Commit e954a038 authored by Luke Plant's avatar Luke Plant
Browse files

Added warning about the performance of large IN clauses in prefetch_related docs 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17147 bcc190cf-cafb-0310-a4f2-bffc1f526a37
parent 648adcd3

docs/ref/models/querysets.txt

+6 −0
Original line number Diff line number Diff line
@@ -821,6 +821,12 @@ referenced is needed, rather than one query for all the items. There could be 
additional queries on the ``ContentType`` table if the relevant rows have not

already been fetched.



``prefetch_related`` in most cases will be implemented using a SQL query that

uses the 'IN' operator. This means that for a large QuerySet a large 'IN' clause

could be generated, which, depending on the database, might have performance

problems of its own when it comes to parsing or executing the SQL query. Always

profile for your use case!



extra

~~~~~