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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
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Luke Plant 2011-11-24 17:18:56 +00:00
parent 648adcd35f
commit e954a03871
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6
docs/ref/models/querysets.txt
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@ -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 additional queries on the ``ContentType`` table if the relevant rows have not
already been fetched. 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 extra
~~~~~ ~~~~~