post_versions/index.html.erb:5: avoid extraneous COUNT.

@post_versions.empty? caused this query:

    SELECT COUNT(count_column) FROM (SELECT 1 AS count_column FROM "post_versions" WHERE (true) LIMIT 20 OFFSET 0) subquery_for_count

which is unnecessary since @post_versions can be counted directly.

app/views/post_versions/index.html.erb

@@ -2,7 +2,7 @@
   <div id="a-index">
     <h1>Changes</h1>
 
-    <% if @post_versions.empty? %>
+    <% if @post_versions.length == 0 %>
       <%= render "post_sets/blank" %>
     <% else %>
       <%= render "listing", :post_versions => @post_versions %>