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sql: update the step of the "Auto Increment ID" #842

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Aug 20, 2018
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sql: update the step of the "Auto Increment ID" #842

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sql: update the step of auto ID
zimulala Aug 16, 2018
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sql: address a comment
zimulala Aug 17, 2018
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4 changes: 2 additions & 2 deletions sql/mysql-compatibility.md
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Expand Up @@ -43,12 +43,12 @@ TiDB 的自增 ID (Auto Increment ID) 只保证自增且唯一，并不保证连
>
> TiDB 实现自增 ID 的原理是每个 tidb-server 实例缓存一段 ID 值用于分配，用完这段值再去取下一段。

> 假设集群中有两个 tidb-server 实例 A 和 B（A 缓存 [1,5000] 的自增 ID，B 缓存 [5001,10000] 的自增 ID），
> 假设集群中有两个 tidb-server 实例 A 和 B（A 缓存 [1,30000] 的自增 ID，B 缓存 [30001,60000] 的自增 ID），
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在上面一行中增加 batch size的描述吧：“每个 tidb-server 实例对每一个表缓存一段 ID 值用于分配（目前一段是 30000 个 ID）”

>
> 依次执行操作：
>
> 1. 客户端向 B 插入一条将 `id` 设置为 1 的语句 `insert into t values (1, 1)`，并执行成功。
> 2. 客户端向 A 发送 Insert 语句 `insert into t (c) (1)`，这条语句中没有指定 `id` 的值，所以会由 A 分配，当前 A 缓存了 [1, 5000] 这段 ID，所以会分配 1 为自增 ID 的值，并把本地计数器加 1。而此时数据库中已经存在 `id` 为 1 的数据，最终返回 `Duplicated Error` 错误。
> 2. 客户端向 A 发送 Insert 语句 `insert into t (c) (1)`，这条语句中没有指定 `id` 的值，所以会由 A 分配，当前 A 缓存了 [1, 30000] 这段 ID，所以会分配 1 为自增 ID 的值，并把本地计数器加 1。而此时数据库中已经存在 `id` 为 1 的数据，最终返回 `Duplicated Error` 错误。


### 内建函数
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