应该为更快的查询创建多少索引

Question

我的对象模型如下所示，并希望您输入要创建的索引数量以加快查询响应（在 h2、mysql 上）。以下模型给出了假设和问题。

@Entity
@Table(name = "user")
public class User  {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id", unique = true, nullable = false, insertable = false, updatable = false)
    private Integer id;

    @ManyToOne(fetch = FetchType.LAZY)
    @ForeignKey(name = "fk_user_org_id")
    @Index(name = "idx_user_org_id")
    @JoinColumn(name = "org_id", nullable = false, referencedColumnName = "id")
    @NotNull
    private Organization organization;

    @ManyToOne(fetch = FetchType.LAZY)
    @ForeignKey(name = "fk_user_year_id")
    @Index(name = "idx_user_year_id")
    @JoinColumn(name = "year", nullable = false, referencedColumnName = "id")
    @NotNull
    private Year year;

    @ManyToOne(fetch = FetchType.LAZY)
    @ForeignKey(name = "fk_user_created_by")
    @Index(name = "idx_user_created_by")
    @JoinColumn(name = "created_by", nullable = false, referencedColumnName = "id")
    @NotNull
    private User createdBy;

    @Column(name = "name", nullable = false)
    private String name;

    @Column(name = "desc")
    private String desc;

    @Column(name = "is_system", length = LEN_1)
    @Type(type = "org.hibernate.type.YesNoType")
    private boolean isSystem = false;

    @Column(name = "user_type", nullable = false)
    private UserType userType;

    @Column(name = "status", nullable = false)
    @NotNull
    private Status status;

}

我们的计划是使用多列索引而不是单列索引（即根据 (organization, year, isSystem, status, userType, createdBy) 创建索引 user_idx）。假设我有这个索引，我会得到下面列出的查询的优化响应吗？

1. select * from user where organization=1 and year=2010;
2. select * from user where organization=1 and year=2010 and isSytem=true or false； （即系统用户或应用程序定义的用户）
3. select * from user where organization=1 and year=2010 and isSytem=false and userType=Manager（即所有经理）
4. select * from user where organization=1 and year=2010 and isSytem=false and userType=Employee（即所有员工）
5. select * from user where organization=1 and year=2010 and isSytem=false and userType=Manager and status=ACTIVE（即活跃用户）

6. select * from user where organization=1 and year=2010 and createdBy='Sam' or 'Joe' [6] 是否需要不同的多列索引，由上述 3 列组成？

7. 由于我们按照我最初的假设创建了一个多列索引，我可以安全地删除模型中当前定义的单个索引（idx_user_org_id、idx_user_year_id、idx_user_created_by）吗？

Answer 1

您应该切换索引中列的顺序：

(organization, year, isSystem, userType, status, createdBy)

这使它可以更好地服务于这两个查询：

select * from user where organization=1 and year=2010 and isSystem=false and userType=Manager
select * from user where organization=1 and year=2010 and isSystem=false and userType=Employee

[6] 是否需要不同的多列索引，由上述 3 列组成？

它不需要需要新索引 - 它可以使用现有索引，但效率较低 - 只会使用前两列。不过，为此查询添加新索引似乎是个好主意。

我可以安全地删除单个索引

是的。您应该删除未使用的索引，否则它们只会占用磁盘空间并减慢表修改速度，而不会带来任何好处。