id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
29254	Improve paginator count performance by removing select_related from the query	hakib	nobody	"The default paginator is initiated with an `object_list` and uses `.count` or ` len` on it to get the total number of rows required to calculate the current page, next page etc...

Most of the time the object_list will be a QuerySet. When this is the case we can get an easy performace boost by removing any select_related from the query. select_related are adding an outer join so it cannot effect the total number of rows in query set - **select_related is basically dead weight if you only want to count the rows**.

`Paginator.count` is pretty simple:

{{{
    def count(self):
        """"""
        Returns the total number of objects, across all pages.
        """"""
        try:
            return self.object_list.count()
        except (AttributeError, TypeError):
            # AttributeError if object_list has no count() method.
            # TypeError if object_list.count() requires arguments
            # (i.e. is of type list).
            return len(self.object_list)
}}}


By changing to this:

{{{
            return self.object_list.select_related(None).count()
}}}

**we can significantly improve the performace of the count when the original query has many select_related models** (as often seen in admin pages with admin_select_related and views that show tables of data).

Only reservation I have is that `Paginator` is currently designed to support any kind of object that implements either `*.count` or `len(*)` - the tests also include some tests for random objects that implement count. This can easily be solved using `isinstance(object_list,  QuerySet)`, sniffing for `hasatrr(select_related, object_list)` or similar to how it's implemented today using `AttributeError`.



"	Cleanup/optimization	closed	Core (Other)	2.0	Normal	invalid	pagination		Unreviewed	0	0	0	0	1	0