9.5.10-1PGDG.rhel7
postgresql95-server.x86_64 9.5.10-1PGDG.rhel7
How can I fix this issue?
➤ psql://[email protected]:5432/postgres
# \dx
List of installed extensions
Name | version | schema | Description
--------------------+---------+------------+----- -------------------------------------------------- ----
pg_buffercache | 1.1 | public | examine the shared buffer cache
pg_stat_statements | 1.3 | public | track execution statistics of all SQL statements executed
plpgsql | 1.0 | pg_catalog | PL/pgSQL procedural language
(3 rows)