Solr - Subtract 2 columns and store the result in the 3rd column

I am trying to compare 2 columns in solr. After searching a bit I realised that it is not possible in solr and i need to make a new field and store the result at the time of indexing .

So far i have created a new field called compare of type Boolean , but i have this question when will the compare field get the subtracted value . I know it is at the time of indexing , but i don't know how. I tried putting column1-column2 as the default field when i was creating the 3rd column , but that didn't work out either.

So any help regarding this will be highly appreciated.