There are situations your JEE application needs to support Postgres and Oracle as a Database.
Hibernate should do the job here, however there are some specifics worth mentioning.
While enabling Postgres for application already running Oracle I came across following tricky parts:
BLOBs support,CLOBs support,- Oracle not knowing
Boolean type (using Integer) instead and
DUAL table.
These were the tricks I had to apply to make the @Entity classes running on both of these.
Please note I’ve used Postgres 9.3 with Hibernate 4.2.1.SP1.
BLOBs support
The problem with Postgres is that it offers 2 types of BLOB storage:
bytea - data stored in tableoid - table holds just identifier to data stored elsewhere
I guess in the most of the situations you can live with the bytea as well as I did. The other one as far as I’ve read is to be used for some huge data (in gigabytes) as it supports streams for IO operations.
Well, it sounds nice there is such a support, however using Hibernate in this case can make things quite problematic (due to need to use the specific annotations), especially if you try to achieve compatibility with Oracle.
To see the trouble here, see StackOverflow: proper hibernate annotation for byte[]
All- the combinations are described there:
annotation postgres oracle works on
-------------------------------------------------------------
byte[] + @Lob oid blob oracle
byte[] bytea raw(255) postgresql
byte[] + @Type(PBA) oid blob oracle
byte[] + @Type(BT) bytea blob postgresql
where @Type(PBA) stands for: @Type(type="org.hibernate.type.PrimitiveByteArrayBlobType") and @Type(BT) stands for: @Type(type="org.hibernate.type.BinaryType").
These result in all sorts of Postgres errors, like:
ERROR: column “foo” is of type oid but expression is of type bytea
or
ERROR: column “foo” is of type bytea but expression is of type oid
Well, there seems to be a solution, still it includes patching of Hibernate library (something I see as the last option when playing with 3.rd party library).
There is also a reference to official blog post from the Hibernate guys on the topic: PostgreSQL and BLOBs. Still solution described in blog post seems not working for me and based on the comments, seems to be invalid for more people.
BLOBs solved
OK, so now the optimistic part.
After quite some debugging I ended up with the Entity definition like this :
@Lob
private byte[] foo;
Oracle has no trouble with that, moreover I had to customize the Postgres dialect in a way:
public class PostgreSQLDialectCustom extends PostgreSQL82Dialect {
@Override
public SqlTypeDescriptor remapSqlTypeDescriptor(SqlTypeDescriptor sqlTypeDescriptor) {
if (sqlTypeDescriptor.getSqlType() == java.sql.Types.BLOB) {
return BinaryTypeDescriptor.INSTANCE;
}
return super.remapSqlTypeDescriptor(sqlTypeDescriptor);
}
}
That’s it! Quite simple right? That works for persisting to bytea typed columns in Postgres (as that fits my usecase).
CLOBs support
The errors in misconfiguration looked something like this:
org.postgresql.util.PSQLException: Bad value for type long : ...
So first I’ve found (on String LOBs on PostgreSQL with Hibernate 3.6) following solution:
@Lob
@Type(type = "org.hibernate.type.TextType")
private String foo;
Well, that works, but for Postgres only.
Then there was a suggestion (on StackOverflow: Postgres UTF-8 clobs with JDBC) from to go for:
@Lob
@Type(type="org.hibernate.type.StringClobType")
private String foo;
That pointed me the right direction (the funny part was that it was just a comment to some answers). It was quite close, but didn’t work for me in all cases, still resulted in errors in my tests.
CLOBs solved
The important was @deprecation javadocs in the org.hibernate.type.StringClobType that brought me to working one:
@Lob
@Type(type="org.hibernate.type.MaterializedClobType")
private String foo;
That works for both Postgres and Oracle, without any further hacking (on Hibernate side) needed.
Boolean type
Oracle knows no Boolean type and the trouble is that Postgres does. As there was also some plain SQL present, I ended up In Postgres with error:
ERROR: column “foo” is of type boolean but expression is of type integer
I decided to enable cast from Integer to Boolean in Postgres rather than fixing all the plain SQL places (in a way found in Forum: Automatically Casting From Integer to Boolean):
update pg_cast set castcontext = 'i' where oid in ( select c.oid from pg_cast c inner join pg_type src on src.oid = c.castsource inner join pg_type tgt on tgt.oid = c.casttarget where src.typname like 'int%' and tgt.typname like 'bool%');
Please note you should run the SQL update by user with provileges to update catalogs (probably not your postgres user used for DB connection from your application), as I’ve learned on Stackoverflow: Postgres - permission denied on updating pg_catalog.pg_cast.
DUAL table
There is one more specific in the Oracle I came across. If you have plain SQL, in Oracle there is DUAL table provied (see more info on Wikipedia on that) that might harm you in Postgres.
Still the solution is simple. In Postgres create a view that would fill the similar purpose. It can be created like this:
create or replace view dual as select 1;
Conclusion
Well that should be it. Enjoy your cross DB compatible JEE apps.
