How does Django's QuerySet laziness work, and when does a queryset actually hit the database? Django का QuerySet लेज़ीनेस कैसे काम करता है, और QuerySet वास्तव में डेटाबेस को कब हिट करता है?

A Django QuerySet is lazy: creating or chaining it (filter(), exclude(), order_by()) only builds up a SQL query description without executing anything against the database. This lets querysets be composed and passed around cheaply, and lets Django combine multiple chained filters into a single efficient query rather than running one per method call.

The query only actually executes when the queryset is evaluated — iterating over it, calling len(), slicing with a step, calling list(), printing/repr in a shell, or calling a method that must return a concrete value like .count(), .exists(), .first(), or .get(). Understanding this distinction is essential for spotting accidental double-evaluation.

qs = Product.objects.filter(price__gt=100)  # no query yet
qs = qs.order_by('name')                     # still no query
list(qs)                                      # query executes now