An application of Function Queries in Solr

Suppose you have a set of Solr documents. You wish to push the subset of documents having a particular field with value X to the top of the search results, effectively superseding Solr's default scoring mechanism. For example, your index contains a collection of computers for sale and the particular field's name is "operatingsystemtype" . Possible values for this field are: 1 (Windows XP), 2 (Vista), 5 (Mac OS 10), and 7 (Debian). You wish to push all the documents with operatingsystemtype = 5 to the top.

One way to implement this is to embed a _val_ hook right after the q parameter in the HTTP query string you send to Solr. For example: http://search.buycomputers.com/select/?q=(price:[1000 TO 1500])+_val_="map(operatingsystemtype,5,5,1000)"&fl=id,name,price,details.

• Note the '+' instead of the usual '&'.
  
• The map function will map all values between 5 and 5 inclusive to 1000. (See here for more details.) Although this 1000 is normalized to a smaller value, it is added to the document's score. Basically, all documents with operatingsystemtype = 5 will have a very large constant added to their score, effectively pushing all these documents to the top.
  
• Please note that this method relies on Solr's default sort (i.e. score desc). If you provide a sort parameter, thus overriding the default order, this method won't work.
• This is called a Function Query in Solr parlance.