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              實例詳解Django的 select_related 和 prefetch_related 函數對 QuerySet 查

              作者:admin 發表于:2014-08-18 點擊:831  保護視力色:

              這是本系列的最后一篇,主要是select_related() 和 prefetch_related() 的最佳實踐。

              第一篇在這里?講例子和select_related()

              第二篇在這里?講prefetch_related()

               

              4. 一些實例

              選擇哪個函數

              如果我們想要獲得所有家鄉是湖北的人,最無腦的做法是先獲得湖北省,再獲得湖北的所有城市,最后獲得故鄉是這個城市的人。就像這樣:
              >>> hb = Province.objects.get(name__iexact=u"湖北省")
              >>> people = []
              >>> for city in hb.city_set.all():
              ...   people.extend(city.birth.all())
              ...
              顯然這不是一個明智的選擇,因為這樣做會導致1+(湖北省城市數)次SQL查詢。反正是個反例,導致的查詢和獲得掉結果就不列出來了。
              prefetch_related() 或許是一個好的解決方法,讓我們來看看。
              >>> hb = Province.objects.prefetch_related("city_set__birth").objects.get(name__iexact=u"湖北省")
              >>> people = []
              >>> for city in hb.city_set.all():
              ...   people.extend(city.birth.all())
              ...
              因為是一個深度為2的prefetch,所以會導致3次SQL查詢:
              SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name` 
              FROM `QSOptimize_province` 
              WHERE `QSOptimize_province`.`name` LIKE '湖北省' ;
              
              SELECT `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` 
              FROM `QSOptimize_city` 
              WHERE `QSOptimize_city`.`province_id` IN (1);
              
              SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`, 
              `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id` 
              FROM `QSOptimize_person` 
              WHERE `QSOptimize_person`.`hometown_id` IN (1, 3);
              嗯…看上去不錯,但是3次查詢么?倒過來查詢可能會更簡單?
              >>> people = list(Person.objects.select_related("hometown__province").filter(hometown__province__name__iexact=u"湖北省"))
              SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`, 
              `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id`, `QSOptimize_city`.`id`, 
              `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id`, `QSOptimize_province`.`id`, `QSOptimize_province`.`name`
              FROM `QSOptimize_person` 
              INNER JOIN `QSOptimize_city` ON (`QSOptimize_person`.`hometown_id` = `QSOptimize_city`.`id`) 
              INNER JOIN `QSOptimize_province` ON (`QSOptimize_city`.`province_id` = `QSOptimize_province`.`id`) 
              WHERE `QSOptimize_province`.`name` LIKE '湖北省';
              +----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
              | id | firstname | lastname | hometown_id | living_id | id | name   | province_id | id | name   |
              +----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
              |  1 | 張        | 三       |           3 |         1 |  3 | 十堰市 |           1 |  1 | 湖北省 |
              |  2 | 李        | 四       |           1 |         3 |  1 | 武漢市 |           1 |  1 | 湖北省 |
              |  3 | 王        | 麻子     |           3 |         2 |  3 | 十堰市 |           1 |  1 | 湖北省 |
              +----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
              3 rows in set (0.00 sec)
              完全沒問題。不僅SQL查詢的數量減少了,python程序上也精簡了。
              select_related()的效率要高于prefetch_related()。因此,最好在能用select_related()的地方盡量使用它,也就是說,對于ForeignKey字段,避免使用prefetch_related()。

              聯用

              對于同一個QuerySet,你可以同時使用這兩個函數。
              在我們一直使用的例子上加一個model:Order (訂單)
              class Order(models.Model):
                  customer   = models.ForeignKey(Person)
                  orderinfo  = models.CharField(max_length=50)
                  time       = models.DateTimeField(auto_now_add = True)
                  def __unicode__(self):
                      return self.orderinfo
              如果我們拿到了一個訂單的id 我們要知道這個訂單的客戶去過的省份。因為有ManyToManyField顯然必須要用prefetch_related()。如果只用prefetch_related()會怎樣呢?
              >>> plist = Order.objects.prefetch_related('customer__visitation__province').get(id=1)
              >>> for city in plist.customer.visitation.all():
              ...   print city.province.name
              ...
              顯然,關系到了4個表:Order、Person、City、Province,根據prefetch_related()的特性就得有4次SQL查詢
              SELECT `QSOptimize_order`.`id`, `QSOptimize_order`.`customer_id`, `QSOptimize_order`.`orderinfo`, `QSOptimize_order`.`time` 
              FROM `QSOptimize_order` 
              WHERE `QSOptimize_order`.`id` = 1 ;
              
              SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`, `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id` 
              FROM `QSOptimize_person` 
              WHERE `QSOptimize_person`.`id` IN (1);
              
              SELECT (`QSOptimize_person_visitation`.`person_id`) AS `_prefetch_related_val`, `QSOptimize_city`.`id`,
              `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` 
              FROM `QSOptimize_city` 
              INNER JOIN `QSOptimize_person_visitation` ON (`QSOptimize_city`.`id` = `QSOptimize_person_visitation`.`city_id`) 
              WHERE `QSOptimize_person_visitation`.`person_id` IN (1); 
              
              SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name` 
              FROM `QSOptimize_province`
              WHERE `QSOptimize_province`.`id` IN (1, 2);
              +----+-------------+---------------+---------------------+
              | id | customer_id | orderinfo     | time                |
              +----+-------------+---------------+---------------------+
              |  1 |           1 | Info of Order | 2014-08-10 17:05:48 |
              +----+-------------+---------------+---------------------+
              1 row in set (0.00 sec)
              
              +----+-----------+----------+-------------+-----------+
              | id | firstname | lastname | hometown_id | living_id |
              +----+-----------+----------+-------------+-----------+
              |  1 | 張        | 三       |           3 |         1 |
              +----+-----------+----------+-------------+-----------+
              1 row in set (0.00 sec)
              
              +-----------------------+----+--------+-------------+
              | _prefetch_related_val | id | name   | province_id |
              +-----------------------+----+--------+-------------+
              |                     1 |  1 | 武漢市 |           1 |
              |                     1 |  2 | 廣州市 |           2 |
              |                     1 |  3 | 十堰市 |           1 |
              +-----------------------+----+--------+-------------+
              3 rows in set (0.00 sec)
              
              +----+--------+
              | id | name   |
              +----+--------+
              |  1 | 湖北省 |
              |  2 | 廣東省 |
              +----+--------+
              2 rows in set (0.00 sec)

              更好的辦法是先調用一次select_related()再調用prefetch_related(),最后再select_related()后面的表

              >>> plist = Order.objects.select_related('customer').prefetch_related('customer__visitation__province').get(id=1)
              >>> for city in plist.customer.visitation.all():
              ...   print city.province.name
              ...

              這樣只會有3次SQL查詢,Django會先做select_related,之后prefetch_related的時候會利用之前緩存的數據,從而避免了1次額外的SQL查詢:

              SELECT `QSOptimize_order`.`id`, `QSOptimize_order`.`customer_id`, `QSOptimize_order`.`orderinfo`,?
              `QSOptimize_order`.`time`, `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`,?
              `QSOptimize_person`.`lastname`, `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id`?
              FROM `QSOptimize_order`?
              INNER JOIN `QSOptimize_person` ON (`QSOptimize_order`.`customer_id` = `QSOptimize_person`.`id`)?
              WHERE `QSOptimize_order`.`id` = 1 ;
              
              SELECT (`QSOptimize_person_visitation`.`person_id`) AS `_prefetch_related_val`, `QSOptimize_city`.`id`,?
              `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id`?
              FROM `QSOptimize_city`?
              INNER JOIN `QSOptimize_person_visitation` ON (`QSOptimize_city`.`id` = `QSOptimize_person_visitation`.`city_id`)?
              WHERE `QSOptimize_person_visitation`.`person_id` IN (1);
              
              SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name`?
              FROM `QSOptimize_province`?
              WHERE `QSOptimize_province`.`id` IN (1, 2);
              +----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
              | id | customer_id | orderinfo     | time                | id | firstname | lastname | hometown_id | living_id |
              +----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
              |  1 |           1 | Info of Order | 2014-08-10 17:05:48 |  1 | 張        | 三       |           3 |         1 |
              +----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
              1 row in set (0.00 sec)
              
              +-----------------------+----+--------+-------------+
              | _prefetch_related_val | id | name ? | province_id |
              +-----------------------+----+--------+-------------+
              | ? ? ? ? ? ? ? ? ? ? 1 | ?1 | 武漢市 | ? ? ? ? ? 1 |
              | ? ? ? ? ? ? ? ? ? ? 1 | ?2 | 廣州市 | ? ? ? ? ? 2 |
              | ? ? ? ? ? ? ? ? ? ? 1 | ?3 | 十堰市 | ? ? ? ? ? 1 |
              +-----------------------+----+--------+-------------+
              3 rows in set (0.00 sec)
              
              +----+--------+
              | id | name   |
              +----+--------+
              |  1 | 湖北省 |
              |  2 | 廣東省 |
              +----+--------+
              2 rows in set (0.00 sec)

              值得注意的是,可以在調用prefetch_related之前調用select_related,并且Django會按照你想的去做:先select_related,然后利用緩存到的數據prefetch_related。然而一旦prefetch_related已經調用,select_related將不起作用。

               

              小結

              1. 因為select_related()總是在單次SQL查詢中解決問題,而prefetch_related()會對每個相關表進行SQL查詢,因此select_related()的效率通常比后者高。
              2. 鑒于第一條,盡可能的用select_related()解決問題。只有在select_related()不能解決問題的時候再去想prefetch_related()。
              3. 你可以在一個QuerySet中同時使用select_related()和prefetch_related(),從而減少SQL查詢的次數。
              4. 只有prefetch_related()之前的select_related()是有效的,之后的將會被無視掉。
              關于這兩個函數,我能想到的東西目前只有這么多。不過基于一些個人原因,寫第三篇時間比較短,寫的有些倉促。如果什么時候又想起了什么,我會在這篇博文中添加。

              實例詳解Django的 select_related 和 prefetch_related 函數對 QuerySet 查詢的優化(三),首發于博客 - 伯樂在線。

              丰满少妇人妻久久久久久,性XXXXBBBB农村小树林,欧美激情性A片在线观看中文