Saturday, May 26, 2012

Oracle Shared pool layers

The shared pool caches various types of program data. For example, the shared pool stores parsed SQL, PL/SQL code, system parameters, and data dictionary information. The shared pool is involved in almost every operation that occurs in the database. For example, if a user executes a SQL statement, then Oracle Database accesses the shared pool.

Library Cache

The library cache is a shared pool memory structure that stores executable SQL and PL/SQL code. This cache contains the shared SQL and PL/SQL areas and control structures such as locks and library cache handles. In a shared server architecture, the library cache also contains private SQL areas.
When a SQL statement is executed, the database attempts to reuse previously executed code. If a parsed representation of a SQL statement exists in the library cache and can be shared, then the database reuses the code, known as a soft parse or a library cache hit. Otherwise, the database must build a new executable version of the application code, known as a hard parse or a library cache miss.















Data Dictionary Cache

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.

Server Result Cache

Unlike the buffer pools, the server result cache holds result sets and not data blocks. The server result cache contains the SQL query result cache and PL/SQL function result cache, which share the same infrastructure.
A client result cache differs from the server result cache. A client cache is configured at the application level and is located in client memory, not in database memory.

Reserved Pool

The reserved pool is a memory area in the shared pool that Oracle Database can use to allocate large contiguous chunks of memory.
Allocation of memory from the shared pool is performed in chunks. Chunking allows large objects (over 5 KB) to be loaded into the cache without requiring a single contiguous area. In this way, the database reduces the possibility of running out of contiguous memory because of fragmentation.
Infrequently, Java, PL/SQL, or SQL cursors may make allocations out of the shared pool that are larger than 5 KB. To allow these allocations to occur most efficiently, the database segregates a small amount of the shared pool for the reserved pool.

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