support cross-shard avg

[TDavid2509]

Hi Vitess developers,

I have question about SQL plan/engine. Do u have any plan about supportting cross-shards avg function? From my perspective, this is similar to the distinct_count or distinct_sum.

Feature Description

If there is a cross shards SQL:

SELECT user.name, avg(user.age) FROM user GROUP BY user.name

we can modify it into Route as:

SELECT user.name, sum(user.age), count(user.age) FROM user GROUP BY user.name ORDER BY user.name

And the OrderedAggregate can compute avg value by utilizing the ordered result from Route, which is similar to the distinct_count.

Use Case(s)

SELECT user.name, avg(user.age) FROM user GROUP BY user.name

My solution is just a discussion. I know there are some other challenges I haven't considered. If there are some incorrect points, pls figure them out.

thx