1. Oracle Memory Internals
INSYS366
Oracle Memory Internals
1/16/2019
ISYS366
, Oracle Memory Internals

2. Oracle Memory Structures
Oracle instance
Have names
Memory buffers (SGA)
Background processes (Oracle processes)
Parallel servers may share a database
1/16/2019

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
1/16/2019

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
1/16/2019

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
1/16/2019

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
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