SOLR dynamic fields and random filtering

I would like to perform a random sort of data coming from database using SOLR. I found that SOLR has already a dynamicField that I can use with random field type :

<types>
   ...
   <fieldType name="random" class="solr.RandomSortField" />
  ... 
</types>
<fields>
    ...
   <dynamicField name="random*" type="random" indexed="true" stored="false"/>
    ...
</fields>

But I don't understant how to fill the random* column. My data are issues from data import using SQL query.

Must I fill the column in db-data-config.xml and in which way ?

Thanks for any help.