Hibernate IDENTITY 与 SEQUENCE 实体标识符生成器

Question

This article says:

Unlike identity, the next number for the column value will be retrieved from memory rather than from the disk – this makes Sequence significantly faster than Identity

Does it mean that ID comes from disk in case of identity? If yes, then which disk and how ?

Using sequence, I can see in the log, an extra select query to DB while inserting a new record. But I didn't find that extra select query in the log in case of identity. Then how sequence becomes faster than identity ?

Answer 1

Strategy used by sequence:

Before inserting a new row, ask the database for the next sequence value, then insert this row with the returned sequence value as ID.

Strategy used by identity:

Insert a row without specifying a value for the ID. After inserting the row, ask the database for the last generated ID.

The number of queries is thus the same in both cases. But , Hibernate uses by default a strategy that is more efficient for the sequence generator. In fact, when it asks for the next sequence value, it keeps th 50 (that's the dafault, IIRC, and it's configurable) next values in memory, and uses these 50 next values for the next 50 inserts. Only after 50 inserts, it goes to the database to get the 50 next values. This tremendously reduces the number of needed SQL queries needed for automatic ID generation.

The identity strategy doesn't allow for such an optimization.

Answer 2

The IDENTITY generator will always require a database hit for fetching the primary key value without waiting for the flush to synchronize the current entity state transitions with the database.

So the IDENTITY generator doesn't play well with Hibernate write-behind first level cache strategy, therefore JDBC batching is disabled for the IDENTITY generator.

The sequence generator can benefit from database value preallocation and you can even employ a hi/lo optimization strategy.

In my opinion, the best generators are the pooled and pooled-lo sequence generators. These generators combine the batch-friendly sequence generator with a client-side value generation optimization that's compatible with other DB clients that may insert rows without knowing anything about our generation strategy.

Anyway, you should never choose the TABLE generator because it performs really bad.

Answer 3

Though I'm personally new to Hibernate, from what I can recall, using Identity basically means that Hibernate will check what is the next possible id value from your DB and keep a value for it.

For sequence, you basically tell Hibernate to generate the next value based on a particular sequence you provide it. So it has to actually calculate the next id by looking at the next possible id value. Hence, the extra query is fired.

Answer 4

maybe this will answer your question :

Unlike identity column values, which are generated when rows are inserted, an application can obtain the next sequence number before inserting the row by calling the NEXT VALUE FOR function. The sequence number is allocated when NEXT VALUE FOR is called even if the number is never inserted into a table. The NEXT VALUE FOR function can be used as the default value for a column in a table definition. Use sp_sequence_get_range to get a range of multiple sequence numbers at once.

you can find the detail here

Identity doesnt need that extra select query because Identity is a table dependent and Sequence is independent from table, but because of this we can get sequence even before creating a row(when you do session.save(T entity), sequence is generated even before you commit the transaction).

sequence : you create or update entities -> each time you save entity -> hibernate get next sequence value -> your program return the value after all process complete without exception or rollback -> you commit all transaction -> hibernate insert all complete entity

identity : when commit transaction, insert incomplete entity(must get it from identity column). so the INSERT command of sequence is definitely slower, but the advantages is if you cancel the insert the count doesn't increasing.