When you use SELECT *, MySQL retrieves every column from a table — even if your application only needs a few of them. While this might seem convenient during development, it can seriously affect performance in production environments.

Why it’s a problem:

• Increased I/O and memory usage: Retrieving unnecessary columns increases the size of the result set, consuming more memory on both the database server and client side.

• Slower network transfer: The more data sent over the network, the longer it takes for queries to complete, especially when working with large tables or remote servers.

• Reduced query optimization: MySQL’s query optimizer performs better when it knows exactly which columns are needed. With SELECT *, the optimizer has to work harder, sometimes generating less efficient execution plans.

• Fragile code maintenance: If new columns are added to the table later, queries using SELECT * will automatically return them — which might break application logic or display unwanted data.

   

Instead of:

SELECT * FROM customers;

( Use only required columns: )

SELECT name, email FROM customers;

This reduces the amount of data transferred and processed, especially in large tables.