added null checks
Add comments to distinct values cases
Merge branch 'piggyback_master' of https://github.com/mpws2013n1/postgres into piggyback_master
refactoring, first steps for fd checks
Merge branch 'piggyback_master' of https://github.com/mpws2013n1/postgres into piggyback_master
Merge branch 'piggyback_master' of https://github.com/mpws2013n1/postgres into prefinal_stats
refactored: removed old min/max/count that were directly in piggyback struct