Depends from your disk system and data set size ... It can also mean 2-5 hours or days versus subsecond query. Try having 6 TB database with integer column. Th

Depends from your disk system and data set size ... It can also mean 2-5 hours or days versus subsecond query. Try having 6 TB database with integer column. Then select just one row from it. Either it's indexed or not. You don't need to explain that, if query ends up taking forever it wasn't indexed.