While sorting results over a numeric field, documents which do not contain a value for the sorting field seem to get 0 (ZERO) value in the sort. (Tested against Double, Float, Int & Long numeric fields ascending and descending order).
This behavior is unexpected, as zero is "comparable" to the rest of the values. A better solution would either be allowing the user to define such a "non-value" default, or always bring those document results as the last ones.
Adding 3 documents, 1st with value 3.5d, 2nd with -10d, and 3rd without any value.
Searching with MatchAllDocsQuery, with sort over that field in descending order yields the docid results of 0, 2, 1.
Asking for the top 2 documents brings the document without any value as the 2nd result - which seems as a bug?