elasticsearch/elasticsearch — pull request #7005 by rmuir (2014-07-24 09:28:16+00:00)
Title: Speed up string sort with custom missing value

Today if the user supplies a custom missing value for a string sort, we do it in an extremely slow way, not using ordinals but dereferencing bytes for every document. Ordinals are only used if the missing value is _first or _last.

Instead, use ordinals with custom missing values too.