mysql-compatibility: clarify multi schema change DDL limits#23170
mysql-compatibility: clarify multi schema change DDL limits#23170joechenrh wants to merge 1 commit into
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This pull request updates mysql-compatibility.md to document two additional restrictions on multi-schema ALTER TABLE DDL operations in TiDB compared to MySQL: dependency validation order and modifying the same index multiple times in a single statement. The review feedback suggests minor phrasing adjustments to adhere to the style guide, specifically using the second person ("you"), avoiding passive voice, and correcting a minor grammatical article omission.
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| In TiDB, all supported DDL changes can be performed online. However, there are some major restrictions on DDL operations in TiDB compared to MySQL: | ||
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| * When using a single `ALTER TABLE` statement to alter multiple schema objects (such as columns or indexes) of a table, specifying the same object in multiple changes is not supported. For example, if you execute the `ALTER TABLE t1 MODIFY COLUMN c1 INT, DROP COLUMN c1` command, the `Unsupported operate same column/index` error is output. | ||
| * When using a single `ALTER TABLE` statement to alter multiple schema objects, TiDB validates the dependencies in the statement order. MySQL might allow some multi-schema-change statements based on the final table definition. For example, MySQL supports `ALTER TABLE t ADD INDEX idx_c(c), ADD COLUMN c INT`, but TiDB returns an error because column `c` does not exist when the `ADD INDEX` clause is processed. Rewrite it as `ALTER TABLE t ADD COLUMN c INT, ADD INDEX idx_c(c)`. |
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To adhere to the style guide, we should write in the second person ("you") and avoid passive voice ("is processed").
| * When using a single `ALTER TABLE` statement to alter multiple schema objects, TiDB validates the dependencies in the statement order. MySQL might allow some multi-schema-change statements based on the final table definition. For example, MySQL supports `ALTER TABLE t ADD INDEX idx_c(c), ADD COLUMN c INT`, but TiDB returns an error because column `c` does not exist when the `ADD INDEX` clause is processed. Rewrite it as `ALTER TABLE t ADD COLUMN c INT, ADD INDEX idx_c(c)`. | |
| * When you use a single `ALTER TABLE` statement to alter multiple schema objects, TiDB validates the dependencies in the statement order. MySQL might allow some multi-schema-change statements based on the final table definition. For example, MySQL supports `ALTER TABLE t ADD INDEX idx_c(c), ADD COLUMN c INT`, but TiDB returns an error because column `c` does not exist when TiDB processes the `ADD INDEX` clause. You can rewrite the statement as `ALTER TABLE t ADD COLUMN c INT, ADD INDEX idx_c(c)`. |
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| * When using a single `ALTER TABLE` statement to alter multiple schema objects (such as columns or indexes) of a table, specifying the same object in multiple changes is not supported. For example, if you execute the `ALTER TABLE t1 MODIFY COLUMN c1 INT, DROP COLUMN c1` command, the `Unsupported operate same column/index` error is output. | ||
| * When using a single `ALTER TABLE` statement to alter multiple schema objects, TiDB validates the dependencies in the statement order. MySQL might allow some multi-schema-change statements based on the final table definition. For example, MySQL supports `ALTER TABLE t ADD INDEX idx_c(c), ADD COLUMN c INT`, but TiDB returns an error because column `c` does not exist when the `ADD INDEX` clause is processed. Rewrite it as `ALTER TABLE t ADD COLUMN c INT, ADD INDEX idx_c(c)`. | ||
| * If table `t` already has an index named `idx`, MySQL can treat `ADD INDEX idx(...), DROP INDEX idx` in a single `ALTER TABLE` statement as replacing the existing index. TiDB does not support modifying the same index multiple times in a single `ALTER TABLE` statement. Rewrite it as separate statements, such as `ALTER TABLE t DROP INDEX idx` followed by `ALTER TABLE t ADD INDEX idx(...)`. |
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To adhere to the style guide, we should write in the second person ("you") and improve the grammar ("If table t" -> "If a table t").
| * If table `t` already has an index named `idx`, MySQL can treat `ADD INDEX idx(...), DROP INDEX idx` in a single `ALTER TABLE` statement as replacing the existing index. TiDB does not support modifying the same index multiple times in a single `ALTER TABLE` statement. Rewrite it as separate statements, such as `ALTER TABLE t DROP INDEX idx` followed by `ALTER TABLE t ADD INDEX idx(...)`. | |
| * If a table `t` already has an index named `idx`, MySQL can treat `ADD INDEX idx(...), DROP INDEX idx` in a single `ALTER TABLE` statement as replacing the existing index. TiDB does not support modifying the same index multiple times in a single `ALTER TABLE` statement. You can rewrite the statement as separate statements, such as `ALTER TABLE t DROP INDEX idx` followed by `ALTER TABLE t ADD INDEX idx(...)`. |
References
- Write in second person ("you") when addressing users. (link)
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What is changed, added or deleted? (Required)
Clarify TiDB DDL compatibility limits for multi-schema-change
ALTER TABLEstatements:ADD INDEX idx_c(c), ADD COLUMN c INTandADD INDEX idx(...), DROP INDEX idxwithidxalready existing.Which TiDB version(s) do your changes apply to? (Required)
What is the related PR or file link(s)?
Do your changes match any of the following descriptions?