diff --git a/app/logical/related_tag_calculator.rb b/app/logical/related_tag_calculator.rb
index d99d287c3..9905d21aa 100644
--- a/app/logical/related_tag_calculator.rb
+++ b/app/logical/related_tag_calculator.rb
@@ -1,11 +1,5 @@
 class RelatedTagCalculator
-  def self.find_tags(tag, limit)
-    CurrentUser.without_safe_mode do
-      Post.with_timeout(5_000, [], {:tags => tag}) do
-        Post.tag_match(tag).limit(limit).reorder("posts.md5").pluck(:tag_string)
-      end
-    end
-  end
+  MAX_RESULTS = 25
 
   def self.calculate_from_sample_to_array(tags, category_constraint = nil)
     convert_hash_to_array(calculate_from_sample(tags, Danbooru.config.post_sample_size, category_constraint))
@@ -60,29 +54,21 @@ class RelatedTagCalculator
     convert_hash_to_array(similar_counts)
   end
 
-  def self.calculate_from_sample(tags, limit, category_constraint = nil)
-    counts = Hash.new {|h, k| h[k] = 0}
+  def self.calculate_from_sample(tags, sample_size, category_constraint = nil, max_results = MAX_RESULTS)
+    Post.with_timeout(5_000, [], {:tags => tags}) do
+      sample = Post.sample(tags, sample_size)
+      posts_with_tags = Post.from(sample).with_unflattened_tags
 
-    find_tags(tags, limit).each do |tags|
-      tag_array = Tag.scan_tags(tags)
       if category_constraint
-        tag_array.each do |tag|
-          category = Tag.category_for(tag)
-          if category == category_constraint
-            counts[tag] += 1
-          end
-        end
-      else
-        tag_array.each do |tag|
-          counts[tag] += 1
-        end
+        posts_with_tags = posts_with_tags.joins("JOIN tags ON tags.name = tag").where("tags.category" => category_constraint)
       end
-    end
 
-    counts
+      counts = posts_with_tags.order("count(*) DESC").limit(max_results).group("tag").count
+      counts
+    end
   end
 
-  def self.convert_hash_to_array(hash, limit = 25)
+  def self.convert_hash_to_array(hash, limit = MAX_RESULTS)
     hash.to_a.sort_by {|x| -x[1]}.slice(0, limit)
   end
 
diff --git a/app/models/post.rb b/app/models/post.rb
index 10c3a863b..f36c7514e 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -1593,6 +1593,17 @@ class Post < ActiveRecord::Base
       where("md5 >= ?", key).reorder("md5 asc").first
     end
 
+    def sample(query, sample_size)
+      CurrentUser.without_safe_mode do
+        tag_match(query).reorder(:md5).limit(sample_size)
+      end
+    end
+
+    # unflattens the tag_string into one tag per row.
+    def with_unflattened_tags
+      joins("CROSS JOIN unnest(string_to_array(tag_string, ' ')) AS tag")
+    end
+
     def pending
       where("is_pending = ?", true)
     end