[参考文档][1]
### 查看 Django queryset 执行的 SQL
在QuerySet最后增加query，即可查询SQL
~~~
Author.objects.all().query
~~~
### extra 实现 别名
原始名称和别名都会显示
~~~
Tag.objects.all().extra(select={'tag_name': 'name'}).query.__str__()
Out[47]: u'SELECT (name) AS "tag_name", "blog_tag"."id", "blog_tag"."name" FROM "blog_tag"'
~~~
不显示原始名称，使用defer除去
~~~
Tag.objects.all().extra(select={'tag_name': 'name'}).defer('name').query.__str__()
Out[49]: u'SELECT (name) AS "tag_name", "blog_tag"."id" FROM "blog_tag"'
~~~
### defer 排除不需要的字段
在复杂的情况下，表中可能有些字段内容非常多，取出来转化成 Python 对象会占用大量的资源。
这时候可以用 defer 来排除这些字段，比如我们在文章列表页，只需要文章的标题和作者，没有必要把文章的内容也获取出来（因为会转换成python对象，浪费内存）
### only 仅选择需要的字段
和 defer 相反，only 用于取出需要的字段，假如我们只需要查出 作者的名称
~~~
Author.objects.all().only('name')
~~~
>[info] only类似于value_list只获取一个字段
### select_related 优化一对一，多对一查询
查询了两次
~~~
>>> art = Article.objects.filter(id=1).first()
(0.000) SELECT "app_article"."id", "app_article"."title", "app_article"."author_id", "app_article"."content", "app_article"."score" FROM "app_article" WHERE "app_article"."id" = 1 ORDER BY "app_article"."id" ASC LIMIT 1; args=(1
,)
>>> art.title
'Django 教程_1'
>>> art.author
(0.000) SELECT "app_author"."id", "app_author"."name", "app_author"."qq", "app_author"."addr", "app_author"."email" FROM "app_author" WHERE "app_author"."id" = 2; args=(2,)
<Author: twz915>
>>> art.author.name
'twz915'
~~~
查询了一次
~~~
>>> art = Article.objects.filter(id=1).select_related('author').first()
(0.000) SELECT "app_article"."id", "app_article"."title", "app_article"."author_id", "app_article"."content", "app_article"."score", "app_author"."id", "app_author"."name", "app_author"."qq", "app_author"."addr", "app_author"."e
mail" FROM "app_article" INNER JOIN "app_author" ON ("app_article"."author_id" = "app_author"."id") WHERE "app_article"."id" = 1 ORDER BY "app_article"."id" ASC LIMIT 1; args=(1,)
>>> art.author
<Author: twz915>
>>> art.author.name
'twz915'
~~~
### prefetch_related 优化一对多，多对多查询
和 select_related 功能类似，但是实现不同。
select_related 是使用 **SQL JOIN** 一次性取出相关的内容。
prefetch_related 用于 一对多，多对多 的情况，这时 select_related 用不了，因为当前一条有好几条与之相关的内容。
prefetch_related是通过再执行一条额外的SQL语句，然后用 Python 把两次SQL查询的**内容关联**（joining)到一起
>[info]使用和不使用的区别在第二步，会不会有查询`art.tags.all()`
使用prefetch_related
~~~
>>> art = Article.objects.all().prefetch_related('tags').first()
(0.000) SELECT "app_article"."id", "app_article"."title", "app_article"."author_id", "app_article"."content", "app_article"."score" FROM "app_article" ORDER BY "app_article"."id" ASC LIMIT 1; args=()
(0.001) SELECT ("app_article_tags"."article_id") AS "_prefetch_related_val_article_id", "app_tag"."id", "app_tag"."name" FROM "app_tag" INNER JOIN "app_article_tags" ON ("app_tag"."id" = "app_article_tags"."tag_id") WHERE "app_a
rticle_tags"."article_id" IN (1); args=(1,)
>>> art.tags.all()
<QuerySet [<Tag: Django>]>
>>> art.tags.all().first().name
(0.000) SELECT "app_tag"."id", "app_tag"."name" FROM "app_tag" INNER JOIN "app_article_tags" ON ("app_tag"."id" = "app_article_tags"."tag_id") WHERE "app_article_tags"."article_id" = 1 ORDER BY "app_tag"."id" ASC LIMIT 1; args=(
1,)
'Django'
~~~
不使用prefetch_related
~~~
>>> art = Article.objects.all().first()
(0.000) SELECT "app_article"."id", "app_article"."title", "app_article"."author_id", "app_article"."content", "app_article"."score" FROM "app_article" ORDER BY "app_article"."id" ASC LIMIT 1; args=()
>>> art.tags.all()
(0.000) SELECT "app_tag"."id", "app_tag"."name" FROM "app_tag" INNER JOIN "app_article_tags" ON ("app_tag"."id" = "app_article_tags"."tag_id") WHERE "app_article_tags"."article_id" = 1 LIMIT 21; args=(1,)
<QuerySet [<Tag: Django>]>
>>> art.tags.all().first().name
(0.000) SELECT "app_tag"."id", "app_tag"."name" FROM "app_tag" INNER JOIN "app_article_tags" ON ("app_tag"."id" = "app_article_tags"."tag_id") WHERE "app_article_tags"."article_id" = 1 ORDER BY "app_tag"."id" ASC LIMIT 1; args=(
1,)
'Django'
~~~
[1]:http://code.ziqiangxuetang.com/django/django-queryset-advance.html
