Unnecessary query in db cache backend when CULL_FREQUENCY > MAX_ENTRIES + 1

[Jacob Walls]

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:

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 ​forum
• in tests, overriding MAX_ENTRIES to 0 or 1 for determinism, which would then fall below the default CULL_FREQUENCY of 3.