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Document tidb_query_cop_store_limit variable #21909

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Document tidb_query_cop_store_limit variable
solotzg Sep 1, 2026
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Update system-variables.md
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solotzg Sep 1, 2026
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13 changes: 13 additions & 0 deletions system-variables.md
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Expand Up @@ -5048,6 +5048,19 @@ EXPLAIN FORMAT='brief' SELECT COUNT(1) FROM t WHERE a = 1 AND b IS NOT NULL;
- 这个变量用来设置 `Projection` 算子的并发度。
- 默认值 `-1` 表示使用 `tidb_executor_concurrency` 的值。

### `tidb_query_cop_store_limit` <span class="version-mark">从 v9.0.0 版本开始引入</span>
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- 作用域：SESSION | GLOBAL
- 是否持久化到集群：是
- 是否受 Hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value) 控制：是
- 类型：整数型
- 默认值：`15`
- 范围：`[0, 256]`
- 这个变量用于限制单条查询语句发送到每个 TiKV Store 的并发 Coprocessor 请求尝试数。该限制对同一语句创建的多个 DistSQL 请求生效，并且每次重试都会重新按照实际目标 TiKV Store 进行限制。
- 当该变量设置为 `0` 时，不启用查询级别的 TiKV Store 并发限制；当设置为大于 `0` 的值时，单条查询语句对每个 TiKV Store 最多允许相应数量的 Coprocessor 请求尝试同时执行。
- 该变量不会覆盖 [`tidb_distsql_scan_concurrency`](#tidb_distsql_scan_concurrency) 的设置。`tidb_distsql_scan_concurrency` 控制单个 DistSQL 请求的扫描 Worker 并发度，而本变量控制单条查询语句对每个 TiKV Store 的 Coprocessor 请求尝试并发度。单条查询语句包含多个 DistSQL 请求时，本变量仍对这些请求共享生效。
- 将该变量设置为较小的值可能降低 TiKV Store 的瞬时压力，但也可能增加查询延迟。

### `tidb_query_log_max_len`

- 作用域：GLOBAL
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