oracle-database hibernate sequence gaps-in-data

hibernate oracle sequence produces large gap

I am using hibernate 3 , oracle 10g. I have a table: subject. The definition is here

CREATE TABLE SUBJECT
    ( 
     SUBJECT_ID NUMBER (10), 
     FNAME VARCHAR2(30)  not null, 
     LNAME VARCHAR2(30)  not null, 
     EMAILADR VARCHAR2 (40),
     BIRTHDT  DATE       not null,
     constraint pk_sub primary key(subject_id) USING INDEX TABLESPACE data_index
    ) 
;

when insert a new subject, sub_seq is used to create an subject id, the definition is here

create sequence sub_seq
       MINVALUE 1 
       MAXVALUE 999999999999999999999999999 
       START WITH 1
       INCREMENT BY 1 
       CACHE 100 
       NOCYCLE ;

the Subject class is like this:

@Entity
@Table(name="ktbs.syn_subject")
public class Subject {

    @Id 
    @Column(name="subject_id")
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SUB_SEQ")
    @SequenceGenerator(name="SUB_SEQ", sequenceName = "SUB_SEQ")
    private long subjectId;
    private String fname;
    private String lname;
    private String emailadr;
    private Date birthdt;
}

in the subject table , there have been 4555 subjects in the database loaded by plsql scripts from excel and the sub_sequence worked fine. subject ids range from 1--4555.

however, when i added a subject from my application using hibernate, the sequence number jumped to 255050. After several days running, the subject ids generated by hibernate look like this

There are several large gaps: 4555 to 255051, 255067 to 260051, 265057 to 270051

this is a waste and not a desired behavior.

does anyone know why this happens and hot to fix it

Thanks

Solution

I think that the problem comes from the fact that the sequence generator is not really a sequence generator, but a sequence hilo generator, with a default allocation size of 50. as indicated by the documentation : http://docs.jboss.org/hibernate/stable/annotations/reference/en/html_single/#entity-mapping-identifier

This means that if the sequence value is 5000, the next generated value will be 5000 * 50 = 250000. Add the cache value of the sequence to the equation, and it might explain your huge initial gap.

Check the value of the sequence. It should be less than the last generated identifier. Be careful not to reinitialize the sequence to this last generated value + 1, because the generated valus would grow exponentially (we've had this problem, and had negative integer ids due to overflow)