ddl: extract data reorg code to separate functions

[tangenta]

What problem does this PR solve?

Issue Number: ref #14766

Problem Summary:

What is changed and how it works?

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:

• Defined2014
• hawkingrei

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[Defined2014]

/run-unit-test

[tangenta]

/merge

[ti-chi-bot]

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

Commit hash: 029e1be

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/c7c2ec656a588ba9b5f11c6e0013b44ce8116890

[hawkingrei]

/run-mysql-test

[ti-chi-bot]

@tangenta: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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