Question

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()

问题是上面的选择代码生成的SQL查询不是FOR SHARE，它是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，在“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。

根据以上所述，编译的SQL语句应为FOR SHARE，但事实并非如此。我的代码中的错误在哪里？

Answer 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，作为独立编译以及Postgresql对话，我们得到FOR SHARE。测试范围为0.7.9和0.8.0b2。如果你能提供一个完整的可运行的测试用例，那可能会有更多的亮点：

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

如何使用SQLAlchemy和PostgreSQL选择共享？

1 个答案: