#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
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Reporter: Thomas Recouvreux | Owner: Shun Yu
Type: Cleanup/optimization | Status: assigned
Component: Migrations | Version: 1.8
Severity: Normal | Resolution:
Keywords: migrations m2m mysql | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Mike Edmunds):
Still seems to be an issue in Django 2.0.5.
The source of the problem appears to be
BaseDatabaseSchemaEditor._alter_field in django.db.backends.base.schema.
It ''unconditionally''
[https://github.com/django/django/blob/2.0.5/django/db/backends/base/schema.py#L530-L542
drops any existing foreign key constraint(s)] and
[https://github.com/django/django/blob/2.0.5/django/db/backends/base/schema.py#L530-L542
later re-creates it]—whether or not anything is changing. (It seems to be
more selective about dropping other types of constraints only if the field
is changing in a way that impacts them.) The
[https://github.com/django/django/commit/a92bae0f0622fb45afb94bf5448b49bc32ebb643
origin of this code] is 2012, I think during initial implementation of
Django migrations.
Are there non-obvious field changes where the field's FK constraint needs
to be dropped and re-created? Or some other reason this code runs
unconditionally? Optimization around other field changes?
A fix might be to defer dropping the FK constraint until just before the
field's actions/null_actions/post_actions are collected and ready to be
applied (around line 650), and then drop it only if there are actually
actions to execute.
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Ticket URL: <https://code.djangoproject.com/ticket/25253#comment:14>
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