orm - What is N+1 SELECT query issue? -

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select n+1 stated problem in object-relational mapping (orm) discussions, , understand has having make lot of database queries seems simple in object world.

does have more detailed explanation of problem?

let's have collection of car objects (database rows), , each car has collection of wheel objects (also rows). in other words, car -> wheel 1-to-many relationship.

now, let's need iterate through cars, , each one, print out list of wheels. naive o/r implementation following:

select * cars;

and for each car:

select * wheel carid = ?

in other words, have 1 select cars, , n additional selects, n total number of cars.

alternatively, 1 wheels , perform lookups in memory:

select * wheel

this reduces number of round-trips database n+1 2. orm tools give several ways prevent n+1 selects.

reference: java persistence hibernate, chapter 13.


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