This article explains the format used for specifying the “Min Number Should Match” criteria of the BooleanQuery objects built by the DisMaxRequestHandler.  Using this it is possible to specify a percentage of query words (or blocks) that should appear in a document.

There are 3 types of “clauses” that Solr (Lucene) knows about: mandatory, prohibited, and ‘optional’.  By default all words or phrases specified in the “q” param are treated as “optional” clauses unless they are preceeded by a “+” or a “-“. When dealing with these “optional” clauses, the “mm” option makes it possible to say that a certain minimum number of those clauses must match (mm).

Specifying this minimum number can be done in complex ways, like…..

1. At least 2 of the optional clauses must match, regardless of how many clauses there are: “2”.
   
2. At least 75% of the optional clauses must match, rounded down: “75%”.
3. If there are less than 3 optional clauses, they all must match; if there are 3 or more, then 75% must match, rounded up: “2<-25%”.
4. If there are less than 3 optional clauses, they all must match; for 3 to 5 clauses, one less than the number of clauses must match, for 6 or more clauses, 80% must match, rounded down: “2<-1 5<80%”
5. Multiple conditional specifications can be separated by spaces, each one only being valid for numbers greater than the one before it. In this example: if there are 1 or 2 clauses both are required, if there are 3-9 clauses all but 25% are required, and if there are more than 9 clauses, all but three are required: “2<-25% 9<-3?