django select_related() 联表查询

最新推荐文章于 2025-08-16 17:02:07 发布

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最新推荐文章于 2025-08-16 17:02:07 发布

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文章标签： python 数据库

原文链接：http://www.cnblogs.com/wc1217/archive/2012/04/09/2438980.html

本文探讨了如何通过使用select_related()方法优化SQL查询，减少数据库访问次数，从而提高性能。详细介绍了其工作原理和应用实例，展示了在联表查询场景下的效率提升。

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`select_related()官方解释`

Returns a QuerySet that will automatically "follow" foreign-key relationships, selecting that additional related-object data when it executes its query. This is a performance booster which results in (sometimes much) larger queries but means later use of foreign-key relationships won't require database queries.

The following examples illustrate the difference between plain lookups and select_related() lookups. Here's standard lookup:

# Hits the database.
e = Entry.objects.get(id=5)

# Hits the database again to get the related Blog object.
b = e.blog

And here's select_related lookup:

# Hits the database.
e = Entry.objects.select_related().get(id=5)

# Doesn't hit the database, because e.blog has been prepopulated
# in the previous query.
b = e.blog

简而言之..
他们由这种

(0.000) SELECT `default_docs`.`id`, `default_docs`.`title`, `default_docs`.`document`, `default_docs`.`user_id` FROM `default_docs` INNER JOIN `default_user` ON (`default_docs`.`user_id` = `default_user`.`id`) WHERE `default_user`.`username` = 'wc' ; args=('wc',)
(0.000) SELECT `default_user`.`id`, `default_user`.`username`, `default_user`.`password` FROM `default_user` WHERE `default_user`.`id` = 1 ; args=(1,)

转成这种

(0.000) SELECT `default_docs`.`id`, `default_docs`.`title`, `default_docs`.`document`, `default_docs`.`user_id`, `default_user`.`id`, `default_user`.`username`, `default_user`.`password` FROM `default_docs` INNER JOIN `default_user` ON (`default_docs`.`user_id` = `default_user`.`id`) WHERE `default_user`.`username` = 'wc' ; args=('wc',)

也就是他一次联表查询(inner join)了,且设置了Field

转载于:https://www.cnblogs.com/wc1217/archive/2012/04/09/2438980.html