Where does the database link SYS_HUB come from?

This blog lives a lot from customer’s feedback and updates. And these days, Nicolas Jardot from dbi-services in Switzerland sent me an email asking: Where does the database link SYS_HUB come from? And if he can delete it or if it has any deeper purpose. Neither Google nor MOS reveal a lot of useful information

Where does the database link SYS_HUB come from?

I took my vanilla 18.3.0 database in our hands-on lab and check:

column owner format a5
column db_link format a12
column username format a12
column host format a12

select con_id, owner, db_link, username, host from cdb_db_links;

    CON_ID OWNER DB_LINK      USERNAME	   HOST
---------- ----- ------------ ------------ ------------
	 1 SYS	 SYS_HUB		   CDB2
	 2 SYS	 SYS_HUB		   CDB2

So there’s a standard out-of-the-box database link in every 12.2 and 18c (and 19c) database. And in a Multitenant environment it is in every container, CDB$ROOT, PDB$SEED and then in all provisioned PDBs as well.

But what is the purpose?

Actually this database link comes in from the RAC side. It is used to allow rerouting of DML from LEAF nodes which
can’t write to the read only instances.You RAC experts may know better about this than I. In case you need to read more about hub and leaf nodes, please see the documentation.

Can you delete this database link?

I’m not Oracle Support giving official recommendations. Hence, if you are seeking for an official statement, please ask via an SR. Currently there’s no MOS note available. But there’s an unpublished bug 23568687. The database link does exist in all Oracle 12.2.0.1, Oracle 18c and Oracle 19c databases.

But in my understanding, as long as you not using RW/RO instance configuration, you can delete the db link with no side effects.

Additional Information

Maybe somebody in Support got triggered by my blog post 😉 A MOS note got published in January 2019 (thanks, Ernest Kalwa!) – but as far as I can see it doesn’t give more background information than I did:

• MOS Note:2413797.1 – What Is The Use Of DataBase Link SYS.SYS_HUB And Can It Be Deleted
• MOS Note: 2442938.1 – How to Drop and Create SYS_HUB (Database Link)

–Mike