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[2.2.x] Fixed #30904 -- Doc'd caveat about using filtered queryset with select_for_update() on MySQL.

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Backport of fc2b1cc926e34041953738e58fa6ad3053059b22 from master
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Saad 2019-10-25 00:39:12 +03:00 committed by Mariusz Felisiak
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commit fb77d53ffe
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docs/ref/databases.txt
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@ -617,6 +617,11 @@ the ``SELECT ... FOR UPDATE`` statement. If ``select_for_update()`` is used
with ``nowait=True``, ``skip_locked=True``, or ``of`` then a with ``nowait=True``, ``skip_locked=True``, or ``of`` then a
:exc:`~django.db.NotSupportedError` is raised. :exc:`~django.db.NotSupportedError` is raised.
When using ``select_for_update()`` on MySQL, make sure you filter a queryset
against at least set of fields contained in unique constraints or only against
fields covered by indexes. Otherwise, an exclusive write lock will be acquired
over the full table for the duration of the transaction.
Automatic typecasting can cause unexpected results Automatic typecasting can cause unexpected results
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