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sql: update the step of the "Auto Increment ID" #577

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Aug 24, 2018
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sql: update the step of the "Auto Increment ID" #577

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6 changes: 3 additions & 3 deletions sql/mysql-compatibility.md
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Expand Up @@ -41,14 +41,14 @@ The auto-increment ID feature in TiDB is only guaranteed to be automatically inc
> create table t(id int unique key auto_increment, c int);
> ```
>
> The principle of the auto-increment ID in TiDB is that each tidb-server instance caches a section of ID values for allocation and fetches the next section after this section is used up.
> The principle of the auto-increment ID in TiDB is that each tidb-server instance caches a section of ID values (currently 30000 IDs are cached) for allocation and fetches the next section after this section is used up.
>
> Assume that the cluster contains two tidb-server instances, namely Instance A and Instance B. Instance A caches the auto-increment ID of [1, 5000], while Instance B caches the auto-increment ID of [5001, 10000].
> Assume that the cluster contains two tidb-server instances, namely Instance A and Instance B. Instance A caches the auto-increment ID of [1, 30000], while Instance B caches the auto-increment ID of [30001, 60000].
>
> The operations are executed as follows:
>
> 1. The client issues the `insert into t values (1, 1)` statement to Instance B which sets the `id` to 1 and the statement is executed successfully.
> 2. The client issues the `insert into t (c) (1)` statement to Instance A. This statement does not specify the value of `id`, so Instance A allocates the value. Currently, Instances A caches the auto-increment ID of [1, 5000], so it allocates the `id` value to 1 and adds 1 to the local counter. However, at this time the data with the `id` of 1 already exists in the cluster, therefore it reports `Duplicated Error`.
> 2. The client issues the `insert into t (c) (1)` statement to Instance A. This statement does not specify the value of `id`, so Instance A allocates the value. Currently, Instances A caches the auto-increment ID of [1, 30000], so it allocates the `id` value to 1 and adds 1 to the local counter. However, at this time the data with the `id` of 1 already exists in the cluster, therefore it reports `Duplicated Error`.

### Built-in functions

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