The query plan for that is simple, but the number of bytes transmitted per query is O(size of table) (as is the disk IO on the DB), which should clearly set off

The query plan for that is simple, but the number of bytes transmitted per query is O(size of table) (as is the disk IO on the DB), which should clearly set off a red flag if you know that size of table can get large. You don't need a large exponent; You need to understand the rough cardinality of values, then you need to look at asymptotic complexity.