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Oracle Shared Pool cache

To see how often Oracle is finding the system references it needs in the data dictionary; you can use the dictdet.sql script, which is sorted from best-hit ratio to worst:

See Code depot for complete script

select
       parameter,
       usage,
       gets,
. . .
       100 - round((getmisses/
       (gets + getmisses) * 100),2) hit_ratio
from  
       sys.v$rowcache 
where
. . .
order by
       5 desc;

 

Figure 5.9 - Drilling down into the dictionary cache

Just as with the library cache, a high data dictionary cache hit ratio is desirable.  You should strive for a hit ratio between 90 - 100%, with 95% being a good rule-of-thumb benchmark. 

Note that when a database is first started, the overall data dictionary cache hit ratio, as well as the individual hit ratios in the above query, will not be at an optimal level.  This is because all references to object definitions will be relatively new, and as such, must be placed into the shared pool.  Look for hit ratios between eighty and ninety percent for new database startups. 

If, however, after a solid hour or two of steady database time, the data dictionary cache hit ratio has not increased to desirable levels, you should look into the possibility of increasing the shared_pool_size parameter. 

While there is certainly more that can be discussed regarding shared pools, the areas covered above are the normal hot spots.  Are there any other SGA issues that should be checked periodically?  One area is the redo log buffer


The above is an excerpt from Oracle Performance Troubleshooting by Robin Schumacher.

It's only $19.95 and you can order it and get instant access to the Oracle scripts here:

http://www.rampant-books.com/book_2003_1_perf.htm

 

 

 

 
 
 

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