Wire up COUNT DISTINCT aggregate

[pauldix]

From a syntax perspective it should look like this:

select count(distinct id) from events

The underlying implementation should use the DISTINCT Map function, but have a new reducer that combines the Map output and outputs a count.

[pauldix]

I should mention that the target could be either a field or a tag key. The implementation would be different depending on which type was passed in.

[corylanou]

Is it more common to see this:

select COUNT(DISTINCT id) as count FROM events

[pauldix]

@beckettsean derivative will actually use the nested syntax. We should just be validating it if they do a nesting that we don't support. See #1477

[pauldix]

@corylanou agreed on your syntax, updated the issue to reflect that

[corylanou]

This issue depends on #1815 being done first.