master: document tidb_foreign_key_check_in_shared_lock#22453
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Summary of ChangesHello @you06, 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 updates the TiDB documentation to introduce and explain the Highlights
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Code Review
This pull request adds documentation for the new system variable tidb_foreign_key_check_in_shared_lock. The changes in system-variables.md are clear and accurate. I've provided a couple of suggestions for foreign-key.md to improve clarity, style, and technical accuracy, ensuring consistency between the two documents.
foreign-key.md
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| Because TiDB currently does not support `LOCK IN SHARE MODE`, if a child table accepts a large number of concurrent writes and most of the referenced foreign key values are the same, there might be serious locking conflicts. It is recommended to disable [`foreign_key_checks`](/system-variables.md#foreign_key_checks) when writing a large number of child table data. | ||
| By default, the locking behavior of foreign key checks is equivalent to performing a `SELECT FOR UPDATE` operation (exclusive lock) on the row where the foreign key value is located in the parent table. In concurrent write scenarios for a child table, if most of the referenced foreign key values are the same, there might be serious locking conflicts. | ||
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| You can enable the system variable [`tidb_foreign_key_check_in_shared_lock`](/system-variables.md#tidb_foreign_key_check_in_shared_lock) to let foreign key checks use shared locks. Shared locks allow multiple transactions to hold locks on the same row simultaneously, reducing lock conflicts and improving the performance of concurrent writes to child tables. |
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For technical accuracy, it's important to mention that this variable applies to pessimistic transactions, as stated in system-variables.md. I've also slightly rephrased the sentence for better flow.
| You can enable the system variable [`tidb_foreign_key_check_in_shared_lock`](/system-variables.md#tidb_foreign_key_check_in_shared_lock) to let foreign key checks use shared locks. Shared locks allow multiple transactions to hold locks on the same row simultaneously, reducing lock conflicts and improving the performance of concurrent writes to child tables. | |
| You can enable the system variable [`tidb_foreign_key_check_in_shared_lock`](/system-variables.md#tidb_foreign_key_check_in_shared_lock) to make foreign key checks use shared locks in pessimistic transactions. Shared locks allow multiple transactions to hold locks on the same row simultaneously, reducing lock conflicts and improving the performance of concurrent writes to child tables. |
References
- The style guide (line 16) requires technical accuracy. (link)
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