On Sat, 2006-07-08 at 21:22 -0700, bradford wrote: > I have a Model Foo, with a FK to user. > > if i do f = Foo.objects.get(id=1) and all i need from that is f.title > and j.user.username in my template... is it returning more info than > needed with User. I mean, is it getting the entire user object for > each returned record? > > If it is returning the entire user object, would it then be best to use > just SQL and a JOIN?
You typically won't see a lot of difference, I would expect. The database still has to read the whole row from disk, unless you are retrieving a field that it can get entirely out of an index (which is a consideration sometimes when designing queries in large systems -- but not something Django worries about, since it's really work out automatically). A general rule of thumb (not quite "urban legend" status, but sometimes a little tricky to measure easily all the same) is that a database will spend more time filtering down to the rowset you want than retrieving the actual data. In other words, the "select" field list is not a big factor in the overall time of the query. Of course, there are exceptions to this: a select clause with a some complex aggregate functions in it (and a lot of rows), or a very wide table (and if that is a problem, moving the large columns into another table if you don't always need them is the solution) are two exceptions, for example. Other cases where you might the data retrieval (as opposed to filtering) taking a more prominent role is when you have smallish data sets that can be filtered very quickly. But in those cases, stop caring! It's so small that it doesn't matter anyway. I would also think the overhead of creating the Python object is not likely to be significant (unless your total time is so small as to be inside the "noise"). Still, the definitive answer is always "time it and see", if it really matters (such as if you really are trying to squeeze every last microsecond out of the processing time). I'm writing all this without having actually benchmarked anything, but the above knowledge is based on my own experience and stuff I've read over the years (and I have worked with large enough data sets that serious query optimisations, such as controlling table join orders, or running over indexes in the right order, made a difference). It shouldn't be too hard to set up a large data set and try out both approaches. The results might be interesting. Regards, Malcolm --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users -~----------~----~----~----~------~----~------~--~---
