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Data Dictionary Cache

Expert Oracle Database Tips by Donald BurlesonMarch 22, 2015

Inside the Data Dictionary Cache

The second major cache area within the shared pool for Oracle is the data dictionary cache.  As a key buffer cache of the shared pool, the data dictionary cache houses the internal data structures for the Oracle database.

The data dictionary is accessed very often by various Oracle processes. Dictionary Cache is the memory areas designated to hold dictionary data. It is also known as the row cache because it holds data as rows instead of buffers (which hold entire blocks of data).

When an Oracle database is first started, no data exists in the data dictionary cache. For this reason, the dictionary cache should be monitored and tuned only after the Oracle 11g database environment has been up and running for a period of time. One way to minimize contention issues on the data dictionary cache is to use Locally Managed Tablespaces (LMT) for Oracle 11g tablespaces instead of dictionary managed tablespaces. This will help reduced contention on the data dictionary cache.

The Data Dictionary Cache Misses Oracle metric is the ratio of the number of data dictionary requests resulting in cache misses.  The data dictionary is used to hold metadata (table and column names, etc. and is heavily referenced during SQL pre-execution processing.)

If you are having difficulties regarding the data dictionary cache misses, note:

"If the Miss Percentage above is greater than 10-15, you may need to tune your library cache usage by adding additional memory to the data dictionary cache. Increasing memory is done by increasing the SHARED_POOL_SIZE parameter in the INITsid.ora file."


 

 
 
 
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