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Fixed #24743, #24745 -- Optimized migration plan handling

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The change partly goes back to the old behavior for forwards migrations
which should reduce the amount of memory consumption (#24745). However,
by the way the current state computation is done (there is no
`state_backwards` on a migration class) this change cannot be applied to
backwards migrations. Hence rolling back migrations still requires the
precomputation and storage of the intermediate migration states.

This improvement also implies that Django does not handle mixed
migration plans anymore. Mixed plans consist of a list of migrations
where some are being applied and others are being unapplied.

Thanks Andrew Godwin, Josh Smeaton and Tim Graham for the review as well
as everybody involved on the ticket that kept me looking into the issue.
This commit is contained in:
Markus Holtermann 2015-08-27 09:49:35 +10:00
parent 186eb21dc1
commit 5aa55038ca
7 changed files with 184 additions and 27 deletions
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19
docs/releases/1.9.txt
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@ -458,6 +458,21 @@ Migrations
* When supplying ``None`` as a value in :setting:`MIGRATION_MODULES`, Django
will consider the app an app without migrations.
* When applying migrations, the "Rendering model states" step that's displayed
when running migrate with verbosity 2 or higher now computes only the states
for the migrations that have already been applied. The model states for
migrations being applied are generated on demand, drastically reducing the
amount of required memory.
However, this improvement is not available when unapplying migrations and
therefore still requires the precomputation and storage of the intermediate
migration states.
This improvement also requires that Django no longer supports mixed migration
plans. Mixed plans consist of a list of migrations where some are being
applied and others are being unapplied. This was never officially supported
and never had a public API that supports this behavior.
Models
^^^^^^
@ -1094,6 +1109,10 @@ Miscellaneous
* The system checks for :class:`~django.contrib.admin.ModelAdmin` now check
instances rather than classes.
* The private API to apply mixed migration plans has been dropped for
performance reasons. Mixed plans consist of a list of migrations where some
are being applied and others are being unapplied.
.. _deprecated-features-1.9:
Features deprecated in 1.9