使用 JPA 指定索引(非唯一密钥)

如何定义字段，例如 email使用 JPA 注释创建索引。我们在 email上需要一个非唯一的键，因为每天在这个字段上有数百万个查询，而且没有键的话有点慢。

@Entity
@Table(name="person",
uniqueConstraints=@UniqueConstraint(columnNames={"code", "uid"}))
public class Person {
// Unique on code and uid
public String code;
public String uid;


public String username;
public String name;
public String email;
}

我看到了一个特定于 hibernate 的注释，但是我试图避免使用特定于供应商的解决方案，因为我们仍然在 hibernate 和 datanucleus 之间做出选择。

更新:

在 JPA 2.1中，您可以这样做，参见: 此位置不允许使用注释@Index

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As far as I know, there isn't a cross-JPA-Provider way to specify indexes. However, you can always create them by hand directly in the database, most databases will pick them up automatically during query planning.

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It's not possible to do that using JPA annotation. And this make sense: where a UniqueConstraint clearly define a business rules, an index is just a way to make search faster. So this should really be done by a DBA.

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I'd really like to be able to specify database indexes in a standardized way but, sadly, this is not part of the JPA specification (maybe because DDL generation support is not required by the JPA specification, which is a kind of road block for such a feature).

So you'll have to rely on a provider specific extension for that. Hibernate, OpenJPA and EclipseLink clearly do offer such an extension. I can't confirm for DataNucleus but since indexes definition is part of JDO, I guess it does.

I really hope index support will get standardized in next versions of the specification and thus somehow disagree with other answers, I don't see any good reason to not include such a thing in JPA (especially since the database is not always under your control) for optimal DDL generation support.

By the way, I suggest downloading the JPA 2.0 spec.

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OpenJPA allows you to specify non-standard annotation to define index on property.

Details are here.

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EclipseLink provided an annotation (e.g. @Index) to define an index on columns. There is an example of its use. Part of the example is included...

The firstName and lastName fields are indexed, together and individually.

@Entity
@Index(name="EMP_NAME_INDEX", columnNames={"F_NAME","L_NAME"})  // Columns indexed together
public class Employee{
@Id
private long id;


@Index                      // F_NAME column indexed
@Column(name="F_NAME")
private String firstName;


@Index                      // L_NAME column indexed
@Column(name="L_NAME")
private String lastName;
...
}
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JPA 2.1 (finally) adds support for indexes and foreign keys! See this blog for details. JPA 2.1 is a part of Java EE 7, which is out .

If you like living on the edge, you can get the latest snapshot for eclipselink from their maven repository (groupId:org.eclipse.persistence, artifactId:eclipselink, version:2.5.0-SNAPSHOT). For just the JPA annotations (which should work with any provider once they support 2.1) use artifactID:javax.persistence, version:2.1.0-SNAPSHOT.

I'm using it for a project which won't be finished until after its release, and I haven't noticed any horrible problems (although I'm not doing anything too complex with it).

UPDATE (26 Sep 2013): Nowadays release and release candidate versions of eclipselink are available in the central (main) repository, so you no longer have to add the eclipselink repository in Maven projects. The latest release version is 2.5.0 but 2.5.1-RC3 is also present. I'd switch over to 2.5.1 ASAP because of issues with the 2.5.0 release (the modelgen stuff doesn't work).

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To sum up the other answers:

I would just go for one of them. It will come with JPA 2.1 anyway and should not be too hard to change in the case that you really want to switch your JPA provider.

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This solution is for EclipseLink 2.5, and it works (tested):

@Table(indexes = {@Index(columnList="mycol1"), @Index(columnList="mycol2")})
@Entity
public class myclass implements Serializable{
private String mycol1;
private String mycol2;
}

This assumes ascendant order.

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最佳答案

With JPA 2.1 you should be able to do it.

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Index;
import javax.persistence.Table;


@Entity
@Table(name = "region",
indexes = {@Index(name = "my_index_name",  columnList="iso_code", unique = true),
@Index(name = "my_index_name2", columnList="name",     unique = false)})
public class Region{


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


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


}

Update: If you ever need to create and index with two or more columns you may use commas. For example:

@Entity
@Table(name    = "company__activity",
indexes = {@Index(name = "i_company_activity", columnList = "activity_id,company_id")})
public class CompanyActivity{
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In JPA 2.1 you need to do the following

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Index;
import javax.persistence.Table;


@Entity(name="TEST_PERSON")
@Table(
name="TEST_PERSON",
indexes = {
@Index(name = "PERSON_INDX_0", columnList = "age"),
@Index(name = "PERSON_INDX_1", columnList = "fName"),
@Index(name = "PERSON_INDX_1", columnList = "sName")  })
public class TestPerson {


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


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


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


@Id
private long id;


public TestPerson() {
}
}

In the above example the table TEST_PERSON will have 3 indexes:

  • unique index on the primary key ID

  • index on AGE

  • compound index on FNAME, SNAME

Note 1: You get the compound index by having two @Index annotations with the same name

Note 2: You specify the column name in the columnList not the fieldName

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A unique hand-picked collection of Index annotations

= Specifications =

= ORM Frameworks =

= ORM for Android =

= Other (difficult to categorize) =

  • Realm - Alternative DB for iOS / Android: Annotation io.realm.annotations.Index;
  • Empire-db - a lightweight yet powerful relational DB abstraction layer based on JDBC. It has no schema definition through annotations;
  • Kotlin NoSQL (GitHub) - a reactive and type-safe DSL for working with NoSQL databases (PoC): ???
  • Slick - Reactive Functional Relational Mapping for Scala. It has no schema definition through annotations.

Just go for one of them.


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