In principle any column whose values can be compared using == can be DISTINCT input to an aggregation, so I'm +1 on having it as a property of aggregate functions instead of having separate {agg}_distinct functions.

I'm -1 on {agg}_distinct. I don't understand the alternative enough to say. When we say a property of invoking the aggregate function are we really talking about a property on the AggregateFunction message? Or is this part of the AggregateRel::Measure messages?

I was thinking it'd be a property on AggregateFunction message.

For example, like this PR: #191