=> CREATE DATABASE wal_buffercache;

CREATE DATABASE

=> \c wal_buffercache

You are now connected to database "wal_buffercache" as user "postgres".

=> CREATE TABLE t(n integer);

CREATE TABLE

=> INSERT INTO t SELECT 1 FROM generate_series(1,10000);

INSERT 0 10000

=> SELECT setting FROM pg_settings WHERE name = 'block_size';

 setting 
---------
 8192
(1 row)

=> SELECT pg_table_size('t') / 8192;

 ?column? 
----------
       43
(1 row)

=> SELECT pg_relation_size('t','main') / 8192;

 ?column? 
----------
       40
(1 row)

=> SELECT pg_relation_size('t','fsm') / 8192;

 ?column? 
----------
        3
(1 row)

=> CREATE EXTENSION pg_buffercache;

CREATE EXTENSION

=> SELECT CASE relforknumber
         WHEN 0 THEN 'main'
         WHEN 1 THEN 'fsm'
         WHEN 2 THEN 'vm'
       END relfork,
       count(*)
FROM   pg_buffercache b,
       pg_class c
WHERE  b.reldatabase = (
         SELECT oid FROM pg_database WHERE datname = current_database()
       )
AND    c.oid = 't'::regclass
AND    b.relfilenode = c.relfilenode
GROUP BY 1;

 relfork | count 
---------+-------
 fsm     |     2
 main    |    40
(2 rows)

=> SELECT count(*)
FROM   pg_buffercache b
WHERE  isdirty;

 count 
-------
    88
(1 row)

=> CHECKPOINT;

CHECKPOINT

=> SELECT count(*)
FROM   pg_buffercache b
WHERE  isdirty;

 count 
-------
     0
(1 row)