The data dictionary is a collection of database tables and views containing reference information about the database, its structures, and its users.
Oracle Database accesses the data dictionary frequently during SQL statement parsing.
The data dictionary is accessed so often by Oracle Database that the following special memory locations are designated to hold dictionary data:
- Data dictionary cache: This cache holds information about database objects. The cache is also known as the row cache because it holds data as rows instead of buffers.
- Library cache:All server processes share these caches for access to data dictionary information.
The redo log buffer is a circular buffer in the SGA that stores redo entries describing changes made to the database.
A redo record is a data structure that contains the information necessary to reconstruct, or redo, changes made to the database by DML or DDL operations. Database recovery applies redo entries to data files to reconstruct lost changes. The database processes copy redo entries from the user memory space to the redo log buffer in the SGA. The redo entries take up continuous, sequential space in the buffer. The background process log writer process (LGWR) writes the redo log buffer to the active online redo log group on disk.