The shared pool in Oracle Database architecture is a mechanism that enables Oracle Database to efficiently manage shared memory across different components of the system. The shared pool is a memory area within the Oracle Database that is used for shared execution of code and queries across multiple users. By sharing memory across components, Oracle Database reduces the need for data transfer between cores and components, resulting in improved performance and scalability.
The shared pool is composed of a number of memory areas known as the library cache, the latches and the shared SQL area. The library cache is the area that holds compiled code known as PL/SQL (or Pro*C) packages, stored procedures and triggers. These objects are pre-compiled and stored in the library cache, reducing the time required to execute the same statement, since the code can be reused if needed. The latches are used to control simultaneous access to objects in the library cache, allowing multiple concurrent users to access the same information without having to wait for each other.
The shared SQL area, on the other hand, is used for executing SQL statements. This area holds information about the execution plan for a particular SQL statement and any results that were returned. By caching this information, Oracle Database can skip the parsing and optimization process for frequently used SQL queries, allowing it to execute faster.
There are several advantages to using a shared pool in Oracle Database architecture. First of all, it allows Oracle Database to reduce memory consumption, by only storing compiled versions of PL/SQL packages, stored procedures and triggers. By not having to store every single version of every statement, Oracle Database can save a lot of memory.
Additionally, the shared pool ensures that commonly used SQL statements are re-executed in a more efficient manner, since the query and execution plans are stored in memory. This helps to improve the performance of your database, since Oracle Database can skip the optimization and parsing process for frequently used queries.
The shared pool can also be used to improve the performance of SQL statements through a process known as SQL tuning. SQL tuning is the process of optimizing the performance of individual SQL statements, and involves setting certain “hints” to indicate the preferred execution plan for a given statement. By doing so, Oracle Database can ensure that the best plan is chosen and that the performance of the query is optimized.
However, the shared pool also has some limitations. One of these is that it can become fragmented over time, as memory is allocated and deallocated from the pool. If the fragmentation becomes severe, then Oracle Database performance may decrease, as the system spends more time searching for available memory, rather than executing queries.
Additionally, the shared pool can become overloaded if there are too many concurrent users accessing it, resulting in reduced performance or even a crash of the system. The system should be monitored closely to ensure that there is sufficient room in the shared pool for concurrent users.
Oracle Database provides several tools for maintaining and monitoring the shared pool. The utility known as V$ Shared Pool Statistics can be used to monitor the usage and fragmentation of the shared pool. Additionally, the ALTER SYSTEM FLUSH SHARED_POOL command can be used to clear the shared pool, freeing up memory and ensuring that the system is running optimally.
The shared pool can also be cleared of unneeded objects, such as packages and triggers that are no longer being used. By removing these objects, the system can reclaim valuable memory and improve the performance of the database. The V$LIBRARYCACHE view can be used to identify the unused objects, and these can then be removed using the DROP PACKAGE and DROP TRIGGER command.
The shared pool can also be used to improve system load balancing. By configuring Oracle Database to run multiple copies of frequently executed SQL statements on different cores or servers, the load can be spread evenly across the system, resulting in improved performance and scalability.
Conclusion
The shared pool is a key part of Oracle Database architecture and can be used to significantly improve the performance and scalability of the system. By caching common statements and keeping track of execution plans, Oracle Database can ensure that the most efficient plan is chosen and that statements are executed as quickly as possible. The shared pool should also be monitored and maintained regularly, to ensure that memory is not being wasted and that the system as a whole is running optimally.
