Index
Problem
A table contains 500 million customers.
You ask:
WHERE email = 'alice@example.com'
Scanning all 500 million rows for every lookup is expensive.
Solution — Index
Maintain an additional structure optimized for finding values.
alice@example.com
↓
row 82741
The database can locate matching rows without scanning everything.
Core idea: indexes trade additional storage and write work for faster retrieval.