On Wed, Nov 2, 2011 at 5:30 PM, Ian Clelland <clell...@gmail.com> wrote: > On Wed, Nov 2, 2011 at 8:25 AM, Thomas Guettler <h...@tbz-pariv.de> wrote: >> >> # This is my current solution >> if get_special_objects().filter(pk=obj.pk).count(): >> # yes, it is special >> > > I can't speak to the "why" of this situation; it seems to me that this could > always be converted into a more efficient database query without any > unexpected side-effects (and if I really wanted the side effects, I would > just write "if obj in list(qs)" instead). In this case, though, I would > usually write something like this: > if get_special_objects().filter(pk=obj.pk).exists(): > # yes, it is special > I believe that in some cases, the exists() query can be optimized to return > faster than a count() aggregation, and I think that the intent of the code > appears more clearly. > Ian
OK, take this example. I have a django model table with 70 million rows in it. Doing any kind of query on this table is slow, and typically the query is date restrained - which mysql will use as the optimum key, meaning any further filtering is a table scan on the filtered rows. Pulling a large query (say, all logins in a month, ~1 million rows) takes only a few seconds longer than counting the number of rows the query would find - after all, the database still has to do precisely the same amount of work, it just doesn't have to deliver the data. Say I have a n entries I want to test are in that resultset, and I also want to iterate through the list, calculating some data and printing out the row, I can do the existence tests either in python or in the database. If I do it in the database, I have n+1 expensive queries to perform. If I do it in python, I have 1 expensive query to perform, and (worst case) n+1 full scans of the data retrieved (and I avoid locking the table for n+1 expensive queries). Depending on the size of the data set, as the developer I have the choice of which will be more appropriate for my needs. Sometimes I need "if qs.filter(pk=obj.pk).exists()", sometimes I need "if obj in qs". Cheers Tom -- 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 django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
