Fix cast datetime to decimal wrong result bug (#4152)

[ti-chi-bot]

This is an automated cherry-pick of #4152

Signed-off-by: guo-shaoge shaoge1994@163.com

What problem does this PR solve?

Issue Number: close #4151

Problem Summary:

1. wrong const multipiler
2. overflow may hanppens

What is changed and how it works?

1. correct const multipiler
2. use Int128 instead of UInt64 if fsp > 0

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

Fix cast datetime to decimal wrong result bug

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• SeaRise
• mengxin9014

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[guo-shaoge]

/run-all-tests

[guo-shaoge]

/run-all-tests

[guo-shaoge]

/merge

[ti-chi-bot]

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/run-all-tests

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[ti-chi-bot]

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

Commit hash: ed0893b