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Snippet Name: Measure the Buffer Cache Hit Ratio

Description: Note that hit ratio based tuning is not recommended.

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Comment: (none)

Language: PL/SQL
Highlight Mode: PLSQL
Last Modified: March 02nd, 2009

-- Get initial Buffer Hit Ratio reading...
SELECT ROUND((1-(phy.VALUE / (cur.VALUE + con.VALUE)))*100,2) "Cache Hit Ratio"
  FROM v$sysstat cur, v$sysstat con, v$sysstat phy
 WHERE cur.name = 'db block gets'
   AND con.name = 'consistent gets'
   AND phy.name = 'physical reads'
/
 
-- Let's artificially increase the buffer hit ratio...
DECLARE
  v_dummy dual.dummy%TYPE;
BEGIN
  FOR I IN 1..1000 LOOP
    SELECT dummy INTO v_dummy FROM dual;
  END LOOP;
END;
/
 
-- Let's measure it again...
SELECT ROUND((1-(phy.VALUE / (cur.VALUE + con.VALUE)))*100,2) "Cache Hit Ratio"
  FROM v$sysstat cur, v$sysstat con, v$sysstat phy
 WHERE cur.name = 'db block gets'
   AND con.name = 'consistent gets'
   AND phy.name = 'physical reads'
/
 


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