Your query is slow. You check: the database is fine. Connections are fine. The query is just... slow.

You look at the query:

SELECT * FROM orders WHERE user_id = 123

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Seems simple. Why is it slow?

Because there's no index on user_id . The database scans every row to find matching ones.

With a million rows, that's a million comparisons.

Add an index. Instant 100x speedup.

The Index

An index is a lookup table.

Without index:

SELECT * FROM orders WHERE user_id = 123 → Database scans all 1 M rows → Finds matches → Returns results

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With index:

SELECT * FROM orders WHERE user_id = 123 → Index lookup : user_id = 123 → row IDs [ 5 , 12 , 99 , ...] → Fetch those rows → Returns results

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Index is pre-sorted.…