[FLINK-20345][table-planner-blink] Adds an Expand node only if there are multiple distinct aggregate functions in an Aggregate when executes SplitAggregateRule

[beyond1920]

What is the purpose of the change

An Expand node is only necessary when multiple aggregate function with different distinct keys appear in an Aggregate. However an unnecessary Expand node will be generated under some special cases, like the following sql.
SELECT COUNT(c) AS pv, COUNT(DISTINCT c) AS uv FROM T GROUP BY a.
This pull request aims to avoid generate unnecessary Expand node.

Brief change log

• SplitAggregateRule check only need expand if there are multiple distinct aggregate functions in an Aggregate

Verifying this change

This change is already covered by existing tests, such as SplitAggregateITCase, DistinctAggregateTest, SplitAggregateRuleTest . Besides, this change added IT and UT in SplitAggregateITCase and SplitAggregateRuleTest

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[flinkbot]

Thanks a lot for your contribution to the Apache Flink project. I'm the @flinkbot. I help the community
to review your pull request. We will use this comment to track the progress of the review.

Automated Checks

Last check on commit c8b66d4 (Fri May 28 08:15:45 UTC 2021)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

_{Mention the bot in a comment to re-run the automated checks.}

Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

Please see the Pull Request Review Guide for a full explanation of the review process.

Details

The Bot is tracking the review progress through labels. Labels are applied according to the order of the review items. For consensus, approval by a Flink committer of PMC member is required

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot approve description to approve one or more aspects (aspects: description, consensus, architecture and quality)
• @flinkbot approve all to approve all aspects
• @flinkbot approve-until architecture to approve everything until architecture
• @flinkbot attention @username1 [@username2 ..] to require somebody's attention
• @flinkbot disapprove architecture to remove an approval you gave earlier

[flinkbot]

CI report:

• c8b66d4 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run travis re-run the last Travis build
• @flinkbot run azure re-run the last Azure build

[JingsongLi]

Thanks @beyond1920 for the contribution, can you modify comments too?

[beyond1920]

@JingsongLi Done. please have a look, thanks.

[JingsongLi]

Thanks for the update, looks good to me.