COUNT(1) in pre-agg for AVG is wrong

[anhqle]

Currently, when we perform pre-aggregation for AVG, we use SUM(expression) / COUNT(1).

COUNT(1) will give the count of another column, which may have a different number of non-null values than the one we're interested in.

For example:

day | measure

1 | 1
2 | 1
3 | NULL

The correct COUNT(measure) is 2, but COUNT(1) is 3 instead.