FIX: custom field index must only index short values

db/migrate/20160520022627_shorten_topic_custom_fields_index.rb

@@ -0,0 +1,12 @@
+class ShortenTopicCustomFieldsIndex < ActiveRecord::Migration
+  def up
+    remove_index :topic_custom_fields, :value
+    add_index :topic_custom_fields, [:value, :name],
+                name: 'topic_custom_fields_value_key_idx',
+                where: 'value IS NOT NULL AND char_length(value) < 400'
+  end
+  def down
+    remove_index :topic_custom_fields, :value, name: 'topic_custom_fields_value_key_idx'
+    add_index :topic_custom_fields, :value
+  end
+end