[Rust] [DataFusion] Introduce support for DISTINCT by partially implementing COUNT(DISTINCT)

[asfimport]

I am unsure where support for DISTINCT may be on the DataFusion roadmap, so I've filed this with the "Wish" type and "Minor" priority to reflect that this is a proposal:

Introduce DISTINCT into DataFusion by partially implementing COUNT(DISTINCT). The ultimate goal is to fully support the DISTINCT keyword, but to get implementation started, limit the scope of this work to:

• the COUNT() aggregate function
• a single expression in COUNT(), i.e., COUNT(DISTINCT c1), but not COUNT(DISTINCT c1, c2)
• only queries with a GROUP BY clause
• integer types

Reporter: Daniel Russo / @drusso
Assignee: Daniel Russo / @drusso

Related issues:

• [Rust] [DataFusion] Add benchmarks for COUNT(DISTINCT) (relates to)

PRs and other links:

• GitHub Pull Request #8222

_{Note: This issue was originally created as ARROW-10043. Please see the migration documentation for further details.}

[asfimport]

Jorge Leitão / @jorgecarleitao:
I assigned @drusso , as he is spearheading this on the PR above :)

[asfimport]

Andy Grove / @andygrove:
Issue resolved by pull request 8222
#8222