sys-var: change tidb_scatter_region usage definition
#18626
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@@ -4149,12 +4149,16 @@ EXPLAIN FORMAT='brief' SELECT COUNT(1) FROM t WHERE a = 1 AND b IS NOT NULL; | |
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| ### `tidb_scatter_region` | ||
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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：否 | ||
| - 类型：枚举型 | ||
| - 默认值：`` | ||
| - 可选值：``，`TABLE`，`GLOBAL` | ||
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There was a problem hiding this comment. @hfxsd 后续需要关注下 PR 合并后官方文档的效果，第一个空的可选值导致 GitHub 页面解析有问题： |
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| - TiDB 默认会在建表时为新表分裂 Region，使用该变量控制表分裂打散策略，TiDB 会根据选择的打散策略进行 Region 打散。适用于批量建表后紧接着批量写入数据，能让刚分裂出的 Region 先在 TiKV 分散而不用等待 PD 进行调度。为了保证后续批量写入数据的稳定性，建表语句会等待打散 Region 完成后再返回建表成功，建表语句执行时间会是该变量关闭时的数倍。可选值包括： | ||
| - 默认值为 ``，表示对于表 Region 不进行打散。 | ||
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| - `TABLE` 表示 TiDB 会按照表数据分布情况对于表数据进行打散。 | ||
| - `GLOBAL` 表示 TiDB 会按照集群整体的数据分布情况对于数据进行打散。在建表语句执行时，同步打散刚分裂出的 Region。 | ||
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| - 如果建表时设置了 `SHARD_ROW_ID_BITS` 和 `PRE_SPLIT_REGIONS`，建表成功后会均匀切分出指定数量的 Region。 | ||
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| ### `tidb_schema_version_cache_limit` <span class="version-mark">从 v7.4.0 版本开始引入</span> | ||
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should we keep the original global level?
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Yes，Current implement should add session level sys var change and also keep global level implementation.