#29254: Improve paginator count performance by removing select_related from the
query
-------------------------------------+-------------------------------------
Reporter: hakib | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Core | Version: 2.0
(Other) |
Severity: Normal | Keywords: pagination
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 1
UI/UX: 0 |
-------------------------------------+-------------------------------------
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`.
--
Ticket URL: <https://code.djangoproject.com/ticket/29254>
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