Why can count(x > 5) not return 0?

When using the count aggregation operator you may have noticed that it sometimes returns nothing rather than 0. Why is this?

To explain, let's start with an example instant vector:

x{l="foo"} 2
x{l="bar"} 4

If you were to evaluate count(x) you would get {}: 2, which is to say a single sample with no labels and the value 2. This is what you'd expect.

If you were to evaluate count by (l)(x) you would get an instant vector with two elements, {l="foo"}: 1 and {l="bar"}: 1. This all seems fine so far.

Now what if you do count by (l)(x > 3)? This makes the result a single sample of {l="bar"}: 1. This is as the sample with l="foo" would be filtered away, and the count aggregator would only be applied to the l="bar" sample. count can't invent a label out of nowhere after all.

The same applies to count(x > 5). The instant vector returned by x > 5 is empty so the result of the count is also going to be an empty instant vector.

The good news is that there is a way to get a 0 in this situation, by taking advantage of the bool modifier of comparison operators. Unlike normal comparison operators which filter if the comparison fails, the bool modifier will  return a 0 if the comparison fails and a 1 if it succeeds. Then you can add these up using sum.

So sum(x > bool 5) would return {}: 0. Similarly sum by (l)(x > bool 3) would return an instant vector with two elements, {l="foo"}: 0 and {l="bar"}: 1.