#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>
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