The Dictionary Cache is one of the two main components of the Shared Pool, the other being the Library Cache. The Dictionary Cache is accessed frequently during Oracle operations as it holds information about Database Objects, table descriptions, datafile names, block locations and so on. It also holds information concerning Accounts and their privileges.

Every time a statement is processed by Oracle the Dictionary Cache is accessed for the relevant information in order that the statement can be properly executed.

The size of the Dictionary as part of the shared pool is managed internally by the database. The size of the shared pool itself, however, is set globally via an init.ora parameter SHARED_POOL_SIZE . As such this forms another tuneable item which requires monitoring, and, like the buffer cache, changes to this will impact on SGA size.

To monitor the efficiency of the Data Dictionary Cache, use the statement

select  (1-(sum(getmisses) / (sum(gets) + sum(getmisses)))) * 100

        "Dictionary Hit Rate"
from     v$rowcache;

A figure of between 90 - 95 % should be maintained; if the rate starts to drop, SHARED_POOL_SIZE should be increased.