How do you assess your SQL level? How do you find bottlenecks and optimize queries?

My SQL level is confidently at middle+. To identify bottlenecks in queries, I usually use EXPLAIN (or EXPLAIN ANALYZE in PostgreSQL) to understand the execution plan, identify full table scans, lack of indexes, or inefficient joins.

Query optimization includes:

• Adding or adjusting indexes
• Rewriting queries using JOIN instead of subqueries
• Using aggregate functions and grouping wisely
• Limiting the selection to only necessary columns
• Caching results of complex queries

Example of optimization:

-- Non-optimized query
SELECT * FROM orders WHERE customer_id IN (SELECT id FROM customers WHERE region = 'EU');

-- Optimized with JOIN
SELECT o.* FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE c.region = 'EU';

This approach helps the database use indexes and execute queries faster.