PERF: batch update post timings

previously we would issue a query per row in post timings,
this batches it

app/models/post_timing.rb

@@ -29,17 +29,7 @@ class PostTiming < ActiveRecord::Base
     TopicUser.ensure_consistency!(topic_id)
   end
 
-  # Increases a timer if a row exists, otherwise create it
+  def self.record_new_timing(args)
-  def self.record_timing(args)
-    rows = exec_sql_row_count("UPDATE post_timings
-                               SET msecs = msecs + :msecs
-                               WHERE topic_id = :topic_id
-                                AND user_id = :user_id
-                                AND post_number = :post_number",
-                                args)
-
-    if rows == 0
-
     begin
       exec_sql("INSERT INTO post_timings (topic_id, user_id, post_number, msecs)
                 SELECT :topic_id, :user_id, :post_number, :msecs
@@ -57,6 +47,17 @@ class PostTiming < ActiveRecord::Base
     Post.where(['topic_id = :topic_id and post_number = :post_number', args]).update_all 'reads = reads + 1'
     UserStat.where(user_id: args[:user_id]).update_all 'posts_read_count = posts_read_count + 1'
   end
+
+  # Increases a timer if a row exists, otherwise create it
+  def self.record_timing(args)
+    rows = exec_sql_row_count("UPDATE post_timings
+                               SET msecs = msecs + :msecs
+                               WHERE topic_id = :topic_id
+                                AND user_id = :user_id
+                                AND post_number = :post_number",
+                                args)
+
+    record_new_timing(args) if rows == 0
   end
 
 
@@ -74,16 +75,46 @@ class PostTiming < ActiveRecord::Base
     account_age_msecs = ((Time.now - current_user.created_at) * 1000.0)
 
     highest_seen = 1
-    timings.each do |post_number, time|
+
-      if post_number >= 0 && time < account_age_msecs
+    join_table = []
-        PostTiming.record_timing(topic_id: topic_id,
+
-                                 post_number: post_number,
+    timings = timings.find_all do |post_number, time|
-                                 user_id: current_user.id,
+      post_number >= 0 && time < account_age_msecs
-                                 msecs: time)
+    end
+
+    timings.each_with_index do |(post_number, time), index|
+        join_table << "SELECT #{topic_id.to_i} topic_id, #{post_number.to_i} post_number,
+                       #{current_user.id.to_i} user_id, #{time.to_i} msecs, #{index} idx"
+
 
         highest_seen = post_number.to_i > highest_seen ?
                        post_number.to_i : highest_seen
     end
+
+    if join_table.length > 0
+      sql = <<SQL
+
+      UPDATE post_timings t
+      SET msecs = t.msecs + x.msecs
+      FROM (#{join_table.join(" UNION ALL ")}) x
+      WHERE x.topic_id = t.topic_id AND
+            x.post_number = t.post_number AND
+            x.user_id = t.user_id
+      RETURNING x.idx
+SQL
+
+      result = exec_sql(sql)
+      result.type_map = SqlBuilder.pg_type_map
+      existing = Set.new(result.column_values(0))
+
+      timings.each_with_index do |(post_number, time),index|
+        unless existing.include?(index)
+          PostTiming.record_new_timing(topic_id: topic_id,
+                                 post_number: post_number,
+                                 user_id: current_user.id,
+                                 msecs: time)
+        end
+      end
     end
 
     total_changed = 0