Document descending-order index support (pingcap/tidb#2519)

[takaidohigasi]

What is changed, added or deleted? (Required)

Add user-facing documentation for the descending-order index feature
introduced in pingcap/tidb#68049 (and the corresponding TiKV change in
tikv/tikv#19558).

Three files updated:

1. system-variables.md — new tidb_enable_descending_index entry
   documenting scope, default OFF, and the TiKV-version prerequisite.
2. sql-statements/sql-statement-create-index.md — replace the
   existing "descending indexes are not supported (similar to MySQL 5.7)"
   note with the new conditional-support description, including the
   mixed-direction composite-index example (INDEX(a ASC, b DESC)
   satisfying ORDER BY a ASC, b DESC without a Sort).
3. mysql-compatibility.md — qualify the existing "Descending Index"
   entry so it is no longer listed as wholly unsupported.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)

The feature is gated behind tidb_enable_descending_index (default
OFF), so the visible behaviour change is opt-in. Backporting to older
release branches is not appropriate because the feature requires a
matching TiKV release that contains tikv/tikv#19558.

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

• TiDB PR: *: support descending-order indexes (#2519) tidb#68049
• TiKV PR: codec: add descending-order index key support (pingcap/tidb#2519) tikv/tikv#19558
• Issue: Support ascending or descending index  tidb#2519

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

🤖 Generated with Claude Code

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[ti-chi-bot]

Hi @takaidohigasi. Thanks for your PR.

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[gemini-code-assist]

Code Review

This pull request introduces documentation for descending index support, a new opt-in feature in TiDB v9.0.0 controlled by the tidb_enable_descending_index system variable. The changes include updates to the MySQL compatibility list, the CREATE INDEX statement documentation, and the addition of the new system variable's details. Review feedback focused on improving conciseness, ensuring consistent terminology, and adhering to style guide principles such as using the active voice and addressing the user in the second person.

[gemini-code-assist]

To maintain consistency with standard MySQL terminology and improve readability, consider using "Descending indexes" and splitting the long final sentence. This also clarifies the limitations for expression and FULLTEXT indexes.

Suggested change

* Descending-order indexes are supported when the [`tidb_enable_descending_index`](/system-variables.md#tidb_enable_descending_index-new-in-v900) system variable is set to `ON` (default `OFF`). When the variable is `OFF`, the `DESC` keyword on individual index columns is parsed but ignored, matching MySQL 5.7 behavior. When `ON`, columns marked `DESC` are stored in descending order, allowing composite indexes such as `INDEX(a ASC, b DESC)` to satisfy `ORDER BY a ASC, b DESC` with a single forward index scan and no additional `Sort` operator. This brings TiDB closer to MySQL 8.0 compatibility for descending indexes; expression and `FULLTEXT` indexes do not support `DESC`.

* Descending indexes are supported when the [`tidb_enable_descending_index`](/system-variables.md#tidb_enable_descending_index-new-in-v900) system variable is set to `ON` (default `OFF`). When this variable is `OFF`, the `DESC` keyword on individual index columns is parsed but ignored, matching MySQL 5.7 behavior. When `ON`, columns marked `DESC` are stored in descending order. This allows composite indexes such as `INDEX(a ASC, b DESC)` to satisfy `ORDER BY a ASC, b DESC` with a single forward index scan and no additional `Sort` operator. This brings TiDB closer to MySQL 8.0 compatibility for descending indexes. Expression and `FULLTEXT` indexes do not support `DESC`.

References

1. Use consistent terminology. ^(link)
2. Avoid unnecessary words and repetition. ^(link)

[takaidohigasi]

Already applied verbatim — see commit d9a65a687. "Descending indexes" terminology and the split sentences (FULLTEXT exclusion on its own line) are in.

[qiancai]

hi @takaidohigasi, thanks for the contribution to the TiDB documentation. I noticed that the corresponding code PRs are still open, so I will hold this doc PR for now.

[qiancai]

/hold