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Databases

N+1 query

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

Why interviewers ask about it

It looks fine on ten rows and collapses on ten thousand. Django's fix is select_related (JOIN, single-valued) or prefetch_related (second query, multi-valued); catch regressions with assertNumQueries.

select_related

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

prefetch_related

Fetches multi-valued relations (ManyToMany, reverse FK) in a second query and joins them in Python.

This term is part of the free Python Full-Stack interview preparation module - browse the full glossary for every definition.

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Why interviewers ask about it

Related terms

select_related

prefetch_related

Why interviewers ask about it

Related terms

select_related

prefetch_related