PERF: batch update post timings

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

app/models/post_timing.rb

@@ -29,6 +29,25 @@ class PostTiming < ActiveRecord::Base
     TopicUser.ensure_consistency!(topic_id)
   end
 
+  def self.record_new_timing(args)
+    begin
+      exec_sql("INSERT INTO post_timings (topic_id, user_id, post_number, msecs)
+                SELECT :topic_id, :user_id, :post_number, :msecs
+                WHERE NOT EXISTS(SELECT 1 FROM post_timings
+                                 WHERE topic_id = :topic_id
+                                  AND user_id = :user_id
+                                  AND post_number = :post_number)",
+             args)
+    rescue PG::UniqueViolation
+       # concurrency is hard, we are not running serialized so this can possibly
+       # still happen, if it happens we just don't care, its an invalid record anyway
+       return
+    end
+
+    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
@@ -38,25 +57,7 @@ class PostTiming < ActiveRecord::Base
                                 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
-                  WHERE NOT EXISTS(SELECT 1 FROM post_timings
-                                   WHERE topic_id = :topic_id
-                                    AND user_id = :user_id
-                                    AND post_number = :post_number)",
-               args)
-      rescue PG::UniqueViolation
-         # concurrency is hard, we are not running serialized so this can possibly
-         # still happen, if it happens we just don't care, its an invalid record anyway
-         return
-      end
-
-      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
+    record_new_timing(args) if rows == 0
   end
 
 
@@ -74,15 +75,45 @@ 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
-        PostTiming.record_timing(topic_id: topic_id,
-                                 post_number: post_number,
-                                 user_id: current_user.id,
-                                 msecs: time)
+
+    join_table = []
+
+    timings = timings.find_all do |post_number, time|
+      post_number >= 0 && time < account_age_msecs
+    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