*: add doc for tikv_client_read_timeout system variable
#14964
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Signed-off-by: crazycs520 <[email protected]>
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optimizer-hints.md
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| @@ -742,6 +742,16 @@ SELECT /*+ MAX_EXECUTION_TIME(1000) */ * FROM t1 inner join t2 WHERE t1.id = t2. | |||
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| 除了 Hint 之外，系统变量 `global.max_execution_time` 也能对语句执行时间进行限制。 | |||
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| ### TIDB_KV_READ_TIMEOUT(N) | |||
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| `TIDB_KV_READ_TIMEOUT(N)` 用于设置查询语句中 TiDB 发送 KV RPC 读请求的超时时间。通常不建议用户设置该变量的值。当 TiDB 集群在网络不稳定，或者 I/O 延迟抖动严重的环境下，且用户对查询 SQL 的延迟比较敏感时，可以通过设置 `TIDB_KV_READ_TIMEOUT(N)` 调小 KV RPC 读请求的超时时间，这样当某个 TiKV 出现 I/O 延迟抖动时，TiDB 侧可以快速超时并重新发送 KV RPC 请求给下一个 KV Region Peer 所在的 TiKV。如果所有 KV Region Peer 都请求超时，则会用默认的超时时间进行重试。 | |||
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不建议用户使用
我们需要有最佳实践和适配场景指引，一般副作用较大只解决极个别场景的用法才作为 “不推荐、不建议使用”
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Co-authored-by: xixirangrang <[email protected]>
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system-variables.md
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| > - 一般情况下一个普通查询的常规耗时是几毫秒，但偶尔可能会出现某个 TiKV 的网络不稳定、或者某个 TiKV 的 I/O 抖动，导致查询耗时超过 1 秒甚至 10 秒。此时你可以尝试通过 Optimizer Hint `/*+ SET_VAR(TIKV_CLIENT_READ_TIMEOUT=100) */` 为该查询语句设置 TiKV RPC 读请求超时时间为 100 毫秒，这样即使遇到某个 TiKV 查询慢，也可以快速超时然后重新发送 RPC 请求给下一个 TiKV Region Peer 所在的 TiKV。由于两个 TiKV 同时出现 I/O 抖动的概率较低，所以该查询语句的耗时通常可以预期在几毫秒到 110 毫秒之间。 | ||
| > - 不建议将 `tikv_client_read_timeout` 的值设置的太小，否则 TiDB 集群在负载压力较大时会很容易导致请求超时，然后重试会进一步增加 TiDB 集群的压力。 | ||
| > - 建议使用 Optimizer Hint 为不同类型的查询语句设置不同的超时时间。 |
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后续如果 retry doc 有对应 public doc，这里也可以补充一个 link
/cc @zyguan @crazycs520
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Co-authored-by: Grace Cai <[email protected]>
Co-authored-by: Grace Cai <[email protected]>
Co-authored-by: Grace Cai <[email protected]>
Co-authored-by: xixirangrang <[email protected]>
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tikv_client_read_timeout system variable
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[APPROVALNOTIFIER] This PR is APPROVED This pull-request has been approved by: easonn7, hfxsd The full list of commands accepted by this bot can be found here. The pull request process is described here
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add doc for tidb_kv_read_timeout system variable and optimizer hint
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