This commit vastly reduces the impact of auto
explain logging when enabled, while keeping
a negligible cost when disabled.
The first implementation was based on the idea
of subscribing to "sql.active_record" when
needed, and unsubscribing once done. This is
the idea behind AR::Relation#explain. Subscribe,
collect, unsubscribe.
But with the current implementation of notifications
unsubscribing is costly, because it wipes an internal
cache and that puts a penalty on the next event.
So we are switching to an approach where a long-running
subscriber is listening. Instead of collecting the
queries with a closure in a dedicated subscriber, now
we setup a thread local.
If the feature is disabled by setting the threshold
to nil, the subscriber will call a method that does
nothing. That's totally cheap.
Rationale: As discussed with José and Jon, this convenience
shortcut is not clearly justified and it could let the user
thing the disabled EXPLAINs are related to the model instance
rather than being globally disabled.
Xavier Noria