Add explicit option to enable IF EXISTS / IF NOT EXISTS in PostgresQL index operations

[Laurent Tramoy]

Hello,

I'd like to be able to add a IF NOT EXISTS clause when creating indexes (both with AddIndex and AddIndexConcurrently).

I've read this ticket https://code.djangoproject.com/ticket/34298 where Mariusz considers it's a bad idea (for constraints), and I get his point, however I've a few counter arguments:

• I feel like the workflow "add an index in a deployed environment / check it works as expected / add it to Django code" is quite common in "real-life" applications, for now we have to write a custom SeparateDatabaseAndState each time we do this
• The idea would be to enable this explicitly (either by adding a AddIndexIfNotExists operation, or by adding a parameter to AddIndex / AddIndexConcurrently), so people should know what they're doing when using this.
• The IF EXISTS clause is already used when dropping the index, it would be more consistent to be able to allow it when adding the index too ​https://github.com/django/django/blob/afc880571d5f5a16b977070b15014f9583524898/django/db/backends/postgresql/schema.py#L25

[Mariusz Felisiak]

I still think it is a bad idea for exactly the same reasons I already mentioned in the #34298. Also, I don't think that separate operations or extra arguments to control this behavior is a good idea, following this path we could add the same to all migration operations. Why should index creation be treated differently? Last but not least this syntax is not supported by all database engines with built-in backends.

Please first start a discussion on the DevelopersMailingList, where you'll reach a wider audience and see what other think, and [​https://docs.djangoproject.com/en/stable/internals/contributing/triaging-tickets/#closing-tickets follow the triaging guidelines with regards to wontfix tickets.