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sql: add new tidb specific system variables #589

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Sep 3, 2018
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sql: add new tidb specific system variables #589

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sql: add new tidb specific system variables
Aug 28, 2018
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sql: address the comment
Aug 29, 2018
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45 changes: 45 additions & 0 deletions sql/tidb-specific.md
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Expand Up @@ -116,6 +116,26 @@ set @@global.tidb_distsql_scan_concurrency = 10
- This variable is used to set the concurrency of the `serial scan` operation.
- Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.

### tidb_projection_concurrency

- Scope: SESSION | GLOBAL
- Default value: 4
- This variable is used to set the concurrency of the `Projection` operator.

### tidb_hashagg_partial_concurrency

- Scope: SESSION | GLOBAL
- Default value: 4
- This variable is used to set the concurrency of executing the concurrent `hash aggregation` algorithm in the `partial` phase.
- When the parameter of the aggregate function is not distinct, `HashAgg` is run concurrently and respectively in two phases - the `partial` phase and the `final` phase.

### tidb_hashagg_final_concurrency

- Scope: SESSION | GLOBAL
- Default value: 4
- This variable is used to set the concurrency of executing the concurrent `hash aggregation` algorithm in the `final` phase.
- When the parameter of the aggregate function is not distinct, `HashAgg` is run concurrently and respectively in two phases - the `partial` phase and the `final` phase.

### tidb_index_join_batch_size

- Scope: SESSION | GLOBAL
Expand Down Expand Up @@ -237,6 +257,31 @@ set @@global.tidb_distsql_scan_concurrency = 10
- Default: 0
- This variable is used to set whether to disable automatic retry of explicit transactions. If you set this variable to 1, the transaction does not retry automatically. If there is a conflict, the transaction needs to be retried at the application layer. To decide whether you need to disable automatic retry, see [description of optimistic transactions](transaction-isolation.md#description-of-optimistic-transactions).

## tidb_enable_table_partition

- Scope: SESSION
- Default value: 0
- This variable is used to set whether to enable the `TABLE PARTITION` feature.

## tidb_backoff_lock_fast

- Scope: SESSION | GLOBAL
- Default value: 100
- This variable is used to set the `backoff` time when the read request meets a lock.

## tidb_ddl_reorg_worker_cnt

- Scope: SESSION | GLOBAL
- Default value: 16
- This variable is used to set the concurrency of the DDL operation in the `re-organize` phase.

## tidb_ddl_reorg_priority

- Scope: SESSION | GLOBAL
- Default value: `PRIORITY_NORMAL`
- This variable is used to set the priority of executing the `ADD INDEX` operation in the `re-organize` phase.
- You can set the value of this variable to `PRIORITY_LOW`, `PRIORITY_NORMAL` or `PRIORITY_HIGH`.

## 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.
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