The next serious question to ask about each variable is whether and how it will be indexed.
Variables where searches are not to be conducted should not be indexed, as these would eat up ressources uselessly.
Do you want to index the entire entry or do you want each individual word to be searchable independantly ? Consider town names City on River. Do you want it to be found by typing 'River' or only by typing 'City in River' literally.
Do you want the search to be case sensitive ? Using our previous example, if the case is search sensitive and indexed 'as one string', someone typing City In River will not find that record...
The system can also conduct checks in such a manner as to ensure that the record being input has a unique value for that variable... things like serial numbers...