1. INSYS365 Oracle Memory Internals ISYS365, Oracle Memory Internals

2. Oracle Memory Structures • Oracle instance • Have names • Memory buffers (SGA) • Background processes (Oracle processes) • Parallel servers may share a database ISYS365, Oracle Memory Internals

3. Oracle Memory Structures • PGA (Program/Private/Process Global Area) • Stores data and control information for a single user process • Created when a user process connects to database (e.g., SQL*Plus) • Stack space, containing contents of variables and arrays that the session is using • Session information • All this depends on whether you are running a multi-threaded server ISYS365, Oracle Memory Internals

4. Oracle Memory Structures • SGA (System/Shared Global Area) • Stores data that the user has retrieved from the database or that the user wants to stored in the database. The importance of this is due to the high cost of disk I/O. • Contains • Database buffer cache • Redo log buffer • Shared pool • Cursors ISYS365, Oracle Memory Internals

5. Oracle Memory Structures • SGA (System/Shared Global Area) • Contains • Database buffer cache • 1. Data blocks (written by DBWR process). Buffer size and number of buffers are specified in INIT<SID>.ORA • 2. Index blocks • 3. Rollback segment blocks • 4. Temporary segment blocks • Redo log buffer • Written to the Redo logfiles by LGWR ISYS365, Oracle Memory Internals

6. Oracle Memory Structures • SGA (System/Shared Global Area) • Contains • Shared pool • 1. Data dictionary cache: most frequently asked questions of the data dictionary • 2. Library cache: PL/SQL statements and its execution plan (e.g., whether indexes will be used) • 3. Control structures: locks, cache handles, etc. • Cursors • 1. Parsed SQL statements (the parsed representation of a SQL statement) • 2. Execution plan (algorithm for executing the statement) • 3. Number of rows affected • 4. In the case of a SELECT, the rows returned ISYS365, Oracle Memory Internals