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  Oracle Database Tips by Donald Burleson

Advanced Tuning of the Shared Pool

Perhaps one of the least-understood aspects of Oracle Shared Global Area tuning is tuning the shared pool. The generally accepted tuning methodology involves throwing memory into the pool until either the problem goes under or the problem is masked. Here we will examine the shared pool and define a method for tuning it that uses measurement, not guesswork, to drive the tuning methodologies. Numerous scripts for examining the shared pool are provided.

What Is the shared pool?

Many people know that the shared pool is a part of the Oracle SGA but little else, so to begin this discussion it's necessary to answer exactly,What is the shared pool? The shared pool contains several key Oracle performance-related memory areas. If the shared pool is improperly sized, then overall database performance will suffer, sometimes dramatically. Figure 13.1 diagrams the shared pool structure located inside Oracle 8i and 9i SGAs.

As you can see from the structures pictured in Figure 13.1, the shared pool is separated into many substructures. The substructures of the shared pool fall into two broad areas: the fixed-size areas, which, for a given database at a given point in time stay relatively constant in size, and the variable-size areas, which grow and shrink according to user and program requirements.

In Figure 13.1, the areas inside the library caches' substructure are variable in size, while those outside the library caches (with the exception of the request and response queues used with MTS) stay relatively fixed in size. The sizes are determined based on an Oracle internal algorithm that ratios out the fixed areas based on overall shared pool size, a few of the initialization parameters, and empirical determinations from previous versions. In early versions of Oracle (notably 6.2 and earlier), the dictionary caches could be sized individually allowing a finer control of this aspect of the shared pool. With Oracle 7, the internal algorithm for sizing the data dictionary caches took control from the DBA.

The major difference between the shared pools in Oracle8i and Oracle9i is that any excess memory specified by the SGA_MAX_CACHE parameter and not used in the actual cache and buffer definitions will be placed in the miscellaneous area of the shared pool.

The shared pool is used for objects that can be shared among all users, such as table definitions, reusable SQL (although nonreusable SQL is also stored there), PL/SQL packages, procedures, and functions. Cursor information is also stored in the shared pool. At a minimum, the shared pool must be sized to accommodate the needs of the fixed areas, plus a small amount of memory reserved for use in parsing SQL and PL/SQL statements. If this is not done, ORA-04031 and ORA-07445 errors will result.

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