SGA Hit Ratio’s

Buffer Hit Ratio

BUFFER HIT RATIO NOTES:

• Consistent Gets – The number of accesses made to the block buffer to retrieve data in a consistent mode.
• DB Blk Gets – The number of blocks accessed via single block gets (i.e. not through the consistent get mechanism).
• Physical Reads – The cumulative number of blocks read from disk.

• Logical reads are the sum of consistent gets and db block gets.
• The db block gets statistic value is incremented when a block is read for update and when segment header blocks are accessed.
• Hit Ratio should be > 80%, else increase DB_BLOCK_BUFFERS in init.ora

select    sum(decode(NAME, 'consistent gets',VALUE, 0)) "Consistent Gets",
        sum(decode(NAME, 'db block gets',VALUE, 0)) "DB Block Gets",
        sum(decode(NAME, 'physical reads',VALUE, 0)) "Physical Reads",
        round((sum(decode(name, 'consistent gets',value, 0)) + 
               sum(decode(name, 'db block gets',value, 0)) - 
               sum(decode(name, 'physical reads',value, 0))) / 
              (sum(decode(name, 'consistent gets',value, 0)) + 
               sum(decode(name, 'db block gets',value, 0))) * 100,2) "Hit Ratio"
from   v$sysstat

Data Dict Hit Ratio

DATA DICTIONARY HIT RATIO NOTES:

• Gets – Total number of requests for information on the data object.
• Cache Misses – Number of data requests resulting in cache misses

• Hit Ratio should be > 90%, else increase SHARED_POOL_SIZE in init.ora

select    sum(GETS),
        sum(GETMISSES),
        round((1 - (sum(GETMISSES) / sum(GETS))) * 100,2)
from    v$rowcache

SQL Cache Hit Ratio

SQL CACHE HIT RATIO NOTES:

• Pins – The number of times a pin was requested for objects of this namespace.
• Reloads – Any pin of an object that is not the first pin performed since the object handle was created, and which requires loading the object from disk.

• Hit Ratio should be > 85%

select    sum(PINS) Pins,
        sum(RELOADS) Reloads,
        round((sum(PINS) - sum(RELOADS)) / sum(PINS) * 100,2) Hit_Ratio
from    v$librarycache

Library Cache Miss Ratio

LIBRARY CACHE MISS RATIO NOTES:

• Executions – The number of times a pin was requested for objects of this namespace.
• Cache Misses – Any pin of an object that is not the first pin performed since the object handle was created, and which requires loading the object from disk.

• Hit Ratio should be < 1%, else increase SHARED_POOL_SIZE in init.ora

select    sum(PINS) Executions,
        sum(RELOADS) cache_misses,
        sum(RELOADS) / sum(PINS) miss_ratio
from    v$librarycache