Opened 14 months ago
Closed 14 months ago
#34874 closed Cleanup/optimization (wontfix)
Add table_exists() to optimize checks for django_migrations table.
Reported by: | Pierre | Owned by: | nobody |
---|---|---|---|
Component: | Database layer (models, ORM) | Version: | 4.2 |
Severity: | Normal | Keywords: | |
Cc: | Triage Stage: | Unreviewed | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
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.
Change History (1)
comment:1 by , 14 months ago
Resolution: | → wontfix |
---|---|
Status: | new → closed |
Summary: | the construction "table_name" in introspection.table_names() is suboptimal → Add table_exists() to optimize checks for django_migrations table. |
As far as I'm aware, tables introspection is extremely fast on PostgreSQL. This is proven by 21ms for 8800 tables.
This is cached in Django 5.0+ (ea8cbca579cc6742e119747fc1eb6ecf90638bce.)
I don't think it's worth adding for a single (already cached) call.