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.
This means we never have to rely on define_method (which is slower and
uses more memory), even when we have attributes containing characters
that are not allowed in standard method names.
(I am mainly changing this because the duplication annoys me, though.)
Also make it actually work.
It slows down all read_attribute accesses to map 'id' to whatever the PK
actually is, inside read_attribute. So instead make sure the necessary
methods are defined and that they redirect wherever they need to go.
Implement a mini state machine for serialized attributes. This means we
do not have to deserialize the values upon initialization, which means
that if we never actually access the attribute, we never have to
deserialize it.
the following code is deprecated:
Thread.new { Post.find(1) }.join
It should be changed to close the database connection at the end of
the thread:
Thread.new {
Post.find(1)
Post.connection.close
}.join
Only people who spawn threads in their application code need to worry
about this change.
Rationale: this is more readable if serveral queries
are involved in one call. Also, it will be possible
to let AR log EXPLAINs automatically in production
mode, where queries are not even around.
