的SQLAlchemy：joinedload +极限

考虑下面的语句：的SQLAlchemy：joinedload +极限

p = db.query(Profile).options(joinedload('*')).filter_by(id=p.id).limit(1).one()

我会得到一个子查询+联接，而不是一个 “纯” 加盟：

SELECT [...] 
FROM (SELECT profile.id AS profile_id, ... 
FROM profile 
WHERE profile.id = %(id_1)s 
LIMIT %(param_1)s) AS anon_1 LEFT OUTER JOIN city AS city_1 ON city_1.id = anon_1.profile_city LEFT OUTER JOIN country AS country_1 ON country_1.id = city_1.country LEFT OUTER JOIN state AS state_1 ON country_1.id = state_1.country LEFT OUTER JOIN state AS state_2 ON state_2.id = city_1.state LEFT OUTER JOIN country AS country_2 ON country_2.id = state_2.country LEFT OUTER JOIN state AS state_3 ON state_3.id = city_1.state LEFT OUTER JOIN country AS country_3 ON country_3.id = state_3.country LEFT OUTER JOIN starred AS starred_1 ON anon_1.profile_id = starred_1.star LEFT OUTER JOIN profiletext AS profiletext_1 ON anon_1.profile_id = profiletext_1.profile LEFT OUTER JOIN starred AS starred_2 ON anon_1.profile_id = starred_2.idprofile LEFT OUTER JOIN photo AS photo_1 ON anon_1.profile_id = photo_1.profile LEFT OUTER JOIN gps AS gps_1 ON anon_1.profile_id = gps_1.idprofile

但我真正需要的是：

SELECT ... 
FROM profile LEFT OUTER JOIN city AS city_1 ON city_1.id = profile.city LEFT OUTER JOIN country AS country_1 ON country_1.id = city_1.country LEFT OUTER JOIN state AS state_1 ON country_1.id = state_1.country LEFT OUTER JOIN state AS state_2 ON state_2.id = city_1.state  
LEFT OUTER JOIN country AS country_2 ON country_2.id = state_2.country LEFT OUTER JOIN state AS state_3 ON state_3.id = city_1.state LEFT OUTER JOIN country AS country_3 ON country_3.id = state_3.country LEFT OUTER JOIN starred AS starred_1 ON profile.id = starred_1.star LEFT OUTER JOIN profiletext AS profiletext_1 ON profile.id = profiletext_1.profile LEFT OUTER JOIN starred AS starred_2 ON profile.id = starred_2.idprofile LEFT OUTER JOIN photo AS photo_1 ON profile.id = photo_1.profile LEFT OUTER JOIN gps AS gps_1 
ON profile.id = gps_1.idprofile                                                  
WHERE profile.id = 4 
limit 1;

即没有子查询。

数据库：在PostgreSQL 9.2

来源

2013-03-21 jbastos

你使用的是哪个数据库？ – 2013-03-21 16:42:13

postgresql 9.2。 – jbastos 2013-03-21 16:47:30

这似乎是根据The Zen of Eager Loading

当使用加入预先加载，如果查询包含影响的行外返回加入改性剂预期的行为，这样的在使用 DISTINCT，LIMIT，OFFSET或同等语句时，完成的语句是首先被包装在子查询中，并且专用于的连接被加入预先加载的子查询。 SQLAlchemy的加入急于加载走多余的一英里，然后进一步十英里，到绝对保证它不会影响查询的最终结果，只有收集方式和相关对象被加载，不管什么格式该查询是。

我意识到这是一个老问题，但是有没有原因生成的SQL不起作用？

来源

2013-05-06 16:49:56 Ben

这是否意味着如果在查询中某处存在具有限制/ distinct/offset与查询的查询中某处的查询，那么该连接的数据被视为子查询？ – adarsh 2014-07-21 11:42:31

看来是这样，是的。我不确定sqlalchemy的内部是否将它转换为[subqueryload]（http://docs.sqlalchemy.org/en/latest/orm/loading.html#sqlalchemy.orm.subqueryload），但似乎就像它有相同的结果。 – Ben 2014-07-22 18:03:37

的SQLAlchemy：joinedload +极限

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