[Django] #34874: the construction "table_name" in introspection.table_names() is suboptimal

[Django]

#34874: the construction "table_name" in introspection.table_names() is suboptimal
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Reporter: pinaraf | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: 4.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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On databases with a lot of tables, even in several schemas (with django-
tenant-schema for instance), the `introspection.table_names()` call can
take tens of ms to run. For instance on a PG database with 8800 relations,
the query takes 21ms.
table_names() is called at least by the migration recorder in order to
verify the presence of the django_migrations table. It thus slows down
every migration execution uselessly.
Fixing the migrations recorder to keep the information cached would be a
quick work around this, but it would still execute the long query at least
once, but especially with django-tenant-schema it would end up taking tens
of seconds (at least one call per tenant).

I suggest adding an `table_exists(table_name)` function in
db.introspection. Its base implementation could be simply a `return
table_name in self.table_names()`, but specific DB engines like PostgreSQL
could have an optimized variant.
The SQL query from table_names function modified to add a `relname =
'XXX'` criteria uses the index on relname and is thus over 20 times
faster.

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Ticket URL: <https://code.djangoproject.com/ticket/34874>
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