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                # 數據庫索引 GORM 允許通過 `index`、`uniqueIndex` 標簽創建索引,這些索引將在使用 GORM 進行[AutoMigrate 或 Createtable ](http://v2.gorm.io/zh_CN/docs/migration.html)時創建 ## 索引標簽 GORM 可以接受很多的索引設置,例如:`class`、`type`、`where`、`comment`、`expression`、`sort`、`collate` 下面的示例演示了如何使用它: ```go type User struct { Name string `gorm:"index"` Name2 string `gorm:"index:idx_name,unique"` Name3 string `gorm:"index:,sort:desc,collate:utf8,type:btree,length:10,where:name3 != 'jinzhu'"` Name4 string `gorm:"uniqueIndex"` Age int64 `gorm:"index:,class:FULLTEXT,comment:hello \\, world,where:age > 10"` Age2 int64 `gorm:"index:,expression:ABS(age)"` } ``` ### 唯一索引 ```go uniqueIndex` 標簽的作用與 `index` 類似,它等效于 `index:,unique type User struct { Name1 string `gorm:"uniqueIndex"` Name2 string `gorm:"uniqueIndex:idx_name,sort:desc"` } ``` ## 復合索引 兩個字段使用同一個索引名將創建復合索引,例如: ```go type User struct { Name string `gorm:"index:idx_member"` Number string `gorm:"index:idx_member"` } ``` ### 字段優先級 復合索引列的順序會影響其性能,因此必須仔細考慮 您可以使用 `priority` 指定順序,默認優先級值是 `10`,如果優先級值相同,則順序取決于模型結構體字段的順序 ```go type User struct { Name string `gorm:"index:idx_member"` Number string `gorm:"index:idx_member"` } // column order: name, number type User struct { Name string `gorm:"index:idx_member,priority:2"` Number string `gorm:"index:idx_member,priority:1"` } // column order: number, name type User struct { Name string `gorm:"index:idx_member,priority:12"` Number string `gorm:"index:idx_member"` } // column order: number, name ``` ## 多索引 一個字段接受多個 `index`、`uniqueIndex` 標簽,這會在一個字段上創建多個索引 ```go type UserIndex struct { OID int64 `gorm:"index:idx_id;index:idx_oid,unique"` MemberNumber string `gorm:"index:idx_id"` } ```
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