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, 3 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/log_subscriber.rb b/activerecord/lib/active_record/log_subscriber.rb
index 3a015ee8c2..a25f2c7bca 100644
--- a/activerecord/lib/active_record/log_subscriber.rb
+++ b/activerecord/lib/active_record/log_subscriber.rb
@@ -26,9 +26,9 @@ module ActiveRecord
 
       return if 'SCHEMA' == payload[:name]
 
-      name    = '%s (%.1fms)' % [payload[:name], event.duration]
-      sql     = payload[:sql].squeeze(' ')
-      binds   = nil
+      name  = '%s (%.1fms)' % [payload[:name], event.duration]
+      sql   = payload[:sql].squeeze(' ')
+      binds = nil
 
       unless (payload[:binds] || []).empty?
         binds = "  " + payload[:binds].map { |col,v|