**Optimization Checklist:**

1. **Indexing Strategy**

- Index all WHERE clause columns

- Index all JOIN columns

- Create composite indexes for multi-column filters

- Use covering indexes to avoid table lookups

- Monitor index usage and remove unused indexes

2. **Query Structure**

- Filter early with WHERE clauses

- Select only needed columns (avoid SELECT *)

- Use EXISTS instead of IN for subqueries

- Avoid functions on indexed columns

- Use UNION ALL instead of UNION when duplicates are acceptable

3. **Join Optimization**

- Join smaller tables first

- Use appropriate join types (INNER vs LEFT)

- Ensure join columns are indexed

- Avoid Cartesian products

- Consider denormalization for frequently joined tables

4. **Data Management**

- Update statistics regularly (ANALYZE in PostgreSQL, UPDATE STATISTICS in SQL Server)

- Partition large tables by date or key ranges

- Archive old data

- Use appropriate data types (INT vs BIGINT, VARCHAR vs TEXT)

5. **Monitoring & Maintenance**

- Set up query performance monitoring

- Log slow queries (slow_query_log in MySQL, pg_stat_statements in PostgreSQL)

- Review execution plans regularly

- Benchmark before and after changes

- Document optimization decisions

**Performance Impact Estimates:**

- Adding missing index: 10-1000x improvement

- Removing SELECT *: 2-5x improvement

- Optimizing join order: 2-10x improvement

- Updating statistics: 2-20x improvement

- Query rewriting: 2-100x improvement

Adding missing indexes: 10-1000x improvement possible

Removing SELECT *: 2-5x improvement in data transfer

Optimizing join order: 2-10x improvement for complex queries

Updating outdated statistics: 2-20x improvement in plan selection