Write efficient database queries by leveraging proper indexes, using performance-optimized ActiveRecord methods, and avoiding unnecessary database operations. Key practices include: 1. **Use indexes effectively**: Structure queries to utilize existing indexes. For example, use `Account.arel_table[:username].lower.in(usernames.map(&:downcase))` to leverage lowered username indexes instead of queries that cause sequential scans. 2. **Prefer exists() over count()**: Use `exists?` instead of `count.zero?` to avoid unnecessary COUNT operations: ```ruby # Instead of Follow.where(conditions).count.zero? # Use !Follow.where(conditions).exists? ``` 3. **Remove redundant indexes**: Avoid creating single-column indexes when composite indexes already cover them. An index on `["account_id", "group_id"]` makes a separate `["account_id"]` index unnecessary. 4. **Simplify complex queries**: When Arel becomes overly complex, consider raw SQL for better readability and maintainability: ```ruby # Instead of complex Arel constructions scope :matches_partially, ->(str) { where("'%' || username || '%'") } ``` 5. **Handle query ordering conflicts**: Use `reorder(nil)` to clear existing ORDER BY clauses when they conflict with new ordering requirements, preventing incorrect results from multiple conflicting sort orders.