rewrite avg for algebraic optimization

[ucasfl]

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• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Perform algebraic optimizations of arithmetic expressions inside avg aggregate function. close #20092.

[alexey-milovidov]

I have some doubts about the cases when result may change. Maybe add a separate option for it?

[ucasfl]

I have some doubts about the cases when result may change. Maybe add a separate option for it?

There is already a setting optimize_arithmetic_operations_in_aggregate_functions, you mean add another setting for avg?

[alexey-milovidov]

Yes, let's name it optimize_arithmetic_operations_in_aggregate_functions_avg.

[alexey-milovidov]

Why I'm worried?

avg(x / 2) does not overflow right now but may start to overflow when optimization is enabled.
It is still Ok to enable it by default but we need a separate setting.

[ucasfl]

Why I'm worried?

avg(x / 2) does not overflow right now but may start to overflow when optimization is enabled.
It is still Ok to enable it by default but we need a separate setting.

But I think sum(x / 2) also has the problem.

[alexey-milovidov]

Ok, reasonable. Let's merge as is.