Can you explain the concept of lazy queries in Django and how they differ from regular ones?

In Django, a QuerySet is lazy, meaning that the database query is not executed immediately when the QuerySet is created, but deferred until the data is actually needed (for example, during iteration, conversion to a list, calling methods like .count(), .exists(), etc.).

This allows for optimizing database operations by avoiding unnecessary queries and enabling the combination of filters and other operations before executing the query.

Example:

# Create a QuerySet, but the database query is not executed yet
qs = MyModel.objects.filter(active=True)

# The query will execute here, during iteration
for obj in qs:
    print(obj.name)

Regular (non-lazy) queries are executed immediately, for example, when calling list(qs) or methods that require data right away.

Thus, lazy queries allow writing more efficient and flexible code by controlling when the database is accessed.