Shared Pool

🧠 Introduction
The Shared Pool is a vital part of the System Global Area (SGA) in an Oracle Database instance. It caches parsed SQL, PL/SQL code, system parameters, and data dictionary info, helping speed up query execution and reduce repetitive work.

Almost every database operation relies on the Shared Pool to perform efficiently.

🧩 Key Subcomponents of the Shared Pool

  1. Library Cache
  • Stores executable SQL and PL/SQL code, including shared SQL areas, PL/SQL areas, and control structures.
  • When you run a SQL statement, Oracle tries to reuse an existing parsed version (soft parse) to save time.
  • If no match is found, it does a hard parse, which creates a new execution plan but costs more time.
  1. Reserved Pool
  • Allocates large chunks of memory inside the shared pool for big objects (over 5 KB).
  • Prevents fragmentation by efficiently loading large objects.
  1. Data Dictionary Cache
  • Also called the row cache.
  • Stores metadata about tables, indexes, users, and more.
  • Reduces disk reads by keeping this info in memory.
  1. Server Result Cache
  • Caches results of queries and PL/SQL functions.
  • Useful for speeding up repeated queries that depend on relatively static data.

🧩 Other Shared Pool Components

  • Enqueues: Locks in shared memory that control access to resources (like recovery and job queues).
  • Latches: Lightweight locks preventing concurrent access to shared data structures (e.g., cache buffers chains).
  • ASH Buffers: Store Active Session History info for performance monitoring.
  • ILM Bitmap Tables: Track data access patterns for Information Lifecycle Management (ILM).

🛠️ DBA Tip of the Day
Monitoring the shared pool hit ratio helps you understand how often Oracle reuses cached SQL vs. parsing new statements. A high hit ratio means better performance!

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