fix decimal cast bug

[windtalker]

What problem does this PR solve?

Issue Number: close #6994

Problem Summary:

What is changed and how it works?

Fix bug in round when cast decimal as decimal

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• guo-shaoge
• yibin87

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[yibin87]

LGTM

[windtalker]

/merge

[ti-chi-bot]

@windtalker: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 6a9fed1

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.0: #7027.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #7028.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.2: #7029.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.3: #7030.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #7031.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #7032.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.4: #7033.