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| 在 `INSERT` 或者 `UPDATE` 子表时，外键约束会检查父表中是否存在对应的外键值，并对父表中的该行数据上锁，避免该外键值被其他操作删除，导致破坏外键约束。 | ||
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| 因为 TiDB 目前暂不支持 `LOCK IN SHARE MODE`，所以，在并发写入子表场景，如果引用的外键值大部分都一样，可能会有比较严重的锁冲突。建议在大批量写入子表数据时，关闭 [`foreign_key_checks`](/system-variables.md#foreign_key_checks)。 | ||
| 默认情况下，外键检查对父表的上锁行为等同于对父表中外键值所在行做 `SELECT FOR UPDATE` 操作（即排他锁），在并发写入子表场景中，如果引用的外键值大部分都一样，可能会有比较严重的锁冲突。 |
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| 默认情况下，外键检查对父表的上锁行为等同于对父表中外键值所在行做 `SELECT FOR UPDATE` 操作（即排他锁），在并发写入子表场景中，如果引用的外键值大部分都一样，可能会有比较严重的锁冲突。 | |
| 默认情况下，在悲观事务中，外键检查对父表中行的加锁行为等价于对该行执行一次 `SELECT ... FOR UPDATE` 的锁定读（即加排他锁）。在子表高并发写入的场景下，如果大量事务反复引用相同的父表行，可能出现较严重的锁冲突。 |
| 因为 TiDB 目前暂不支持 `LOCK IN SHARE MODE`，所以，在并发写入子表场景，如果引用的外键值大部分都一样，可能会有比较严重的锁冲突。建议在大批量写入子表数据时，关闭 [`foreign_key_checks`](/system-variables.md#foreign_key_checks)。 | ||
| 默认情况下，外键检查对父表的上锁行为等同于对父表中外键值所在行做 `SELECT FOR UPDATE` 操作（即排他锁），在并发写入子表场景中，如果引用的外键值大部分都一样，可能会有比较严重的锁冲突。 | ||
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| 你可以通过开启系统变量 [`tidb_foreign_key_check_in_shared_lock`](/system-variables.md#tidb_foreign_key_check_in_shared_lock-从-v856-和-v900-版本开始引入) 来让外键检查使用共享锁。共享锁允许多个事务同时持有同一行的锁，从而减少锁冲突，提升并发写入子表的性能。 |
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| 你可以通过开启系统变量 [`tidb_foreign_key_check_in_shared_lock`](/system-variables.md#tidb_foreign_key_check_in_shared_lock-从-v856-和-v900-版本开始引入) 来让外键检查使用共享锁。共享锁允许多个事务同时持有同一行的锁，从而减少锁冲突，提升并发写入子表的性能。 | |
| 你可以通过开启系统变量 [`tidb_foreign_key_check_in_shared_lock`](/system-variables.md#tidb_foreign_key_check_in_shared_lock-从-v856-和-v900-版本开始引入) 来让外键检查使用共享锁。共享锁允许多个事务在同一父表行同时完成外键检查，从而减少锁冲突，提升子表并发写入性能。 |
| - 是否受 Hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value) 控制：否 | ||
| - 类型：布尔型 | ||
| - 默认值：`OFF` | ||
| - 这个变量用于控制在悲观事务中，外键约束检查是否使用共享锁（而非排他锁）。开启后，外键检查对父表行加共享锁，允许多个并发事务同时检查同一父表行而不互相阻塞，从而提升并发写入子表的性能。 |
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| - 这个变量用于控制在悲观事务中，外键约束检查是否使用共享锁（而非排他锁）。开启后，外键检查对父表行加共享锁，允许多个并发事务同时检查同一父表行而不互相阻塞，从而提升并发写入子表的性能。 | |
| - 该变量用于控制在悲观事务中，外键约束检查对父表中的行加锁时是否使用共享锁（而非排他锁）。开启后，多个并发事务可以同时对同一父表行执行外键检查而不互相阻塞，从而降低锁冲突并提升子表并发写入性能。 |
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ref tikv/tikv#19087
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