N+1 selection of DGS

N+1 Choice problem

[ORM（ Object relation mapping ） Medium “N+1 Choice problem ” What is it? ？](https://stackoverflow.com/questions/97197/what-is-the-n1-selects-problem-in-orm-object-relational-mapping)

Suppose you have a collection of car objects （ Database line ）, Each car has a set of wheel objects （ It's OK ）. let me put it another way , automobile → The wheel is a 1 Relationship to many .

Now? , Suppose you need to traverse all the cars , And print out a list of wheels for each car . Naive O/R The implementation will do the following .

SELECT * FROM Cars;

Then traverse each car

SELECT * FROM Wheel WHERE CarId = ?

let me put it another way , You have a choice about cars , Then there is N Two additional options , among N Is the total number of cars .

in addition , We can also get all the wheels , And search in memory .

SELECT * FROM Wheel

This reduces the number of round trips to the database from N+1 Reduced to 2. majority ORM Tools give you several ways to prevent N+1 choice .

Get by CarId Sort of all wheel （1 A choice ）, If you need to compare CarId More details , Yes, please. all The car makes a second query （ in total 2 A query ）. It's best to print it out now , No indexing or secondary storage is required