Why Cursor declaration and usage in PostgreSQL? - Purpose & Use Cases

The Big Idea

What if you could handle thousands of records one by one without breaking a sweat or crashing your system?

The Scenario

Imagine you have a huge list of customer orders in a database, and you want to check each order one by one to apply some special discount or update its status.

Doing this manually means opening the entire list at once and trying to handle all orders in your head or on paper.

The Problem

Handling all orders at once is slow and confusing. You might miss some orders or make mistakes updating them.

Also, loading all data at once can crash your system if the list is very large.

The Solution

Cursors let you open the list and look at one order at a time, like flipping through pages of a book.

This way, you can carefully check and update each order without getting overwhelmed or crashing your system.

Before vs After

✗ Before

SELECT * FROM orders; -- then manually process all rows at once

✓ After

BEGIN
DECLARE order_cursor CURSOR FOR SELECT * FROM orders;
FETCH NEXT FROM order_cursor;
-- process the fetched order
CLOSE order_cursor;
END;

What It Enables

Cursors enable smooth, step-by-step processing of large data sets without overloading your system or losing track.

Real Life Example

A store manager uses a cursor to review each online order individually to apply personalized discounts before shipping.

Key Takeaways

Cursors help process large data sets one row at a time.

They prevent system overload and reduce errors.

They make complex data handling easier and safer.