There is a large table, and queries from it are slow. What will you do?

If queries to a large table are slow, the following steps can be taken:

1. Query analysis: Check the execution plan (EXPLAIN) to understand which operations take the most time.
2. Indexing: Add or optimize indexes on columns involved in filters, joins, and sorts.
3. Denormalization: In some cases, consider denormalizing data to reduce the number of joins.
4. Partitioning: Split the table into partitions based on a key (e.g., date) so that queries process less data.
5. Caching: Use caching of query results at the application or database level.
6. Query optimization: Rewrite the query, remove redundant joins, and use more efficient constructs.
7. Materialized views: Create materialized views for frequently used aggregates.

Example of using EXPLAIN in PostgreSQL:

EXPLAIN ANALYZE SELECT * FROM big_table WHERE user_id = 123;

This will show which indexes are used and where the bottlenecks are.