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Don Burleson Blog 







Oracle shared_pool_size tuning

Oracle Tips by Burleson Consulting

Rules for adjusting shared_pool_size

From Oracle8 onward, there are several queries used for determining when the Oracle shared pool is too small.  The library cache miss ratio tells the DBA whether to add space to the shared pool, and it represents the ratio of the sum of library cache reloads to the sum of pins.


In general, if the library cache miss ratio is greater than one, the DBA should consider adding to the shared_pool_size .  Library cache misses occur during the parsing and preparation of the execution plans for SQL statements. 


The compilation of an SQL statement consists of two phases: the parse phase and the execute phase.  When the time comes to parse an SQL statement, Oracle checks to see if the parsed representation of the statement already exists in the library cache.  If not, Oracle will allocate a shared SQL area within the library cache and then parse the SQL statement.  At execution time, Oracle checks to see if a parsed representation of the SQL statement already exists in the library cache.  If not, Oracle will reparse and execute the statement.


The following script will compute the library cache miss ratio.  The script sums all of the values for the individual components within the library cache and provides an instance-wide view of the health of the library cache.




The output is shown below.  This report can easily be customized to alert the DBA when there are excessive executions or library cache misses.


                  Cache Misses  Library Cache


Yr.  Mo Dy  Hr.       execs While Executing   Miss Ratio

---------------- ---------- --------------- ------------------

2001-12-11 10        10,338          3               .00029

2001-12-12 10       182,477        134               .00073

2001-12-14 10       190,707        202               .00106

2001-12-16 10         2,803         11               .00392


Once this report identifies a time period where there may be a problem, STATSPACK provides the ability to run detailed reports to show the behavior of the objects within the library cache. 


In the preceding example, there is clearly a RAM shortage in the shared pool between 10:00 a.m. and 11:00 a.m. each day.  In this case, the shared pool could be reconfigured dynamically with additional RAM memory from the db_cache_size during this period.



This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:




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