There is a predictable pattern we see in growing applications. Everything runs fast with a few thousand rows. Then the data grows and suddenly certain pages time out or background jobs start falling behind. Nine times out of ten the fix is an index that was never added.
The Basics: What an Index Actually Does
An index is a separate data structure that PostgreSQL maintains alongside your table. It lets the database jump straight to matching rows rather than scanning the entire table from top to bottom. The tradeoff is that writes become slightly slower because the index has to be updated too.
Index the Columns You Filter and Sort On
If your application frequently runs WHERE user_id = $1 or ORDER BY created_at DESC, those are the columns that need indexes. Run EXPLAIN ANALYZE in front of any slow query to see exactly where the database is doing sequential scans instead of index lookups.
Composite Indexes Need the Right Column Order
A composite index on (user_id, created_at) helps queries that filter by user_id alone or filter by both. It does not help queries that only filter by created_at. Put the most selective column first unless you have a specific reason to do otherwise.
Partial Indexes Are Often Overlooked
If your application frequently queries active records while inactive ones are rarely touched, a partial index with a WHERE is_active = true condition is smaller and faster than a full index on the entire table.
Conclusion
Add EXPLAIN ANALYZE to your debugging toolkit today if it is not already there. Slow queries leave obvious footprints. Catching them before they become incidents is one of the highest-leverage maintenance habits a backend engineer can build.