Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Oracle data dictionary is a collection of database tables, owned by the SYS and SYSTEM schemas, that contain the metadata about the database, its structures, the privileges and roles of database users. The data dictionary cache holds cached blocks from the data dictionary. Data blocks from tables in the data dictionary are used continually to assist in processing user queries and other DML commands. If the data dictionary cache is too small, requests for information from the data dictionary will cause extra I/O to occur; these I/O-bound data dictionary requests are called recursive calls and should be avoided by sizing the data dictionary cache correctly.

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