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.

Practice

1. What is the primary purpose of declaring a cursor in PostgreSQL?

easy

A. To speed up bulk inserts

B. To process query results one row at a time

C. To create a new table in the database

D. To backup the database automatically

Solution

1. Step 1: Understand what a cursor does

   A cursor allows you to handle query results row by row instead of all at once.

2. Step 2: Compare options with cursor purpose

   Only To process query results one row at a time describes this behavior; others describe unrelated tasks.

3. Final Answer:

   To process query results one row at a time -> Option B

4. Quick Check:

   Cursor purpose = process rows one by one [OK]

Hint: Cursors handle rows stepwise, not bulk operations [OK]

Common Mistakes:

• Confusing cursors with table creation
• Thinking cursors speed up inserts
• Assuming cursors automate backups

2. Which of the following is the correct syntax to declare a cursor named cur_emp for selecting all rows from employees table?

easy

A. CREATE CURSOR cur_emp AS SELECT * FROM employees;

B. OPEN cur_emp CURSOR SELECT * FROM employees;

C. DECLARE cur_emp CURSOR FOR SELECT * FROM employees;

D. FETCH cur_emp FROM employees;

Solution

1. Step 1: Recall cursor declaration syntax

   In PostgreSQL, cursors are declared with DECLARE cursor_name CURSOR FOR query.

2. Step 2: Match syntax with options

   DECLARE cur_emp CURSOR FOR SELECT * FROM employees; matches this exactly; others use incorrect keywords or order.

3. Final Answer:

   DECLARE cur_emp CURSOR FOR SELECT * FROM employees; -> Option C

4. Quick Check:

   DECLARE + CURSOR + FOR + query = correct syntax [OK]

Hint: Use DECLARE ... CURSOR FOR ... to declare [OK]

Common Mistakes:

• Using OPEN instead of DECLARE for declaration
• Confusing FETCH with DECLARE
• Using CREATE CURSOR which is invalid syntax

3. Given the following code snippet, what will be the output after fetching from the cursor?

DECLARE cur_emp CURSOR FOR SELECT id FROM employees ORDER BY id LIMIT 3;
OPEN cur_emp;
FETCH NEXT FROM cur_emp;
FETCH NEXT FROM cur_emp;

medium

A. First two employee ids in ascending order

B. All employee ids from the table

C. Syntax error due to missing CLOSE statement

D. Empty result because cursor is not opened

Solution

1. Step 1: Understand cursor declaration and fetch

   The cursor selects 3 employee ids ordered ascending. FETCH NEXT retrieves one row each time.

2. Step 2: Analyze fetch calls

   Two FETCH NEXT calls return the first two rows from the cursor result.

3. Final Answer:

   First two employee ids in ascending order -> Option A

4. Quick Check:

   Two FETCH NEXT = two rows fetched [OK]

Hint: Each FETCH returns one row in cursor order [OK]

Common Mistakes:

• Assuming FETCH returns all rows at once
• Thinking missing CLOSE causes syntax error
• Believing cursor must be closed before fetching

4. Identify the error in the following cursor usage:

DECLARE cur_dept CURSOR FOR SELECT name FROM departments;
FETCH NEXT FROM cur_dept;
OPEN cur_dept;
CLOSE cur_dept;

medium

A. Cursor declaration syntax is incorrect

B. Cursor is declared after fetching

C. Cursor is closed before declaration

D. Cursor is fetched before it is opened

Solution

1. Step 1: Check the order of cursor operations

   Cursors must be declared, then opened, then fetched, then closed.

2. Step 2: Identify incorrect sequence

   Here, FETCH is called before OPEN, which is invalid.

3. Final Answer:

   Cursor is fetched before it is opened -> Option D

4. Quick Check:

   OPEN must precede FETCH [OK]

Hint: Always OPEN cursor before FETCH [OK]

Common Mistakes:

• Fetching before opening cursor
• Closing cursor before opening
• Misordering declaration and fetch

5. You want to process all rows from orders table one by one using a cursor in a PL/pgSQL function. Which sequence of statements correctly implements this?

hard

A. DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; LOOP FETCH cur_orders INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur_orders;

B. OPEN cur_orders; DECLARE cur_orders CURSOR FOR SELECT * FROM orders; FETCH cur_orders INTO rec; CLOSE cur_orders;

C. DECLARE cur_orders CURSOR FOR SELECT * FROM orders; FETCH cur_orders INTO rec; OPEN cur_orders; CLOSE cur_orders;

D. DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; FETCH ALL FROM cur_orders INTO rec; CLOSE cur_orders;

Solution

1. Step 1: Understand correct cursor usage in PL/pgSQL

   Declare cursor, open it, then loop fetching rows until no more rows, then close cursor.

2. Step 2: Analyze each option

   DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; LOOP FETCH cur_orders INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur_orders; follows correct order and uses LOOP with EXIT WHEN NOT FOUND to process all rows. Others have wrong order or invalid FETCH ALL.

3. Final Answer:

   DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; LOOP FETCH cur_orders INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur_orders; -> Option A

4. Quick Check:

   Declare, Open, Loop Fetch, Close = correct pattern [OK]

Hint: Use LOOP with FETCH and EXIT WHEN NOT FOUND [OK]

Common Mistakes:

• Opening cursor before declaring
• Fetching before opening
• Using FETCH ALL which is invalid for cursors