Shared SQL Area vs. Private SQL Area

A comparison between Shared SQL Area and Private SQL Area in Oracle memory architecture

The Shared SQL Area and the Private SQL Area are fundamental memory structures in the Oracle Database, each playing a distinct role in the execution of SQL statements. Here's a comparison of the two:

Shared SQL Area:

The Shared SQL Area is a component of the Shared Pool, which itself resides within the System Global Area (SGA). The SGA is a memory region shared by all Oracle processes.

Key characteristics of the Shared SQL Area include:

Shared Access: As its name suggests, the Shared SQL Area is accessible to all users and sessions in the database.

Contains Execution Plan: It stores the parsed representation and execution plan of a unique SQL statement. This allows the database to reuse the plan for subsequent identical SQL statements, avoiding the overhead of reparsing.

Reduces Memory Usage: By sharing the SQL statement's parsed form and execution plan, the Shared SQL Area significantly reduces overall memory consumption.

Located in the Library Cache: The Shared SQL Area is part of the Library Cache, a key component of the Shared Pool.

LRU Algorithm: The database uses a Least Recently Used (LRU) algorithm to manage the objects in the Shared Pool, including the Shared SQL Areas. This means that if a SQL statement is not executed for a while, its Shared SQL Area might be aged out to make space for new statements.

Private SQL Area (PGA):

The Private SQL Area (PGA) is a memory region that is private to a specific session or connection. It is not shared with other sessions.


Key characteristics of the Private SQL Area include:

Private to a Session: Each session that issues a SQL statement has its own Private SQL Area.

Contains Session-Specific Data: The Private SQL Area stores data that is specific to a particular execution of a SQL statement by a session.

This includes:

Bind variable values: The values supplied to the SQL statement at runtime.

Query execution state information: Information about the current state of the query execution.

Runtime memory structures: Buffers and work areas used for operations like sorting or hash joins.

Location: The location of the Private SQL Area depends on the server connection model:

Dedicated Server: The Private SQL Area is located in the Program Global Area (PGA) of the server process.

Shared Server: Part of the Private SQL Area is located in the System Global Area (SGA), specifically in the Large Pool or Shared Pool.

Cursor Association: A cursor is essentially a handle to a specific Private SQL Area. Closing a cursor releases the memory in the persistent area of the Private SQL Area.

Two Main Areas: The Private SQL Area is divided into two main parts:

Persistent Area: Contains bind information and is freed only when the cursor is closed.

Runtime Area: Contains information about the execution state and is freed when the execution is complete.

Summary of Differences:

Location:

Shared SQL Area: Resides in the Shared Pool, which is a part of the System Global Area (SGA).

Private SQL Area: Typically located in the Program Global Area (PGA) for a dedicated server connection. For shared server connections, part of it is in the SGA.

Accessibility:

Shared SQL Area: It is a public resource, shared by all sessions connected to the database.

Private SQL Area: It is a private resource, accessible only to the specific session that created it.

Contents:

Shared SQL Area: Contains the parsed SQL text and the execution plan, which are not specific to any single session.

Private SQL Area: Holds session-specific information, such as bind variable values, query execution state, and runtime memory buffers.

Purpose:

Shared SQL Area: Its primary purpose is to enable code sharing and reuse, which reduces the overhead of parsing the same SQL statement multiple times.

Private SQL Area: Its purpose is to store the private data and state needed for a specific execution of a SQL statement by a single session.
Lifespan:

Shared SQL Area: Its lifetime is managed by a Least Recently Used (LRU) algorithm within the Shared Pool. It can be aged out if not used, to make space for other statements.

Private SQL Area: Its lifespan is tied to a cursor. The persistent part is freed when the cursor is closed, and the runtime part is freed when the execution is complete.

In essence, when a SQL statement is executed, Oracle checks the Shared Pool for an existing Shared SQL Area for that statement. If one is found, it's reused.

Regardless, a new Private SQL Area is created for the current session to hold its private data for that execution. This architecture allows Oracle to efficiently manage memory and improve performance by sharing the resource-intensive parts of SQL execution while keeping session-specific data separate.