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 || '%'") }

1. 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.