#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--------------------------------------+------------------------------------
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
--------------------------------------+------------------------------------
Comment (by Shun Yu):
Just for clarity, this seems to be a MySQL specific symptom. I tried using
PostgreSQL 9.5.8 and Django 2.0.dev and could not replicate the add/drop
of indices.
{{{
from django.db import models
from django.contrib.auth.models import User
class Banana(models.Model):
- users = models.ForeignKey(User, on_delete=models.CASCADE)
+ users = models.ForeignKey(User, on_delete=models.PROTECT)
}}}
{{{
class Migration(migrations.Migration):
dependencies = [
('banana', '0001_initial'),
]
operations = [
migrations.AlterField(
model_name='banana',
name='users',
field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT,
to=settings.AUTH_USER_MODEL),
),
]
}}}
{{{
$ python manage.py sqlmigrate --database mysql banana 0001
BEGIN;
--
-- Create model Banana
--
CREATE TABLE `banana_banana` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY
KEY, `users_id` integer NOT NULL);
ALTER TABLE `banana_banana` ADD CONSTRAINT
`banana_banana_users_id_b9f5715e_fk_auth_user_id` FOREIGN KEY (`users_id`)
REFERENCES `auth_user` (`id`);
COMMIT;
$ python manage.py sqlmigrate --database mysql banana 0002
BEGIN;
--
-- Alter field users on banana
--
ALTER TABLE `banana_banana` DROP FOREIGN KEY
`banana_banana_users_id_b9f5715e_fk_auth_user_id`;
ALTER TABLE `banana_banana` ADD CONSTRAINT
`banana_banana_users_id_b9f5715e_fk_auth_user_id` FOREIGN KEY (`users_id`)
REFERENCES `auth_user` (`id`);
COMMIT;
}}}
{{{
$ python manage.py sqlmigrate --database postgres banana 0001
BEGIN;
--
-- Create model Banana
--
CREATE TABLE "banana_banana" ("id" serial NOT NULL PRIMARY KEY, "users_id"
integer NOT NULL);
ALTER TABLE "banana_banana" ADD CONSTRAINT
"banana_banana_users_id_b9f5715e_fk_auth_user_id" FOREIGN KEY ("users_id")
REFERENCES "auth_user" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "banana_banana_users_id_b9f5715e" ON "banana_banana"
("users_id");
COMMIT;
$ python manage.py sqlmigrate --database postgres banana 0002
BEGIN;
--
-- Alter field users on banana
--
COMMIT;
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/25253#comment:12>
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