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  Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Shared Pool and Large Pool

The shared pool segment of the SGA contains three major areas: the library cache, the dictionary cache, and buffers for parallel execution messages.

Library Cache - The library cache includes the shared SQL areas, private SQL areas (in shared server), PL/SQL procedures and packages, and control structures such as library cache handles, locks, synonym translations, and dependency tracking information. It contains parse trees and execution plans for shareable SQL statements, as well as pseudo code for PL/SQL program units. All users access the shared SQL areas.

Dictionary Cache - Includes the usernames, segment information, profile data, tablespace information, and the sequence numbers. The dictionary cache also contains descriptive information or metadata about the schema objects. Oracle uses this metadata when parsing SQL cursors or during the compilation of PL/SQL programs. The dictionary cache is also known as the row cache because it holds the data in rows instead of buffers. It also holds entire blocks of data. This helps to reduce physical access to the data dictionary tables from the system tablespace, and also enables fine-grained locking of individual data dictionary rows.

The large pool is an optional area. If the large_pool_size parameter is set, then the large pool is configured as a separate heap within a variable area of the SGA. The large pool is not a part of the shared pool.

Using the large pool instead of the shared pool decreases fragmentation of the shared pool. Unlike the shared pool, the large pool does not have an LRU list. Oracle does not attempt to age memory out of the large pool.

The above text is an excerpt from:

Oracle 10g Grid & Real Application Clusters
Oracle 10g Grid Computing with RAC
ISBN 0-9744355-4-6

by Mike Ault, Madhu Tumma


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