129 lines
4.0 KiB
Markdown
129 lines
4.0 KiB
Markdown
关于表的索引
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===========================
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可以在表的一个或多个列上创建索引以加速表上的 SQL 语句执行速度。索引使用正确的话，可以减少物理 IO 或者逻辑 IO。
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如果创建表时同时设置了主键，OceanBase 数据库会默认创建一个唯一索引。以下面的 SQL 为例：
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```javascript
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obclient> DROP TABLE IF EXISTS t1;
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Query OK, 0 rows affected (0.01 sec)
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obclient> CREATE TABLE t1(id bigint not null primary key, name varchar(50));
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Query OK, 0 rows affected (0.05 sec)
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obclient> show indexes from t1\G
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*************************** 1. row ***************************
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Table: t1
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Non_unique: 0
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Key_name: PRIMARY
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Seq_in_index: 1
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Column_name: id
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Collation: A
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Cardinality: NULL
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Sub_part: NULL
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Packed: NULL
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Null:
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Index_type: BTREE
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Comment: available
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Index_comment:
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Visible: YES
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1 row in set (0.01 sec)
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```
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新增索引
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-------------
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为表增加索引可以通过 CREATE INDEX 语句。OceanBase 能在普通表和分区表上创建索引，索引可以是本地索引或者全局索引。同时索引可以是唯一索引或者普通索引，如果是分区表的唯一索引，唯一索引必须包含表分区的拆分键。
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创建索引的 SQL 语法格式如下：
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```javascript
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CREATE [UNIQUE] INDEX index_name ON table_name ( column_list ) [LOCAL | GLOBAL] [ PARTITION BY column_list PARTITIONS N ] ;
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```
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MySQL 租户里，索引名称在表范围内不能重复，查看索引可以通过命令 SHOW INDEXES 。
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在 MySQL 租户里，新增索引还有一种方法，SQL 语法格式如下：
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```javascript
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ALTER TABLE table_name
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ADD|DROP INDEX|KEY index_name ( column_list ) ;
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```
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可以一次增加多个索引，索引关键字用 INDEX 或 KEY 都可以。
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* 示例：对分区表新增索引
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```javascript
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obclient> create table t3(
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id bigint not null primary KEY
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, name varchar(50)
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, gmt_create timestamp not null default current_timestamp
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) partition by hash(id) partitions 8;
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Query OK, 0 rows affected (0.14 sec)
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obclient> alter table t3 add unique key t3_uk (name) local;
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ERROR 1503 (HY000): A UNIQUE INDEX must include all columns in the table's partitioning function
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obclient> alter table t3
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add unique key t3_uk (name, id) LOCAL
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, add key t3_ind3(gmt_create) global;
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Query OK, 0 rows affected (18.03 sec)
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obclient> show indexes from t3;
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+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+
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| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible |
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+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+
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| t3 | 0 | PRIMARY | 1 | id | A | NULL | NULL | NULL | | BTREE | available | | YES |
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| t3 | 0 | t3_uk | 1 | name | A | NULL | NULL | NULL | YES | BTREE | available | | YES |
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| t3 | 0 | t3_uk | 2 | id | A | NULL | NULL | NULL | | BTREE | available | | YES |
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| t3 | 1 | t3_ind3 | 1 | gmt_create | A | NULL | NULL | NULL | | BTREE | available | | YES |
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+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+
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4 rows in set (0.01 sec)
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```
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删除索引
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-------------
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删除索引的语法格式如下：
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```javascript
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ALTER TABLE table_name DROP key|index index_name ;
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```
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* 示例：删除表的索引
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```javascript
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obclient> alter table t3 drop key t3_uk, drop key t3_ind3;
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Query OK, 0 rows affected (0.07 sec)
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```
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