从 PostgreSQL 序列中的 nextVal 在 Hibernate 获取序列中多次

Question

For business logic we need to update a record with the next value from a sequence defined directly in database and not in Hibernate (because is not always applied on insert / updates)

For this purpose we have a sequence defined in PostgreSQL witch DDL is:

CREATE SEQUENCE public.facturaproveedor_numeracionperiodofiscal_seq
INCREMENT 1 MINVALUE 1
MAXVALUE 9223372036854775807 START 1
CACHE 1;

Then in DAO, when some conditions are true, we get the nextVal via:

Query query = sesion.createSQLQuery("SELECT nextval('facturaproveedor_numeracionperiodofiscal_seq')");
Long siguiente = ((BigInteger) query.uniqueResult()).longValue();

But the values asigned aren't consecutive. Looking the Hibernate output log we see four fetchs to the sequence in the same transaction:

Hibernate: SELECT nextval('facturaproveedor_numeracionperiodofiscal_seq') as num
Hibernate: SELECT nextval('facturaproveedor_numeracionperiodofiscal_seq') as num
Hibernate: SELECT nextval('facturaproveedor_numeracionperiodofiscal_seq') as num
Hibernate: SELECT nextval('facturaproveedor_numeracionperiodofiscal_seq') as num

Why is this happening? Is this for catching purposes? There is a way to disable this? Or this workaround is not correct?

Answer 1

Hibernate won't usually generate standalone nextval calls that look like that, so I won't be too surprised if it's your application doing the multiple fetches. You'll need to collect more tracing information to be sure.

I think you may have a bigger problem though. If you care about sequences skipping values or leaving holes then you're using the wrong tool for the job, you should be using a counter in a table that you UPDATE , probably UPDATE my_id_generator SET id = id + 1 RETURNING id . This locks out concurrent transactions and also ensures that if the transaction rolls back, the update is undone .

Sequences by contrast operate in parallel, which means that it's impossible to roll back a sequence increment when a transaction rolls back (see the PostgreSQL documentation). So they're generally not suitable for accounting purposes like invoice numbering and other things where you require a gapless sequence.

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For other readers who don't have the specific requirement to only sometimes generate a value: don't generate the sequence values manually; use a @GeneratedValue annotation.

In Hibernate 3.6 and newer, you should set hibernate.id.new_generator_mappings in your Hibernate properties.

Assuming you're mapping a generated key from a PostgreSQL SERIAL column, use the mapping:

@Id
@SequenceGenerator(name="mytable_id_seq",sequenceName="mytable_id_seq", allocationSize=1)
@GeneratedValue(strategy=GenerationType.SEQUENCE, generator="mytable_id_seq")

If you omit the allocationSize then Hibernate assumes it's bizarre default of 50 and fails to check that the sequence actually increments by 50 so it tries to insert already-used IDs and fails.

Hibernate/JPA isn't able to automatically create a value for your non-id-properties. The @GeneratedValue annotation is only used in conjunction with @Id to create auto-numbering

Answer 2

Hibernate caches query results.

Notice the "as num", looks like hibernate is altering the SQL as well.

You should be able to tell Hibernate not to cache results with

query.setCacheable(false);