[Sqlalchemy-tickets] Issue #3270: in the loop of large quantity query result, update operation costs strange time (zzzeek/sqlalchemy)

[awanabe <iss...@bi...>]

New issue 3270: in the loop of large quantity query result,  update operation costs strange time
https://bitbucket.org/zzzeek/sqlalchemy/issue/3270/in-the-loop-of-large-quantity-query-result

awanabe:

When I query a large quantity records, then loop the result. 
if I update one record, it will cost a lot time.

```
_users = db.session.query(WxUser).limit(10000).all()
for _user in _users:
    t1 = time()
    db.session.query(WxUser).filter_by(id=_user.id).update(dict(sex=WxUser.Sex.MALE))
    db.session.commit()
    t2 = time()
    print t2 - t1
```

out put:
```
0.242075920105
15.5323040485
16.6957418919
...
```
the commit operation cost time is more than normal.
when open the 'SQLALCHEMY_ECHO',  shows that SQLAlchemy query the every record by id.