How can you determine which fields or conditions require creating indexes for optimizing large database queries?

To determine which fields or conditions require indexes, you should:

1. Analyze frequently executed queries — which fields are used in WHERE, JOIN, ORDER BY.
2. Use profiling tools and explain plans (e.g., EXPLAIN in SQL) to see where full table scans occur.
3. Pay attention to fields that are often filtered or sorted.

For example, if a query frequently searches by user_id or sorts by created_at, consider creating indexes on these fields.

Example SQL for creating an index:

CREATE INDEX idx_user_id ON orders(user_id);

Thus, indexes are created based on analysis of actual queries and their performance.