[5.6] Math DB functions should return numeric type

[antonkomarev]

I've found one interesting thing when started to declare static types in one of my packages. Maybe I'm missing something, but it's not right that database query math functions like avg and sum returns string instead of float or int.

I understand that min & max functions are math too, but they could be used to get DATE & TIMESTAMP types. Is there any use cases for avg or sum functions where string could be returned?

Average dates are getting by converting them to Unix timestamps, calculating average value and then converting them to date value back.

Notice that we don't need this check: return $result ?: 0;. It's done in numericAggregate method.

[antonkomarev]

One more function is count, but I was unsure if we need to change it. It's using very frequently and it could be bad for performance.

[GrahamCampbell]

I think we did this before, then had to revert?

[taylorotwell]

Yes pretty certain this already had to be reverted once. If you can find out and fix why let me know in another PR.

[antonkomarev]

There were issues with min and max functions. But test for this case already in place and they are failing if these functions will return numeric values: #14991

@taylorotwell I didn't opened new PR as you said, because I don't see anything to fix here. I've changed only avg and sum functions.

Initially changed here: #14793
Reverted here: #14994