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sql: add tidb_retry_limit #501

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Jun 14, 2018
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sql: add tidb_retry_limit #501

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sql: add tidb_retry_limit
lilin90 Jun 14, 2018
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sql: address the comment
lilin90 Jun 14, 2018
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22 changes: 17 additions & 5 deletions sql/tidb-specific.md
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Expand Up @@ -8,14 +8,19 @@ category: user guide
On the basis of MySQL variables and syntaxes, TiDB has defined some specific system variables and syntaxes to optimize performance.

## System variable

Variables can be set with the `SET` statement, for example:

```set @@tidb_distsql_scan_concurrency = 10 ```
```
set @@tidb_distsql_scan_concurrency = 10
```

If you need to set the global variable, run:

```set @@global.tidb_distsql_scan_concurrency = 10 ```

```
set @@global.tidb_distsql_scan_concurrency = 10
```

### tidb_snapshot

- Scope: SESSION
Expand Down Expand Up @@ -218,8 +223,15 @@ If you need to set the global variable, run:
- Scope: SERVER
- Default value: 0
- This variable is used to set whether to enable Streaming.

## Optimizer hint

## tidb_retry_limit

- Scope: SESSION | GLOBAL
- Default value: 10
- When a transaction encounters retriable errors, such as transaction conflicts and TiKV busy, this transaction can be re-executed. This variable is used to set the maximum number of the retries.

## Optimizer Hint

On the basis of MySQL’s `Optimizer Hint` Syntax, TiDB adds some proprietary `Hint` syntaxes. When using the `Hint` syntax, the TiDB optimizer will try to use the specific algorithm, which performs better than the default algorithm in some scenarios.

The `Hint` syntax is included in comments like `/*+ xxx */`, and in MySQL client versions earlier than 5.7.7, the comment is removed by default. If you want to use the `Hint` syntax in these earlier versions, add the `--comments` option when starting the client. For example: `mysql -h 127.0.0.1 -P 4000 -uroot --comments`.
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