Fix migration when running on MySQL databases

db/migrate/20170905202320_add_missing_indexes_to_geo_event_log.rb

@@ -6,14 +6,19 @@ class AddMissingIndexesToGeoEventLog < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
+    # MySQL automatically creates an index on a foreign-key constraint; PostgreSQL does not
+    if Gitlab::Database.postgresql?
       add_concurrent_index :geo_event_log, :repositories_changed_event_id
       add_concurrent_index :geo_event_log, :repository_deleted_event_id
       add_concurrent_index :geo_event_log, :repository_renamed_event_id
     end
+  end
 
   def down
+    if Gitlab::Database.postgresql?
       remove_concurrent_index :geo_event_log, :repositories_changed_event_id if index_exists? :geo_event_log, :repositories_changed_event_id
       remove_concurrent_index :geo_event_log, :repository_deleted_event_id if index_exists? :geo_event_log, :repository_deleted_event_id
       remove_concurrent_index :geo_event_log, :repository_renamed_event_id if index_exists? :geo_event_log, :repository_renamed_event_id
     end
+  end
 end