dismax solr request handler MM , PS and Q.ALT

Question

I'm testing the dismax requesthandler , im trying to customize the mm ( Minimum Match ) Parameter ( i already looked at the documentation )

 <str name="mm">
 2<-2 3<-70% 5<-50%  
 </str>
 <int name="ps">100</int>
 <str name="q.alt">*:*</str>

I have 3 Questions :

• mm , i understood what it does , i want to verify if my param is ok ,

  correct me if im wrong
  2<-2 3<-70% 5<-50% stands for ? :

  • if 1 or 2 terms match 100% of them
  • if 3 to 4 match only 70% of them
  • if 5 or more match only 50%

• ps : what is this param? Is it mandatory ?

• q.alt : the same i did not understand the utility of that one.

Can you provide me some advise for the best configuration of DisMax ?

Thank you !

Answer 1

1: Your string would translate to

1-2 terms: match all terms
3 terms: total number of terms - 2 must match (i.e. 1 term must match :)
4-5 terms: 70% of the terms must match 
6+ terms: 50% of the terms must match

mm-string to achieve your requirement would be

2<%70 4<%50

A very good resource for mm strings can be found here.

2: The pf parameter is used to boost document relevance based on query terms occurring in close proximity of each other (as opposed to being scattered all over the document).

3: The q.alt parameter is used as a fallback query for cases when the client did not provide any search terms. In my interpretation, it is meant for use if you don't do any client-side query processing/transformation, but there could be other practical uses for it.