SQL: Investigate the way text manipulating functions extract the value from the fields

At the moment, if a `text` field is passed to a text manipulating function, we try to see if the fields has a `keyword` subfield and if it does, we use that one and extract the value from doc_values. Should we look into see if the fields has the source stored and extract that instead? (and pay the performance penalty of retrieving the source and extract it)