Simplifying Oracle Cursors & Navigating Cursor Types

 As a database administrator, understanding the ins and outs of Oracle cursors is crucial. Let's dive into the world of SQL cursors and explore how they function, while keeping things engaging and informative.

What is an SQL Cursor? 

An SQL cursor is essentially a memory area in a user session that contains information about a specific SQL statement. Think of it as a workspace where the SQL magic happens. A cursor is divided into two parts:

1. Library Cache Metadata: The first heap, which includes metadata about the SQL statement.
2. Executable Representation: The second, larger heap, known as the SQLAREA, where the executable form of the cursor resides.

Each heap is made up of chunks of memory, and SQLAREA acts like a handle pointing to a private SQL area in the User Global Area (UGA), which in turn points to the shared SQL area.

How SQL Cursors Work

SQL cursors are allocated in 4K chunks. The larger the cursor, the more 4K chunks it requires. An open cursor means the SQL statement has already been parsed, and its handle is in the library cache. However, if the cursor isn't in the shared pool, it needs to be reconstructed through a hard parse, which requires access to the shared pool and library cache latch. A soft parse is preferred as it skips optimization and proceeds directly to execution, saving valuable resources.

Optimizing Cursor Usage

To minimize latching and improve performance, DBAs can configure a session cursor cache. This cache holds instantiations of shared child cursors (closed session cursors), allowing repeatable statements to reuse these cursors. The session_cached_cursors parameter is independent of open_cursors, meaning you can set it higher because session cursors are not cached in an open state. For example, in Oracle 11g R2, the default value for open_cursors is 300, while session_cached_cursors is 50.

When a statement is in the library cache, its associated cursor is placed on a hash chain within the shared pool. The parsing process works as follows:

1. Find and execute an open cursor.
2. Locate a closed cursor in the session cursor cache.
3. Search the hash chains for a soft parse.
4. Construct the cursor via a hard parse if necessary.

Private vs. Shared SQL Areas

A cursor is a handle to a private SQL area, which resides in the UGA. This area contains information about a parsed SQL statement and session-specific data needed for execution, such as bind variable values and query execution details. In contrast, the shared SQL area is in the library cache within the System Global Area (SGA), containing execution plans shared across sessions.

Cursors in PL/SQL

In PL/SQL, cursors are categorized into static and dynamic cursors.

Static Cursors

• Explicit Cursors: These are named and manipulated directly by the programmer. You can open, fetch data from, close, and check attributes of explicit cursors.
• Implicit Cursors: These are automatically created by Oracle for single SQL statements. They require less coding effort and are faster. Implicit cursors do not raise INVALID_CURSOR errors and cannot be opened outside the statement.

Dynamic Cursors

• Ref Cursors: These are only created when opened. A Ref Cursor is a data type, and variables based on this type are known as cursor variables. The primary advantage of cursor variables is their ability to pass result sets between subprograms, such as stored procedures, functions, and packages.

By understanding and utilizing these concepts, you can effectively manage SQL cursors in your database, ensuring optimal performance and resource management. Happy querying!