ALTER PLACEMENT POLICY

ALTER PLACEMENT POLICY 用于修改之前已创建的现有放置策略。所有使用该放置策略的表和分区都会自动更新。

ALTER PLACEMENT POLICY 会用新的定义 替换 之前的策略，而不是将新旧策略 合并。在以下示例中，执行 ALTER PLACEMENT POLICY 后，FOLLOWERS=4 会被丢弃：

CREATE PLACEMENT POLICY p1 FOLLOWERS=4;
ALTER PLACEMENT POLICY p1 PRIMARY_REGION="us-east-1" REGIONS="us-east-1,us-west-1";

语法

AlterPolicyStmt ::=
    "ALTER" "PLACEMENT" "POLICY" IfExists PolicyName PlacementOptionList

PolicyName ::=
    Identifier

PlacementOptionList ::=
    PlacementOption
|   PlacementOptionList PlacementOption
|   PlacementOptionList ',' PlacementOption

PlacementOption ::=
    CommonPlacementOption
|   SugarPlacementOption
|   AdvancedPlacementOption

CommonPlacementOption ::=
    "FOLLOWERS" EqOpt LengthNum

SugarPlacementOption ::=
    "PRIMARY_REGION" EqOpt stringLit
|   "REGIONS" EqOpt stringLit
|   "SCHEDULE" EqOpt stringLit

AdvancedPlacementOption ::=
    "LEARNERS" EqOpt LengthNum
|   "CONSTRAINTS" EqOpt stringLit
|   "LEADER_CONSTRAINTS" EqOpt stringLit
|   "FOLLOWER_CONSTRAINTS" EqOpt stringLit
|   "LEARNER_CONSTRAINTS" EqOpt stringLit
|   "SURVIVAL_PREFERENCES" EqOpt stringLit

示例

CREATE PLACEMENT POLICY p1 PRIMARY_REGION="us-east-1" REGIONS="us-east-1,us-west-1";
CREATE TABLE t1 (i INT) PLACEMENT POLICY=p1; -- 将策略 p1 分配给表 t1
ALTER PLACEMENT POLICY p1 PRIMARY_REGION="us-east-1" REGIONS="us-east-1,us-west-1,us-west-2" FOLLOWERS=4; -- t1 的规则会自动更新。
SHOW CREATE PLACEMENT POLICY p1\G

Query OK, 0 rows affected (0.08 sec)

Query OK, 0 rows affected (0.10 sec)

***************************[ 1. row ]***************************
Policy        | p1
Create Policy | CREATE PLACEMENT POLICY `p1` PRIMARY_REGION="us-east-1" REGIONS="us-east-1,us-west-1,us-west-2" FOLLOWERS=4
1 row in set (0.00 sec)

MySQL 兼容性

该语句是 TiDB 对 MySQL 语法的扩展。

另请参阅

• Placement Rules in SQL
• SHOW PLACEMENT
• CREATE PLACEMENT POLICY
• DROP PLACEMENT POLICY