[Django] #37130: Unnecessary query in db cache backend when CULL_FREQUENCY > MAX_ENTRIES + 1

[Django]

#37130: Unnecessary query in db cache backend when CULL_FREQUENCY > MAX_ENTRIES + 1
-------------------------------------+-------------------------------------
Reporter: Jacob Walls | Type:
| Cleanup/optimization
Status: new | Component: Core
| (Cache system)
Version: dev | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
-------------------------------------+-------------------------------------
The db cache backend deletes expired entries according to a
`CULL_FREQUENCY` denominator after expired entries are deleted. For
example: `2`, means delete 50%. This generates SQL like:

{{{#!sql
SELECT cache_key from ... ORDER_BY cache_key LIMIT 1 OFFSET N
DELETE FROM ... WHERE cache_key < result of query 1
}}}

But where N is 0 (`cull_num` in the code), the SELECT is unnecessary. I'm
suggesting there should be another fast path where `cull_num` is 0 that
simply exits.

I spotted this in a real user's logs, see #32785.

This would happen when `CULL_FREQUENCY` > `MAX_ENTRIES` + 1. I imagine the
use cases for that configuration would be:
- a culling strategy that is age-based instead of random, see
[https://forum.djangoproject.com/t/django-database-cache-untouched-
expired-entries-are-never-culled-unless-max-entries-is-reached/22679
forum]
- in tests, overriding `MAX_ENTRIES` to 0 or 1 for determinism, which
would then fall below the default `CULL_FREQUENCY` of 3.
--
Ticket URL: <https://code.djangoproject.com/ticket/37130>
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[Django]

#37130: Unnecessary query in db cache backend when CULL_FREQUENCY > MAX_ENTRIES + 1
-------------------------------------+-------------------------------------

Reporter: Jacob Walls | Owner: Vishy
Type: | Status: assigned
Cleanup/optimization |

Component: Core (Cache system) | Version: dev

Severity: Normal | Resolution:

Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Vishy):

* owner: (none) => Vishy
* status: new => assigned

--
Ticket URL: <https://code.djangoproject.com/ticket/37130#comment:1>