#34874: the construction "table_name" in introspection.table_names() is 
               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          |
 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

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