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Python Sqlalchemy如何實現(xiàn)select for update

更新時間：2020年10月12日 10:19:45 作者：笨手笨腳越

這篇文章主要介紹了Python Sqlalchemy如何實現(xiàn)select for update,文中通過示例代碼介紹的非常詳細(xì)，對大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價值,需要的朋友可以參考下

sqlalchemy 對于行級鎖有兩種實現(xiàn)方式，with_lockmode(self, mode): 和 with_for_update(self, read=False, nowait=False, of=None)，前者在sqlalchemy 0.9.0 被廢棄，用后者代替。所以我們使用with_for_update !

看下函數(shù)的定義：

@_generative()
  def with_for_update(self, read=False, nowait=False, of=None):
    """return a new :class:`.Query` with the specified options for the
    ``FOR UPDATE`` clause.
 
    The behavior of this method is identical to that of
    :meth:`.SelectBase.with_for_update`. When called with no arguments,
    the resulting ``SELECT`` statement will have a ``FOR UPDATE`` clause
    appended. When additional arguments are specified, backend-specific
    options such as ``FOR UPDATE NOWAIT`` or ``LOCK IN SHARE MODE``
    can take effect.
 
    E.g.::
 
      q = sess.query(User).with_for_update(nowait=True, of=User)
 
    The above query on a Postgresql backend will render like::
 
      SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT
 
    .. versionadded:: 0.9.0 :meth:`.Query.with_for_update` supersedes
      the :meth:`.Query.with_lockmode` method.
 
    .. seealso::
 
      :meth:`.GenerativeSelect.with_for_update` - Core level method with
      full argument and behavioral description.
 
    """
     
read
  是標(biāo)識加互斥鎖還是共享鎖. 當(dāng)為 True 時, 即 for share 的語句, 是共享鎖. 多個事務(wù)可以獲取共享鎖, 互斥鎖只能一個事務(wù)獲取. 有"多個地方"都希望是"這段時間我獲取的數(shù)據(jù)不能被修改, 我也不會改", 那么只能使用共享鎖.
nowait
  其它事務(wù)碰到鎖, 是否不等待直接"報錯".
of
  指明上鎖的表, 如果不指明, 則查詢中涉及的所有表(行)都會加鎖.

q = sess.query(User).with_for_update(nowait=True, of=User)

對應(yīng)于sql：

SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT

mysql 不支持這幾個參數(shù)，轉(zhuǎn)成sql都是:

SELECT users.id AS users_id FROM users FOR UPDATE

范例：

def query_city_for_update():
  session = get_session()
  with session.begin():
    query = session.query(City).with_for_update().filter(City.ID == 8)
    print 'SQL : %s' % str(query)
    print_city_info(query.first())

結(jié)果：

SQL : SELECT city."ID" AS "city_ID", city."Name" AS "city_Name", city."CountryCode" AS "city_CountryCode", city."District" AS "city_District", city."Population" AS "city_Population" 
FROM city 
WHERE city."ID" = :ID_1 FOR UPDATE

{'city': {'population': 234323, 'district': u'Utrecht', 'id': 8, 'country_code': u'NLD', 'name': u'Utrecht'}}

SELECT ... FOR UPDATE 的用法，不過鎖定(Lock)的數(shù)據(jù)是判別就得要注意一下了。由于InnoDB 預(yù)設(shè)是Row-Level Lock，所以只有「明確」的指定主鍵，MySQL 才會執(zhí)行Row lock (只鎖住被選取的數(shù)據(jù)) ，否則mysql 將會執(zhí)行Table Lock (將整個數(shù)據(jù)表單給鎖住)。

以上就是本文的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。

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