Shared buffers increased but cache hit ratio is still 85%

[Hans Schou <hans.schou@xxxxxxxxx>]

Hi

I have this system with some databases and I have run the cache_hit_ratio.sql script on it. It showed that the db acme777booking had a ratio on 85%. I then changed shared_buffer size from 0.5GB to 4GB as the server has 16GB of physical RAM. After 6 days of running I checked the ratio again and it is still 85%.

Am I doing something wrong or should some history be cleared?

cache_hit_ratio.sql
    datname     | blks_read  |   blks_hit   | cachehitratio
----------------+------------+--------------+---------------
 acme777web     |   50225009 |   3157586919 |         98.43
 acmelog        |     462198 |     14332508 |         96.88
 acme777domain  | 7540616252 | 119574349075 |         94.07
 acme777booking |  337915568 |   1902310783 |         84.92
(4 rows)

pg_runtime.sql
           pg_start            |        runtime        
-------------------------------+------------------------
 2018-07-12 06:49:48.654252+02 | 6 days 02:44:09.488978
(1 row)

get_version_num.sql
 Version text |  Num 
--------------+-------
 9.1.9        | 90109
(1 row)

SELECT pg_stat_database.datname,
       pg_stat_database.blks_read,
       pg_stat_database.blks_hit,
       round((pg_stat_database.blks_hit::double precision
              / (pg_stat_database.blks_read
                 + pg_stat_database.blks_hit
                 +1)::double precision * 100::double precision)::numeric, 2) AS cachehitratio
   FROM pg_stat_database
  WHERE pg_stat_database.datname !~ '^(template(0|1)|postgres)$'::text
  ORDER BY round((pg_stat_database.blks_hit::double precision
                 / (pg_stat_database.blks_read
                    + pg_stat_database.blks_hit
                    + 1)::double precision * 100::double precision)::numeric, 2) DESC;

OS: Red Hat Enterprise Linux Server release 6.4 (Santiago)