sql-statements: document ALTER TABLE COMPRESSION='NONE' behavior

[takaidohigasi]

What is changed, added or deleted? (Required)

Document the ALTER TABLE ... COMPRESSION behavior introduced by pingcap/tidb#62305:

• ALTER TABLE ... COMPRESSION only accepts 'NONE' (case-insensitive) and it is a no-op (metadata-only; TiDB does not compress or uncompress any table data). This allows reverting the COMPRESSION attribute for tables migrated from MySQL.
• Other values such as 'ZLIB' and 'LZ4' return the error This type of ALTER TABLE is currently unsupported (error code 8200).
• If an unsupported COMPRESSION value is specified together with other options in one statement, none of the options take effect.

Added one item to the MySQL compatibility section of sql-statements/sql-statement-alter-table.md.

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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): ddl: enable ALTER TABLE COMPRESSION=NONE (no ops) tidb#62305

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

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Summary by CodeRabbit

• Documentation
  • Updated ALTER TABLE MySQL compatibility guidance for the COMPRESSION option.
  • Clarified that only 'NONE' is accepted and treated as a no-op; other values return an unsupported-operation error.
  • Documented that unsupported compression values prevent other options in the same statement from taking effect.

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[gemini-code-assist]

Code Review

This pull request updates the ALTER TABLE documentation to clarify that the COMPRESSION option only accepts NONE as a no-op for compatibility, while other values like ZLIB or LZ4 will return an error. The review feedback suggests formatting SQL values with backticks, using active voice, and addressing the user in the second person to align with the repository style guide.

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[qiancai]

@D3Hunter Please help with the tech review of this PR. Thanks.

[qiancai]

The parser grammar requires COMPRESSION to be followed by stringLit, so bare NONE, ZLIB, and LZ4 do not reflect executable syntax. Keeping the quotes inside the code span also makes the MySQL-migration example immediately usable.

Suggested change

- `ALTER TABLE ... COMPRESSION` only accepts `NONE` (case-insensitive) and is a no-op. TiDB accepts this statement only for compatibility (for example, for tables migrated from MySQL that carry a `COMPRESSION` attribute) and does not compress or uncompress any table data. Specifying other values, such as `ZLIB` or `LZ4`, returns the error `This type of ALTER TABLE is currently unsupported` (error code 8200). If you specify an unsupported `COMPRESSION` value together with other options in a single statement, none of the options take effect.

- `ALTER TABLE ... COMPRESSION` accepts only the string literal `'NONE'` (case-insensitive) and is a no-op. TiDB accepts this statement only for compatibility, for example, for tables migrated from MySQL that carry a `COMPRESSION` attribute. It does not compress or uncompress table data. Other values, such as `'ZLIB'` or `'LZ4'`, return error code 8200 (`This type of ALTER TABLE is currently unsupported`). If you specify an unsupported `COMPRESSION` value together with other options in a single statement, none of the options take effect.

[takaidohigasi]

Applied your suggestion in 375da90 — thanks for catching that the grammar requires a string literal after COMPRESSION.

[ti-chi-bot]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 9b6f832 and 375da90.

📒 Files selected for processing (1)

• sql-statements/sql-statement-alter-table.md

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📝 Walkthrough

Walkthrough

The ALTER TABLE MySQL compatibility restrictions now document COMPRESSION behavior: 'NONE' is accepted as a no-op, unsupported values return error 8200, and combined statements apply none of their other options.

Changes

ALTER TABLE compatibility

Layer / File(s)	Summary
Document COMPRESSION restrictions sql-statements/sql-statement-alter-table.md	Documents accepted 'NONE', unsupported compression values, error 8200, and the behavior of combined ALTER TABLE options.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: d3hunter

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the documentation update about ALTER TABLE COMPRESSION='NONE'.
Description check	✅ Passed	The description includes the required change summary, affected version, reference link, and AI involvement section.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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