What is the difference between unique_index and unique?

Question

What is the difference between unique_index and unique in GORM?
I am using MySQL 8.0, I cannot find the description about the difference between unique_index & unique form manual.
From here, see specifically the Email and MemberNumber fields:

Declaring Models

Models are usually just normal Golang structs, basic Go types, or pointers of them. sql.Scanner and driver.Valuer interfaces are also supported.

Model Example:

type User struct {
    gorm.Model
    Name         string
    Age          sql.NullInt64
    Birthday     *time.Time
    Email        string  `gorm:"type:varchar(100);unique_index"`
    Role         string  `gorm:"size:255"` // set field size to 255
    MemberNumber *string `gorm:"unique;not null"` // set member number to unique and not null
    Num          int     `gorm:"AUTO_INCREMENT"` // set num to auto incrementable
    Address      string  `gorm:"index:addr"` // create index with name `addr` for address
    IgnoreMe     int     `gorm:"-"` // ignore this field
}

Answer 1

unique is a database constraint that (in this case) prevents the multiple record have the same value for MemberNumber. If such an insert or update is made, the operation will not succeed and return an error.

unique_index will create a database index that also ensures that no two values can be the same. It will do the same, but create an index.

In your case: MySQL will use a unique index behind the scenes when using a unique constraint. So when using MySQL, there is no difference when using unique and using unique index.

If you use other database management systems there might be differences.

The differences (if any) will be handled by the database management system internally. For practical purposes you can regard them as the same. The differences will be documented for each database management system.