1 files changed, 82 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/explain.rb b/activerecord/lib/active_record/explain.rb
new file mode 100644
index 0000000000..01cacf6153
--- /dev/null
+++ b/activerecord/lib/active_record/explain.rb
@@ -0,0 +1,82 @@
+require 'active_support/core_ext/class/attribute'
+
+module ActiveRecord
+  module Explain
+    def self.extended(base)
+      # If a query takes longer than these many seconds we log its query plan
+      # automatically. nil disables this feature.
+      base.config_attribute :auto_explain_threshold_in_seconds, :global => true
+    end
+
+    # If auto explain is enabled, this method triggers EXPLAIN logging for the
+    # queries triggered by the block if it takes more than the threshold as a
+    # whole. That is, the threshold is not checked against each individual
+    # query, but against the duration of the entire block. This approach is
+    # convenient for relations.
+    #
+    # The available_queries_for_explain thread variable collects the queries
+    # to be explained. If the value is nil, it means queries are not being
+    # currently collected. A false value indicates collecting is turned
+    # off. Otherwise it is an array of queries.
+    def logging_query_plan # :nodoc:
+      return yield unless logger
+
+      threshold = auto_explain_threshold_in_seconds
+      current   = Thread.current
+      if threshold && current[:available_queries_for_explain].nil?
+        begin
+          queries = current[:available_queries_for_explain] = []
+          start = Time.now
+          result = yield
+          logger.warn(exec_explain(queries)) if Time.now - start > threshold
+          result
+        ensure
+          current[:available_queries_for_explain] = nil
+        end
+      else
+        yield
+      end
+    end
+
+    # Relation#explain needs to be able to collect the queries regardless of
+    # whether auto explain is enabled. This method serves that purpose.
+    def collecting_queries_for_explain # :nodoc:
+      current = Thread.current
+      original, current[:available_queries_for_explain] = current[:available_queries_for_explain], []
+      return yield, current[:available_queries_for_explain]
+    ensure
+      # Note that the return value above does not depend on this assigment.
+      current[:available_queries_for_explain] = original
+    end
+
+    # Makes the adapter execute EXPLAIN for the tuples of queries and bindings.
+    # Returns a formatted string ready to be logged.
+    def exec_explain(queries) # :nodoc:
+      queries && queries.map do |sql, bind|
+        [].tap do |msg|
+          msg << "EXPLAIN for: #{sql}"
+          unless bind.empty?
+            bind_msg = bind.map {|col, val| [col.name, val]}.inspect
+            msg.last << " #{bind_msg}"
+          end
+          msg << connection.explain(sql, bind)
+        end.join("\n")
+      end.join("\n")
+    end
+
+    # Silences automatic EXPLAIN logging for the duration of the block.
+    #
+    # This has high priority, no EXPLAINs will be run even if downwards
+    # the threshold is set to 0.
+    #
+    # As the name of the method suggests this only applies to automatic
+    # EXPLAINs, manual calls to +ActiveRecord::Relation#explain+ run.
+    def silence_auto_explain
+      current = Thread.current
+      original, current[:available_queries_for_explain] = current[:available_queries_for_explain], false
+      yield
+    ensure
+      current[:available_queries_for_explain] = original
+    end
+  end
+end