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Django

QuerySet laziness

A QuerySet builds a query without executing it until evaluated - iterated, sliced with a step, len(), list() or bool().

Why interviewers ask about it

Laziness is why chained filters cost nothing extra, and also why N+1 hides so well: the extra query fires at attribute access, far from where the queryset was defined.

Related terms

N+1 query

One query for a list, then one extra query per row when a related field is accessed.

select_related

Follows single-valued relations (ForeignKey, OneToOne) with a SQL JOIN in the same query.

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