v8.5.5: refine modify column (#22268)

[ti-chi-bot]

This is an automated cherry-pick of #22268

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• v8.1 (TiDB 8.1 versions)
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• This PR is translated from: refine modify column docs-cn#21110
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• Delete files
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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the documentation for the ALTER TABLE ... MODIFY COLUMN statement in TiDB. The updates provide a clearer understanding of how column type changes are handled, particularly regarding Reorg-Data operations. It also introduces significant new information about recent performance optimizations in newer TiDB versions that can reduce the overhead of certain column modifications by avoiding full table rebuilds under specific conditions.

Highlights

• Enhanced MODIFY COLUMN Documentation: The documentation for ALTER TABLE ... MODIFY COLUMN has been significantly expanded to provide a more detailed explanation of Reorg-Data behavior during column type changes.
• Performance Optimizations for MODIFY COLUMN: New information has been added regarding performance optimizations introduced in TiDB v8.5.5 and v9.0.0. These optimizations allow certain column modifications to rebuild only affected indexes instead of the entire table, under specific conditions, thereby improving execution efficiency.
• Conditions for Optimizations: The documentation now clarifies the conditions required for these MODIFY COLUMN optimizations, including the use of a strict SQL mode, the absence of TiFlash replicas, and no risk of data truncation.
• Specific Conversion Notes: A new note has been added to address the behavior of VARCHAR to CHAR conversions, specifically highlighting the impact of trailing spaces on Reorg-Data operations.
• Wording and Clarity Improvements: Minor textual adjustments have been made for consistency, such as changing section headings from 'Meta-Only Change' to 'Meta-only change', and clarifying the DDL statement wait time when Metadata lock is disabled.

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[gemini-code-assist]

Code Review

This pull request refines the documentation for MODIFY COLUMN, providing more details about Reorg-Data operations and related optimizations. The changes are well-written and improve the clarity of the document. I've provided a few suggestions to further enhance readability and consistency, mainly around the usage of the term 'Reorg-Data'.

[ti-chi-bot]

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