The shared pool is a component of the system global area (SGA) within the database instance. It's responsible for caching various types of program data. For example, the shared pool stores parsed SQL, PL/SQL code, system parameters, and data dictionary information. Almost every operation that occurs in the database involves the shared pool. For example, if a user runs a SQL statement, then Oracle Database accesses the shared pool.

The shared pool consists of the following subcomponents:

• 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. 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, the database reuses the code. This action is known as a soft parse or a library cache hit. Otherwise, the database must build a new executable version of the application code, which is known as a hard parse or a library cache miss.
• The reserved pool is a memory area in the shared pool that Oracle Database can use to allocate large contiguous chunks of memory. The database allocates memory from the shared pool 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.
• The data dictionary cache stores information about database objects (that is, dictionary data). This cache is also called the row cache because it holds data as rows instead of buffers.
• The server result cache is a memory pool within the shared pool that contains the SQL query result cache and PL/SQL function result cache, which share the same infrastructure. The SQL query result cache stores the results of queries and query fragments. Most applications benefit from this performance improvement. The PL/SQL function result cache stores function result sets. Good candidates for result caching are frequently invoked functions that depend on relatively static data.
• Other components include enqueues, latches, information lifecycle management (ILM) bitmap tables, active session history (ASH) buffers, and other minor memory structures. Enqueues are shared memory structures (locks) that serialize access to database resources. They can be associated with processes or sessions. Examples are control file transaction, instance recovery, media recovery, job queue, and row cache. Latches are used as a low-level serialization control mechanism to protect shared data structures in the SGA from simultaneous access. Examples are cache buffers chains and redo allocation.

• Data Dictionary Cache
• Library Cache
• Reserved Pool
• Server Result Cache
• Shared Pool