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faq, sql: Document tidb_force_priority #645

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Oct 12, 2018
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faq, sql: Document tidb_force_priority #645

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Documented force-priority option.
morgo Oct 4, 2018
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2 changes: 1 addition & 1 deletion FAQ.md
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Expand Up @@ -531,7 +531,7 @@ When TiDB is executing a SQL statement, the query will be `EXPENSIVE_QUERY` if e

#### How to control or change the execution priority of SQL commits?

TiDB has the following high priority and low priority syntax:
TiDB supports changing the priority on a [per-session](sql/tidb-specific.md#tidb_force_priority), [global](sql/server-command-option.md#force-priority) or [individual statement basis](sql/dml.md). Priority has the following meaning:
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Please delete the extra space before "Priority".


- HIGH_PRIORITY: this statement has a high priority, that is, TiDB gives priority to this statement and executes it first.

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8 changes: 8 additions & 0 deletions sql/server-command-option.md
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Expand Up @@ -157,6 +157,14 @@ Same as the "run-ddl" startup option
- Default: true
- When you execute `join` on tables without any conditions on both sides, the statement can be run by default. But if you set the value to `false`, the server does not run such `join` statement.

### force-priority

- The default priority for statements
- Default: `NO_PRIORITY`
- TiDB supports the priorities `LOW_PRIORITY`, `NO_PRIORITY`, `HIGH_PRIORITY` and `DELAYED` for statements. One use case for changing the priority, is you may choose to dedicated a pool of servers for OLAP queries and set the value to `LOW_PRIORITY` to ensure that TiKV servers will provide priority to OLTP workloads which are routed to a different pool of TiDB servers. This helps ensure more uniform OLTP performance at the risk of slightly slower OLAP performance.
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TiDB supports the priorities LOW_PRIORITY, NO_PRIORITY, HIGH_PRIORITY and DELAYED for statements. One use case for changing the priority, is you may choose to dedicate a pool of servers for OLAP queries and set the value to LOW_PRIORITY to ensure that TiKV servers will provide priority to OLTP workloads which are routed to a different pool of TiDB servers. This helps ensure more uniform OLTP performance at the risk of slightly slower OLAP performance.

(I delete a space before "One" and "This".)

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@jackysp jackysp Oct 8, 2018 •
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It's better to use a union priority order, how about

NO_PRIORITY | LOW_PRIORITY | DELAYED | HIGH_PRIORITY

or

LOW_PRIORITY | DELAYED | HIGH_PRIORITY | NO_PRIORITY


Note that TIDB will automatically set table scans to `LOW_PRIORITY` and overwriting priority on a per-statement basis is possible by using the `HIGH PRIORITY` or `LOW PRIORITY` [dml modifier](dml.md).
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- TiDB will automatically set table scans to LOW_PRIORITY and overwriting priority on a per-statement basis is possible by using the HIGH PRIORITY or LOW PRIORITY DML modifier.


### join-concurrency

- The goroutine number when the `join-concurrency` runs `join`
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7 changes: 7 additions & 0 deletions sql/tidb-specific.md
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Expand Up @@ -282,6 +282,13 @@ set @@global.tidb_distsql_scan_concurrency = 10
- 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`.

### tidb_force_priority

- Scope: SESSION | GLOBAL
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It is a session-only variable though it works on the server level. GLOBAL variable will work on all TiDB servers.

- Default value: `NO_PRIORITY`
- This variable is used to change the default priority for statements executed on a TiDB server. A use case is to ensure that a particular user (or TiDB server) that is performing OLAP queries receives lower priority than users performing OLTP queries.
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  • This variable is used to change the default priority for statements executed on a TiDB server. A use case is to ensure that a particular user (or TiDB server) that is performing OLAP queries receives lower priority than users performing OLTP queries.

- You can set the value of this variable to `PRIORITY_LOW`, `NO_PRIORITY` or `PRIORITY_HIGH`.
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@jackysp jackysp Oct 8, 2018 •
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It's better to use

`NO_PRIORITY`, `LOW_PRIORITY`, `DELAYED` or `HIGH_PRIORITY`

tidb_force_priority uses SQL priority options as its available values which is not the same as tidb_ddl_reorg_priority does.


## 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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