On 12/01/2016 09:37 AM, Adrian wrote:
I would normally do session.query(Foo).count()
COUNT is somewhat expensive compared to just checking whether rows
exist, especially if lots of rows match (2.2M rows in the example):
I would have assumed you had some filtering condition.
Checking an EXISTS for an entire table at once, that may either have
zero or 2.2M rows is a little unusual :) I'd do
session.query(Foo.id).first() in that case.
In [2]: %timeit -n1 -r1 EventPerson.query.count()
1 loop, best of 1: 135 ms per loop
In [3]: %timeit -n1 -r1
db.session.query(EventPerson.query.exists()).scalar()
1 loop, best of 1: 2.44 ms per loop
how does this interact with filtering etc?
http://docs.sqlalchemy.org/en/latest/orm/query.html#sqlalchemy.orm.query.Query.exists
:P
It keeps all the filters present on the query object (in that example,
`self` is the existing query object)
well exists() returns a core SQL element whereas count() is special in
that it invokes a query. You're possibly asking for some
session.query.row_exists() type of thing.
this query is against column expressions, not entities, so eager
loading is not involved.
That's what I first thought, but the related OUTER JOINs are still
added. Maybe a bug?
https://gist.github.com/ThiefMaster/00f812a5be0ce321c91241de297dbdd0
it's not ideal it adds those in, there's the edge case that someone is
joined eager loading with innerjoin=True and is expecting that to
participate in the row being present, that's not the correct use of that
but it could break things for someone.
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