What actions would you take if you notice that an SQL query is running significantly slower than expected?

If an SQL query runs significantly longer than expected, I would take the following steps:

1. Analyze the execution plan — use EXPLAIN or a similar command to check how the DBMS executes the query and identify bottlenecks.
2. Check indexes — ensure that there are appropriate indexes on the fields used in the query.
3. Optimize the query — simplify the query, remove unnecessary JOINs, subqueries, and use more efficient constructs.
4. Check data volume — the query might be processing too much data; consider adding filters or pagination.
5. Monitor locks and contention — verify if the table is being blocked by other transactions.
6. Cache results — if data doesn't change often, consider caching.

Example of using EXPLAIN in PostgreSQL:

EXPLAIN ANALYZE SELECT * FROM users WHERE email = 'example@example.com';