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Added warning about the performance of large IN clauses in prefetch_r…

…elated docs

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17147 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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commit e954a03871cdc1c155ba7a3f3000dcf75998d9f4 1 parent 648adcd
@spookylukey spookylukey authored
Showing with 6 additions and 0 deletions.
  1. +6 −0 docs/ref/models/querysets.txt
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6 docs/ref/models/querysets.txt
@@ -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
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