On Jun 4, 2013, at 8:18 AM, Ladislav Lenart <lenart...@volny.cz> wrote:
> Hello. > > I have a hard time to understand the following comment for Query.yield_per(): > Yield only ``count`` rows at a time. > > WARNING: use this method with caution; if the same instance is present > in more than one batch of rows, end-user changes to attributes will be > overwritten. > > In particular, it's usually impossible to use this setting with > eagerly loaded collections (i.e. any lazy='joined' or 'subquery') > since those collections will be cleared for a new load when > encountered in a subsequent result batch. In the case of 'subquery' > loading, the full result for all rows is fetched which generally > defeats the purpose of :meth:`~sqlalchemy.orm.query.Query.yield_per`. > > Also note that many DBAPIs do not "stream" results, pre-buffering > all rows before making them available, including mysql-python and > psycopg2. :meth:`~sqlalchemy.orm.query.Query.yield_per` will also > set the ``stream_results`` execution > option to ``True``, which currently is only understood by psycopg2 > and causes server side cursors to be used. > > Suppose I have a code like this: > > q = session.query(cls).filter(...) > q = q.options( > joinedload(cls.foo), > subqueryload(cls.bars), > ) > for each in q.yield_per(50): > # access each.foo or each.bars > > Does it work? Is so, why? If not, why? assuming cls.foo is a many-to-one, it will produce the correct result, but will be far worse in terms of memory and performance, as the subqueryload() call will be invoked for each distinct batch of 50 rows, across the *full* result set. So if your result has 1000 rows, and the number of "bars" total is 10000, you will load 10000 additional rows for *each* yield per of 50. -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy?hl=en. For more options, visit https://groups.google.com/groups/opt_out.
