implements a much faster auto EXPLAIN, closes #3843 [José Valim & Xavier Noria]

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.

1 files changed, 13 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/explain_subscriber.rb b/activerecord/lib/active_record/explain_subscriber.rb
new file mode 100644
index 0000000000..8a88be6761
--- /dev/null
+++ b/activerecord/lib/active_record/explain_subscriber.rb
@@ -0,0 +1,13 @@
+module ActiveRecord
+  class ExplainSubscriber < ActiveSupport::LogSubscriber
+    def sql(event)
+      ActiveRecord::Base.collect_queries_for_explain(event.payload)
+    end
+
+    def logger
+      ActiveRecord::Base.logger
+    end
+
+    attach_to :active_record
+  end
+end