Learn why indexes can speed up reads but add storage and write overhead.
The core idea
An index is an additional data structure that helps the database locate rows without scanning the entire table. Indexes are especially useful for selective filters, joins, and some sort operations, but they are not free.
A practical way to approach it
Inspect real query patterns and query plans. Add indexes to columns used frequently in selective filters or joins, consider composite indexes for common combinations, and measure before and after. Keep statistics current so the optimizer can choose appropriate plans.
Try it in a small project
For an orders table, compare a date-range query with and without an index on the date field. Then test a composite index only if real queries filter by multiple columns together.
Common mistakes to avoid
Too many indexes increase storage and slow inserts, updates, and deletes. Do not index every column or assume an index will help a query that returns most rows.
Key takeaway
Keep the first version small, make your assumptions explicit, and verify the result with a repeatable test. For production use, check the current official documentation for the tools and versions you choose.