The large table with promo codes has slowed down. How to understand and what to do?

If a large table with promo codes becomes slow, you should:

1. Analyze queries: see which queries are executed on the table and which ones are the slowest (using EXPLAIN or profiling).
2. Check indexes: ensure that there are appropriate indexes on fields involved in filters and joins.
3. Optimize structure: possibly normalize data or denormalize it for speed.
4. Archive old data: if the table is very large, consider moving outdated promo codes to an archive table.
5. Caching: use caching for frequently repeated query results.
6. Parallelism and sharding: if the load is very high, consider horizontal scaling.

In Go, profiling (pprof) can be used to identify bottlenecks in the application, as well as monitoring the database.