Introduction to Query Optimization

The Slowdown

As our data grew, my queries got slower. What used to take 1 second now took 60. It was time to learn *optimization*.

Optimization is about understanding how the database *executes* your query, not just what the query *says*.

1. **Execution Plan**: The database's internal plan for running your query.

2. **Indexes**: Pre-built shortcuts for finding data.

3. **I/O Cost**: How much data does the database have to read?

4. **Memory/Sort Cost**: How much data has to be held in memory?

EXPLAIN ANALYZE SELECT * FROM orders WHERE customer_id = 100;

This shows you the database's plan: Is it scanning the whole table? Using an index?

Never optimize blindly. Always measure first with `EXPLAIN ANALYZE`.

*Day 72: Understanding the EXPLAIN Output.*