在SQLAlchemy中，.first（）和[0]之间的性能有何不同？

Question

Any difference between

a = session.query(Accounts).filter(Accounts.key = 4).first()

and

a = session.query(Accounts).filter(Accounts.key = 4)[0]

?

Answer 1

apparently, there isn't much difference between useing __gettitem__ or first

hint: both of the above calls basically return the same as:

query = session.query(Accounts).filter(Accounts.key = 4)
a = list(query[0:1])[0]

Answer 2

As @mata indicated and pointed to the source code, there is no difference in the execution path of the two. In fact, you can also enable SQL logging by setting echo=True when you create an engine, and you will see that the both SQL statements are exactly the same (some sort of SELECT ... FROM ... WHERE ....id = ? LIMIT ? OFFSET ? )

But, if the Accounts.key is actually a primary key, the best way to get a persistent instance by the primary key is to use the Query.get method, which will generate SELECT ... FROM ... WHERE ....id = ? and it should be good enough. I assume it is not going to be faster, but is much cleaner.