Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
-+-
 Reporter:  Thomas Recouvreux|Owner:  Çağıl
 Type:   |  Uluşahin
  Cleanup/optimization   |   Status:  closed
Component:  Migrations   |  Version:  2.1
 Severity:  Normal   |   Resolution:  fixed
 Keywords:  migrations m2m   | Triage Stage:  Ready for
  mysql  |  checkin
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Mariusz Felisiak ):

 * status:  assigned => closed
 * resolution:   => fixed


Comment:

 In [changeset:"9159d173c3822312c653db7ff5b9a94b14af1dca" 9159d17]:
 {{{
 #!CommitTicketReference repository=""
 revision="9159d173c3822312c653db7ff5b9a94b14af1dca"
 Fixed #25253 -- Made AlterField operation a noop when changing attributes
 that don't affect the schema.
 }}}

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
-+-
 Reporter:  Thomas Recouvreux|Owner:  Çağıl
 Type:   |  Uluşahin
  Cleanup/optimization   |   Status:  assigned
Component:  Migrations   |  Version:  2.1
 Severity:  Normal   |   Resolution:
 Keywords:  migrations m2m   | Triage Stage:  Ready for
  mysql  |  checkin
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Mariusz Felisiak):

 * needs_better_patch:  1 => 0
 * stage:  Accepted => Ready for checkin


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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
-+-
 Reporter:  Thomas Recouvreux|Owner:  Çağıl
 Type:   |  Uluşahin
  Cleanup/optimization   |   Status:  assigned
Component:  Migrations   |  Version:  2.1
 Severity:  Normal   |   Resolution:
 Keywords:  migrations m2m   | Triage Stage:  Accepted
  mysql  |
Has patch:  0|  Needs documentation:  0
  Needs tests:  1|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Çağıl Uluşahin):

 * owner:  (none) => Çağıl Uluşahin
 * status:  new => assigned
 * needs_tests:  0 => 1


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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  Thomas Recouvreux |Owner:  (none)
 Type:  Cleanup/optimization  |   Status:  new
Component:  Migrations|  Version:  2.1
 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
--+
Changes (by Adam (Chainz) Johnson):

 * cc: Adam (Chainz) Johnson (added)


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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  Thomas Recouvreux |Owner:  (none)
 Type:  Cleanup/optimization  |   Status:  new
Component:  Migrations|  Version:  2.1
 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
--+
Changes (by Phil Krylov):

 * cc: Phil Krylov (added)


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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  Thomas Recouvreux |Owner:  (none)
 Type:  Cleanup/optimization  |   Status:  new
Component:  Migrations|  Version:  2.1
 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
--+
Changes (by JR Heard):

 * version:  1.8 => 2.1


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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  Thomas Recouvreux |Owner:  (none)
 Type:  Cleanup/optimization  |   Status:  new
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
--+
Changes (by JR Heard):

 * status:  assigned => new
 * owner:  Shun Yu => (none)


Comment:

 The issue in this ticket is still happening for me on Django 2.1.9 on
 Postgres. Adding `blank=True` to a `ManyToManyField` drops the FK
 constraints on the join table and creates them back, exactly as described.

 This ticket hasn't been touched in a couple of years so I'm going to
 deassign it. I hope this is the right thing to do here, I'm new to the
 Django community :)

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#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 israel-tk):

 We had this problem with Postgres and Django 1.11 when deleting a not-null
 constraint in a ForeignKey field (adding null=True to a field). Instead of
 just dropping the not-null constraint which is a safe database operation,
 it first dropped the ForeignKey constraint, then dropped the not-null
 constraint, and last re-created the ForeignKey constraint (3 statements).

 Re-creating ForeignKey constraints locks tables and can generate downtime.
 Since we cannot afford the downtime we used a
 [https://docs.djangoproject.com/en/2.1/ref/migration-operations/#runsql
 RunSQL] migration operation with `state_operations` as a **workaround**:

 {{{
 migrations.RunSQL(
"""--
-- Alter field trip on payment
--
ALTER TABLE "app_payment" ALTER COLUMN "trip_id" DROP NOT NULL;
""",
state_operations=[
migrations.AlterField(
model_name='payment',
name='trip',
field=models.ForeignKey(blank=True, null=True,
 on_delete=django.db.models.deletion.PROTECT,
related_name='payments',
 to='app.Trip'),
),
])
 }}}

 This way we only apply the SQL we want and Django won't complain about the
 migration state not matching the current model. :)

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#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 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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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#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):

 I have a pull request: https://github.com/django/django/pull/9269

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#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;
 }}}

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#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
--+
Changes (by Shun Yu):

 * owner:  nobody => Shun Yu
 * status:  new => assigned


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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  Thomas Recouvreux |Owner:  nobody
 Type:  Cleanup/optimization  |   Status:  new
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 Cameron Dawson):

 I'm still hitting this with MySQL 5.7 and Django 1.11.3:

 {{{--
 -- Alter field job on textlogstep
 --
 ALTER TABLE `text_log_step` DROP FOREIGN KEY
 `text_log_step_job_id_698f3bfb_fk_job_id`;
 ALTER TABLE `text_log_step` ADD CONSTRAINT
 `text_log_step_job_id_698f3bfb_fk_job_id` FOREIGN KEY (`job_id`)
 REFERENCES `job` (`id`);}}}

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  trecouvr  |Owner:  nobody
 Type:  Cleanup/optimization  |   Status:  new
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 piquadrat):

 For what it's worth, I'm also seeing this on PostgreSQL 9.1, Django 1.9.5
 and psycopg2 2.6.1. Specifically, adding an `on_delete` argument to a
 `ForeignKey` (but I was also able to reproduce it with just adding
 `blank=True`) and then running `sqlmigrate` on the generated migration
 results in this output:

 {{{
 BEGIN;
 --
 -- Alter field last_occurrence on event
 --
 ALTER TABLE "event_event" DROP CONSTRAINT
 "last_occurrence_id_refs_id_9b97c921";
 ALTER TABLE "event_event" ADD CONSTRAINT
 "event_event_last_occurrence_id_a6d9e0d9_fk_event_instance_id" FOREIGN KEY
 ("last_occurrence_id") REFERENCES "event_instance" ("id") DEFERRABLE
 INITIALLY DEFERRED;

 COMMIT;
 }}}

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Re: [Django] #25253: MySQL migrations drop & recreate constraints unnecessarily when changing attributes that don't affect the schema (was: Migrations do unnecessary work when adding `blank=True` to M

[Django]

#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--+
 Reporter:  trecouvr  |Owner:  nobody
 Type:  Cleanup/optimization  |   Status:  new
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 timgraham):

 #25614 reported the same unnecessary dropping/adding of constraints when
 changing `ForeignKey.on_delete`. I assume that's a duplicate of this
 ticket, but please reopen it if fixing this issue doesn't also fix it.

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