[Django] #28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field attribute is added

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field
attribute is added
-------------------------------------+-------------------------------------
Reporter: Hari - 何 | Owner: nobody
瑞理 |
Type: Bug | Status: new
Component: Database | Version: 1.11
layer (models, ORM) | Keywords:
Severity: Normal | postgresql,migration,index
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
PostgreSQL migration automatically creates an index for fields that set
db_index=True. An example is SlugField, which sets this property
implicitly. Thereafter when the unique=True property is added to the field
the resultant migration script generates an AlterField object to apply
this unique attribute. The schema editor then incorrectly detects this new
unique=True attribute to indicate the need to create a like index
statement on the field which causes an error as it conflicts with the
already existing index.

The offending piece of code seems to be at
django/db/backends/postgresql/schema.py:117.

{{{
if ((not (old_field.db_index or old_field.unique) and new_field.db_index)
or
(not old_field.unique and new_field.unique)):
like_index_statement = self._create_like_index_sql(model,
new_field)
if like_index_statement is not None:
self.execute(like_index_statement)
}}}

If it's changed as:
{{{
if (not (old_field.db_index or old_field.unique) and (new_field.db_index
or new_field.unique)):
like_index_statement = self._create_like_index_sql(model,
new_field)
if like_index_statement is not None:
self.execute(like_index_statement)
}}}

this error won't occur.

PostgreSQL 9.5 introduces 'IF NOT EXISTS' to the CREATE INDEX statement
which if added to the schema template can also address this problem
without changing the above logic.

I encountered the problem with SlugField() which implicitly sets db_index
= True on PostgreSQL 9.4.

Interestingly, I only discovered this when I used django-tenant-schemas
which adds a thin layer on top of the default Database router setting the
schema search path before handing over the work to the default router.
With a vanilla Django installation using default router, the second call
to create a like index does not throw an error. However, upon reviewing
the code, the logic does look incorrect.

I'm still investigating to see if this there's more to this than what I
just described.

--
Ticket URL: <https://code.djangoproject.com/ticket/28646>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field
attribute is added
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.11
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
postgresql,migration,index | Unreviewed
Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Description changed by Hari - 何瑞理:

Old description:

New description:

PostgreSQL migration automatically creates an index for fields that set

`db_index=True`. An example is `SlugField`, which sets this property

`db_index=True` on PostgreSQL 9.4.

Interestingly, I only discovered this when I used `django-tenant-schemas`
which adds a thin layer on top of the default Database router setting the
schema search path before handing over the work to the default router.
With a vanilla Django installation using default router, the second call
to create a like index does not throw an error. However, upon reviewing

the code, the logic does look incorrect. Also issuing the duplicate SQL
statement in PostgreSQL console also throws an error.

I'm still investigating to see if this there's more to this than what I
just described.

--

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:1>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field
attribute is added
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: nobody
Type: Bug | Status: new

Component: Database layer | Version: 1.11
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
postgresql,migration,index | Unreviewed

Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Tim Graham):

That code was last touched in 9356f63a99957f01c14a9788617428a172a29fcb.
Your proposal results in some tests failures. Can you write a test for
`tests/schema/tests.py` that demonstrates this issue?

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:2>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field
attribute is added
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: nobody

Type: Bug | Status: closed

Component: Database layer | Version: 1.11

(models, ORM) | Resolution:
Severity: Normal | worksforme

Keywords: | Triage Stage:
postgresql,migration,index | Unreviewed

Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Tim Graham):

* status: new => closed
* resolution: => worksforme

Comment:

I couldn't reproduce this by changing `SlugField()` to
`SlugField(unique=True)`. Perhaps the bug is in django-tenant-schemas.
Please reopen if you find that Django is at fault and add more specific
steps to reproduce.

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:3>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field
attribute is added
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: nobody
Type: Bug | Status: new

Component: Database layer | Version: 1.11
(models, ORM) |

Severity: Normal | Resolution:

Keywords: | Triage Stage:
postgresql,migration,index | Unreviewed

Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Ariki):

* status: closed => new
* resolution: worksforme =>

Comment:

Django tries to create a like index twice and fails when I try to make
existing SlugField a primary key in a manually written migration. The code
to reproduce:

{{{
import unittest

from django.db import connection, migrations, models
from django.db.migrations.state import ProjectState
from django.test import TestCase

class ChangePrimaryKeyTest(TestCase):

def test_change_primary_key(self):
# Set PostgreSQL messages locale to get error messages
operation0 = migrations.RunSQL("SET lc_messages = 'C';")
# Create a model with two fields
operation1 = migrations.CreateModel(
'SimpleModel',
[
("field1", models.SlugField(max_length=20,
primary_key=True)),
("field2", models.SlugField(max_length=20)),
],
)
# Drop field1 primary key constraint - this doesn't fail
operation2 = migrations.AlterField(
"SimpleModel",
"field1",
models.SlugField(max_length=20, primary_key=False),
)
# Add a primary key constraint to field2 - this fails
operation3 = migrations.AlterField(
"SimpleModel",
"field2",
models.SlugField(max_length=20, primary_key=True),
)

project_state = ProjectState()
with connection.schema_editor() as editor:
new_state = project_state.clone()
operation0.database_forwards(
"migrtest", editor, project_state, new_state)
operation1.state_forwards("migrtest", new_state)
operation1.database_forwards(
"migrtest", editor, project_state, new_state)
project_state, new_state = new_state, new_state.clone()
operation2.state_forwards("migrtest", new_state)
operation2.database_forwards(
"migrtest", editor, project_state, new_state)
project_state, new_state = new_state, new_state.clone()
operation3.state_forwards("migrtest", new_state)
operation3.database_forwards(
"migrtest", editor, project_state, new_state)

}}}

Error message:

{{{
ERROR: test_change_primary_key (migrtest.tests.ChangePrimaryKeyTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/django/db/backends/utils.py",
line 65, in execute
return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: relation
"migrtest_simplemodel_field2_972171aa_like" already exists
}}}

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:4>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)

-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: nobody

Type: Bug | Status: new
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index |
Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Tim Graham):

* component: Database layer (models, ORM) => Migrations
* stage: Unreviewed => Accepted

Comment:

I can reproduce as long as the three operations are in the same migration.
The crash doesn't happen if you put the `AlterField` operations in a
separate migration.

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:5>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: Tomer
| Chachamu
Type: Bug | Status: assigned

Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index |

Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Tomer Chachamu):

* status: new => assigned
* cc: Tomer Chachamu (added)
* owner: nobody => Tomer Chachamu

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:6>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: Tomer
| Chachamu
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index |

Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Tomer Chachamu):

The test case given is incorrect, as Django always uses a fresh schema
editor for each migration step:

https://github.com/django/django/blob/master/django/db/migrations/executor.py#L225

This passes and is similar to other cases in
{{{migrations/test_operations.py}}}:

{{{

def test_change_primary_key(self):

# Create a model with two fields
operation1 = migrations.CreateModel(
'SimpleModel',
[
("field1", models.SlugField(max_length=20,
primary_key=True)),
("field2", models.SlugField(max_length=20)),
],
)
# Drop field1 primary key constraint - this doesn't fail
operation2 = migrations.AlterField(
"SimpleModel",
"field1",
models.SlugField(max_length=20, primary_key=False),
)
# Add a primary key constraint to field2 - this fails
operation3 = migrations.AlterField(
"SimpleModel",
"field2",
models.SlugField(max_length=20, primary_key=True),
)

project_state = ProjectState()
new_state = project_state.clone()
operation1.state_forwards("migrtest", new_state)
with connection.schema_editor() as editor:

operation1.database_forwards("migrtest", editor,
project_state, new_state)
project_state, new_state = new_state, new_state.clone()
operation2.state_forwards("migrtest", new_state)

with connection.schema_editor() as editor:

operation2.database_forwards("migrtest", editor,
project_state, new_state)
project_state, new_state = new_state, new_state.clone()
operation3.state_forwards("migrtest", new_state)

with connection.schema_editor() as editor:

operation3.database_forwards("migrtest", editor,
project_state, new_state)
}}}

I'm going to try working off the original bug description to reproduce the
bug.

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:7>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: (none)
Type: Bug | Status: new

Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index |

Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Tomer Chachamu):

* status: assigned => new
* owner: Tomer Chachamu => (none)
* has_patch: 0 => 1

Comment:

I have added a reasonable PR. https://github.com/django/django/pull/9438

The root cause is that {{{schema_editor.create_model}}} defers the
creation of indexes, so at any time you can either have indexes on the
database (can be found using {{{schema_editor._constraint_names}}}),
deferred or not at all.
https://github.com/django/django/blob/master/django/db/backends/base/schema.py#L300

According to the comment they are deferred for SQLite, so one solution
would be letting schema editors override the behaviour - removing it for
non-sqlite, or just for postgres. I think it will become more difficult to
reason about the schema editor in that case, and if done properly the
deferred sql can also be an optimiser, removing redundant index creations
and deletions.

Every place that an index is added ought to check whether the index
creation is already deferred, and remove the deferred one if so (in favour
of an immediate one). Every place that an index is removed needs to check
both deferred indexes and actual indexes.

We can probably find more bugs by parameterising the schema and migration
tests to try using separate schema_editors (which flushes deferred SQL to
the database every step) or sharing schema_editors.

Not all the lines added have a test backing them up but I think it's ready
for somebody to have a look at and decide whether the approach is good.

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:8>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: (none)

Type: Bug | Status: new

Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index |

Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Carlton Gibson):

* needs_better_patch: 0 => 1

Comment:

Comments on PR: we have an error in the boolean logic, not correctly
distinguishing between the `_unique` and `primary_key` cases.

(The original suggestion leads to just 3 failures a fix should be simple
enough...)

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:9>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: (none)

Type: Bug | Status: new

Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted

postgresql,migration,index,#djangocph|
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Carlton Gibson):

* keywords: postgresql,migration,index =>
postgresql,migration,index,#djangocph

Comment:

I'm going to mark this for #djangocph for the sprint in Copenhagen.

Whilst it's right in the heart of the migration framework, I think it
should be an easy fix.

Here's the GitHub permalink to the problem `if` check:
https://github.com/django/django/blob/fb8fd535c0f47cffb4da0c5900f3f66e1ec8d432/django/db/backends/postgresql/schema.py#L124-L126

If you apply the original suggested fix you get a small number of failures
(3 I think). These relate to needing to add an index due to a `unique`
flag being added. That's the last `or` in the problem `if`.

The test from https://github.com/django/django/pull/9438 checks the new
problem behaviour. (Can it live with the tests that fail if you apply the
suggested fix?)

That new test fails because the `unique` property is essentially `_unique
or primary_key`, which is too wide. As I said above, using
`new_field._unique` was enough to make the test pass.

The task here is to go through that and make sure it's correct. Make sure
the test is in the right place. Add a comment in the code (if it's
needed). Maybe a release note etc.

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:10>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: (none)

Type: Bug | Status: new

Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by bcail):

* cc: bcail (added)
* needs_better_patch: 1 => 0

Comment:

[https://github.com/django/django/pull/17807 PR]
--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:11>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------

Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned

Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by bcail):

* owner: (none) => bcail

* status: new => assigned

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:12>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Mariusz Felisiak):

* needs_better_patch: 0 => 1

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:13>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by bcail):

* needs_better_patch: 1 => 0

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:14>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Mariusz Felisiak):

* needs_better_patch: 0 => 1

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:15>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Mariusz Felisiak):

#35180 was a duplicate. We should add a test for altering
`CharField(db_index=True, ...)` to the `TextField()`.
--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:16>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by bcail):

* needs_better_patch: 1 => 0

Comment:

Hi Mariusz, I added that test. I also updated the code to use "CREATE
INDEX IF NOT EXISTS"... what do you think of going that direction?
--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:17>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Mariusz Felisiak):

* needs_better_patch: 0 => 1

--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:18>

[Django]

#28646: Migration calls "CREATE INDEX" when one already exists when 'unique' field

attribute is added (PostgreSQL)
-------------------------------------+-------------------------------------
Reporter: Hari - 何瑞理 | Owner: bcail
Type: Bug | Status: assigned
Component: Migrations | Version: 1.11
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
postgresql,migration,index,#djangocph|

Has patch: 1 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by bcail):

* needs_better_patch: 1 => 0

Comment:

I reverted the "IF NOT EXISTS" changes, and split out the create-new-index
and recreate-deleted-index conditions into separate methods.
--
Ticket URL: <https://code.djangoproject.com/ticket/28646#comment:19>