ddl: never write untouched index values to temp index (#41879)

[ti-chi-bot]

This is an automated cherry-pick of #41879

What problem does this PR solve?

Issue Number: close #41880

Problem Summary:

Previously, for untouched temp index key, TiDB writes it to the memory buffer and never commits it to TiKV(by checking if the last byte is the uncommited flag).

#40749 introduced a new temp index value encoding: for each insertion, TiDB appends the new temp index value to the end based on the old value. As a result, the "uncommitted flag" is not the last byte anymore and the key-value will be incorrectly committed to TiKV. The encoding becomes invalid.

What is changed and how it works?

Check the untouched values before writting to the temp index. Because the untouched values is only used for speeding up the index reading, and the temp index is never visible to the users, we can skip writting a untouched value to the temp index.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	195a45a	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	195a45a	link	true	/test check-dev
idc-jenkins-ci-tidb/build	195a45a	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	195a45a	link	true	/test check-dev2

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