如何在 PostgreSQL 中使用 SQLAlchemy 选择 FOR SHARE？答案

【问题标题】：How to SELECT FOR SHARE using SQLAlchemy with PostgreSQL?如何在 PostgreSQL 中使用 SQLAlchemy 选择 FOR SHARE？
【发布时间】：2012-12-08 14:45:53
【问题描述】：

我正在尝试 SELECT FOR SHARE 表中的一组行，以便它们被锁定直到事务结束。我正在使用 SQLAlchemy 0.7.9 在 PostgreSQL 9.1.6 数据库中执行此操作。这是有问题的python代码：

NUM_TERMS = 10
conn = engine.connect()
get_terms = select([search_terms.c.term_id, search_terms.c.term],
                   and_(search_terms.c.lock==False,
                   search_terms.c.status==False),
                   order_by=search_terms.c.term,
                   limit=NUM_TERMS, for_update="read")
trans = conn.begin()
try:
    search_terms = conn.execute(get_terms).fetchall()
    for term in search_terms:
        lock_terms = update(search_terms).\
                     where(search_terms.c.term_id==term.term_id).\
                     values(lock=True)
        conn.execute(lock_terms)
    if trans.commit():
        <do things with the search terms>
except:
    trans.rollback()

问题是上面select代码生成的SQL查询不是FOR SHARE，是FOR UPDATE：

SELECT search_terms.term_id, search_terms.term
FROM search_terms
WHERE search_terms.lock = :lock_1 AND search_terms.status = :status_1     
ORDER BY search_terms.term
LIMIT :param_1 FOR UPDATE

根据SQLAlchemy API docs，“for_update”参数说明下：

使用 Postgresql 方言，值“read”和“read_nowait”分别转换为 FOR SHARE 和 FOR SHARE NOWAIT。

根据上面，编译好的SQL语句应该是FOR SHARE，其实不是。我的代码哪里出错了？

【问题讨论】：

标签： python sqlalchemy postgresql-9.1

【解决方案1】：

无法重现：

from sqlalchemy import *

m = MetaData()
t = Table('t', m, Column('x', Integer))

s = select([t], for_update="read")

from sqlalchemy.dialects import postgresql
print s.compile(dialect=postgresql.dialect())

e = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
with e.begin() as conn:
    m.create_all(conn)
    conn.execute(s)

输出显示，无论是作为独立编译还是在 Postgresql 对话中，我们都会得到 FOR SHARE。在 0.7.9 和 0.8.0b2 中测试。如果您可以提供完整的可运行测试用例，那可能会更清楚：

SELECT t.x 
FROM t FOR SHARE
2012-12-20 23:53:33,670 INFO sqlalchemy.engine.base.Engine select version()
2012-12-20 23:53:33,671 INFO sqlalchemy.engine.base.Engine {}
2012-12-20 23:53:33,672 INFO sqlalchemy.engine.base.Engine select current_schema()
2012-12-20 23:53:33,672 INFO sqlalchemy.engine.base.Engine {}
2012-12-20 23:53:33,674 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2012-12-20 23:53:33,674 INFO sqlalchemy.engine.base.Engine select relname from pg_class c join pg_namespace n on n.oid=c.relnamespace where n.nspname=current_schema() and relname=%(name)s
2012-12-20 23:53:33,675 INFO sqlalchemy.engine.base.Engine {'name': u't'}
2012-12-20 23:53:33,676 INFO sqlalchemy.engine.base.Engine SELECT t.x 
FROM t FOR SHARE
2012-12-20 23:53:33,676 INFO sqlalchemy.engine.base.Engine {}
2012-12-20 23:53:33,676 INFO sqlalchemy.engine.base.Engine COMMIT

【讨论】：