Why is the Hibernate default generator for PostgreSql "SequenceGenerator", not "IdentityGenerator"?

Question

The default identifier generator for Postgresql in Hibernate is SequenceGenerator [1]. i.e. Hibernate will do SELECT nextval('hibernate_sequence') to generate an ID before doing an INSERT foo (id, ...) VALUES (123, ...) on session commit.

However, PostgreSql supports autoincrement id columns (see e.g. [2]), and the default generator for all other databases which support autoincrement is to use that feature [3], and to perform inserts omitting the id value and to query the database for the new id (before session commit, but within the session's transaction).

I have seen some recent discussion [4] suggesting that the former strategy is better in general, due to the insert-before-session-commit mismatch.

If SequenceGenerator is better (as per [4]), why is it not the default for databases which support it (see [3])?

If IdentityGenerator is better, why does PostgreSql explicitly choose SequenceGenerator when Postgres does support the former (as per [2])?

I tried to find the history of the decision to override the default in the Postgres dialect (see [1]), but I couldn't find the relevant commit in GitHub. I have followed the code back to the SVN repository, but the trail goes cold where the PostgreSQLDialect file is added at r11563 with an unhelpful commit message of "maven migration" [5]. I can't seem to follow the history any further back. Can anyone find the commit which added this override? Perhaps there is more info in the commit message.

Thanks in advance.

[1] https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/dialect/PostgreSQL81Dialect.java#L267

[2] PostgreSQL Autoincrement

[3] https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/dialect/Dialect.java#L639

[4] http://nhforge.org/blogs/nhibernate/archive/2009/03/20/nhibernate-poid-generators-revealed.aspx

[5] https://source.jboss.org/browse/Hibernate/core/trunk/core/src/main/java/org/hibernate/dialect/PostgreSQLDialect.java?focusedRev=14993&fromRev=11563&toRev=14993#r14993

Answer 1

Maybe because afterInsert generators are generally broken for PG in NHibernate because it uses OracleStyle out-parameter style which is not supported by npgsql-ADONET driver which returns the result as query result and not out parameter.

SQL: INSERT INTO .... returning id into nhoutparameter; :nhoutparameter = null;

using Oracle this works

command.Execute();
object id =  command.Parameter["nhoutparameter"].Value;
Assert.NotNull(id);

in PG not. It should be

object id = command.ExecuteScalar();