let automatic EXPLAIN ignore CACHE notifications

activerecord/lib/active_record/explain_subscriber.rb

@@ -11,7 +11,10 @@ module ActiveRecord
 
     # SCHEMA queries cannot be EXPLAINed, also we do not want to run EXPLAIN on
     # our own EXPLAINs now matter how loopingly beautiful that would be.
-    IGNORED_PAYLOADS = %w(SCHEMA EXPLAIN)
+    #
+    # On the other hand, we want to monitor the performance of our real database
+    # queries, not the performance of the access to the query cache.
+    IGNORED_PAYLOADS = %w(SCHEMA EXPLAIN CACHE)
     def ignore_payload?(payload)
       payload[:exception] || IGNORED_PAYLOADS.include?(payload[:name])
     end

activerecord/test/cases/explain_subscriber_test.rb

@@ -0,0 +1,48 @@
+require 'cases/helper'
+
+if ActiveRecord::Base.connection.supports_explain?
+  class ExplainSubscriberTest < ActiveRecord::TestCase
+    SUBSCRIBER = ActiveRecord::ExplainSubscriber.new
+
+    def test_collects_nothing_if_available_queries_for_explain_is_nil
+      with_queries(nil) do
+        SUBSCRIBER.call
+        assert_nil Thread.current[:available_queries_for_explain]
+      end
+    end
+
+    def test_collects_nothing_if_the_payload_has_an_exception
+      with_queries([]) do |queries|
+        SUBSCRIBER.call(:exception => Exception.new)
+        assert queries.empty?
+      end
+    end
+
+    def test_collects_nothing_for_ignored_payloads
+      with_queries([]) do |queries|
+        ActiveRecord::ExplainSubscriber::IGNORED_PAYLOADS.each do |ip|
+          SUBSCRIBER.call(:name => ip)
+        end
+        assert queries.empty?
+      end
+    end
+
+    def test_collects_pairs_of_queries_and_binds
+      sql   = 'select 1 from users'
+      binds = [1, 2]
+      with_queries([]) do |queries|
+        SUBSCRIBER.call(:name => 'SQL', :sql => sql, :binds => binds)
+        assert_equal 1, queries.size
+        assert_equal sql, queries[0][0]
+        assert_equal binds, queries[0][1]
+      end
+    end
+
+    def with_queries(queries)
+      Thread.current[:available_queries_for_explain] = queries
+      yield queries
+    ensure
+      Thread.current[:available_queries_for_explain] = nil
+    end
+  end
+end