Sunday, July 24, 2011

Difference between Select Any Dictionary and Select_Catalog_Role

When you want to give a user the privilege to select from data dictionary and dynamic performance views such as V$DATAFILE, you have two options:

grant select any dictionary to ;
grant select_catalog_role to ;

Did you ever wonder why there are two options for accomplishing the same objective? Is one of them redundant? Won't it make sense for Oracle to have just one privilege? And, most important, do these two privileges produce the same result?

The short answer to the last question is -- no; these two do not produce the same result. Since they are fundamentally different, there is a place of each of these. One is not a replacement for the other. In this blog I will explain the subtle but important differences between the two seemingly similar privileges and how to use them properly.

Create the Test Case

First let me demonstrate the effects by a small example. Create two users called SCR and SAD:

SQL> create user scr identified by scr;
SQL> create user sad identified by sad;

Grant the necessary privileges to these users, taking care to grant a different one to each user.

SQL> grant create session, select any dictionary to sad;

Grant succeeded.

SQL> grant create session, select_catalog_role to scr;

Grant succeeded.

Let's test to make sure these privileges work as expected:

SQL> connect sad/sad

SQL> select * from v$session;
 ... a bunch of rows come here ...

SQL> connect scr/scr

SQL> select * from v$datafile;
 ... a bunch of rows come here ...

Both users have the privilege to select from the dictionary views as we expected. So, what is the difference between these two privileges? To understand that, let's create a procedure on the dictionary tables/views on each schema. Since we will create the same procedure twice, let's first create a script which we will call p.sql. Here is the script:

create or replace procedure p as
    l_num number;
    select count(1)
    into l_num
    from v$session;

The procedure is very simple; it merely counts the number of connected sessions by querying V$SESSION. When you connect as SAD and create the procedure by executing p.sql:

SQL> @p.sql

Procedure created.

The procedure was created properly; but when you connect as SCR and execute the script:

SQL> @p.sql

Warning: Procedure created with compilation errors.

SQL> show error
Errors for PROCEDURE P:

-------- ------------------------------------------------
4/2      PL/SQL: SQL Statement ignored
6/7      PL/SQL: ORA-00942: table or view does not exist

That must be perplexing. We just saw that the user has the privilege to select from the V$SESSION view. You can double check that by selecting from the view one more time. So, why did it report ORA-942: table does not exist?

Not All Privileges have been Created Equal

The answer lies in the way Oracle performs compilations. To compile a code with a named object, the user must have been granted privileges by direct grants; not through the roles. Selecting or performing DML statements do not care how the privileges were received. The SQL will work as long as the privileges are there. The privilege SELECT ANY DICTIONARY is a system privilege, similar to create session or unlimited tablespace. This is why the user SAD, which had the system privilege, could successfully compile the   procedure P.

The user SCR had the role SELECT_CATALOG_ROLE, which allowed it to SELECT from V$SESSION but not to create the procedure. Remember, to create another object on the base object, the user must have the direct grant on the base object; not through a role. Since SCR had the role not the direct grant on V$DATAFILE, it can't compile the procedure.

So while both the privileges allow the users to select from v$datafile, the role does not allow the users to create objects; the system privilege does.

Why the Role?

Now that you know how the privileges are different, you maybe wondering why the role is even there. It seems that the system grant can do everything and there is no need for a role. Not quite.The role has a very different purpose. Roles provide privileges; but only when they are enabled. To see what roles are enabled in a session, use this query:

SQL> connect scr/oracle
SQL> select * from session_roles
  2  /


2 rows selected.

We see that two roles - SELECT_CATALOG_ROLE and HS_ADMIN_SELECT_ROLE - have been enabled in the session. The first one was granted to the user. The other one is granted to the first one; so that was also enabled.

Just because a role was granted to the user does not necessarily mean that the role would be enabled. The roles which are marked DEFAULT by the user will be enabled; the others will not be. Let's see that with an example. As SYS user, execute the following:

SQL> alter user scr default role none;

User altered.

Now connect as SCR user and see which roles have been enabled:

SQL> connect scr/oracle
SQL> select * from session_roles;

no rows selected

None of the roles have been enabled. Why? That's  because none of the roles are default for the user (effected by the alter user statement by SYS). At this point when you select from a dynamic performance view:

SQL> select * from v$datafile;
select * from v$datafile
ERROR at line 1:
ORA-00942: table or view does not exist

You will get this error because the role is not enabled, or active. Without the role the user does not have any privilege to select from the data dictionary or dynamic performance view. To enable the role, the user has to execute the SET ROLE command:


Role set.

Checking the enabled roles:

SQL> select * from session_roles;


2 rows selected.

Now the roles have been enabled. Since the roles are not default, the user must explicitly enable them using the SET ROLE command. This is a very important characteristic of the roles. We can control how the user will get the privilege. Merely granting a role to a user will not enable the role; the user's action is required and that can be done programmatically. In security conscious environments, you may want to take advantage of that property. A user does not always have the to have to privilege; but when needed it will be able to do so.

The SET ROLE command is an SQL*Plus command. To call it from SQL, use this:


   dbms_session.set_role ('SELECT_CATALOG_ROLE');

You can also set a password for the role. So it will be set only when the correct password is given;

SQL> alter role SELECT_CATALOG_ROLE identified by l       
  2  /

Role altered.

To set the role, you have to give the correct password:

SQL> set role SELECT_CATALOG_ROLE identified by l;

Role set.

If you give the wrong password:

SQL> set role SELECT_CATALOG_ROLE identified by fl
  2  /
set role SELECT_CATALOG_ROLE identified by fl
ERROR at line 1:
ORA-01979: missing or invalid password for role 'SELECT_CATALOG_ROLE'

You can also revoke the execute privilege on dbms_session from public. After that the user will not be able to use it to set the role. You can construct another wrapper procedure to call it. Inside the wrapper, you can have all sort of checks and balances to make sure the call is acceptable.

We will close this discussion with a tip. How do you know which roles are default? Simply use the following query:

  2  from dba_role_privs
  3  where GRANTEE = 'SCR';

GRANTED_ROLE                   DEF
------------------------------ ---


Thanks to Randolph Geist ( and Pavel Ruzicka ( for pointing out yet another important difference. SELECT ANY DICTIONARY allows select from all SYS owner tables such as TAB$, USER$, etc. This is not possible in the SELECT_CATALOG_ROLE. This difference may seem trivial; but is actually quite important in some cases. For instance, latest versions of Oracle do not show the password column from DBA_USERS; but the hashed password is visible in USER$ table. It's not possible to reverse engineer the password from the hash value; but it is possible to match it to a similar entry and guess the password. A user with the system privilege will be able to do that; but a user with the role will not be.


In this blog entry I started with a simple question - what is the difference between two seemingly similar privileges - SELECT ANY DICTIONARY and SELECT_CATALOG_ROLE. The former is a system privilege, which remains active throughout the sessions and allows the user to create stored objects on objects on which it has privileges as a result of the grant. The latter is not a system grant; it's a role which does not allow the grantee to build stored objects on the granted objects. The role can also be non-default which means the grantee must execute a set role or equivalent command to enable it. The role can also be password protected, if desired.

The core message you should get from this is that roles are different from privileges. Privileges allow you to build stored objects such as procedures on the objects on which the privilege is based. Roles do not.

Who Manages the Exadata Machine?

For organizations that just procured an Exadata machine, one of the big questions is bound to be about the group supporting it. Who should it be - the DBAs, Sys Admins, Network Admins, or some blend of multiple teams?

The conventional Oracle database system is a combination of multiple distinct components - servers, managed by system admins; storage units, managed by SAN admins; network components such as switches and routers, managed by network admins; and, of course, the database itself, managed by the DBAs. Exadata has all those components - servers, storage (as cell servers), infiniband network, ethernet network, flash disks, the whole nine yards; but packaged inside a single physical frame representing a single logical unit - a typical engineered system. (For a description of the components inside the Exadata system, please see my 4-part article series on Oracle Technology Network) None of these conventional technology groups posses the skillsets to the manage all these components. That leads to a difficult but important decision - how the organization should assign the operational responsibilities.


There are two choices for organizations to assign administrative responsibilities.

  1. Distributed - Have these individual groups manage the respective components, e.g. Sys Admins managing the Linux servers, the storage admins managing the storage cells, network admins managing the network components and finally DBAs managing the database and the cluster.
  2. Consolidated - Create a specialized group - Database Machine Administrator (DMA) and have one of these groups expand the skillset to include the other non-familiar areas.

Each option has its own pros and cons. Let's examine them and see if we can get the right fit for our specific case.

Distributed Management

Under this model each component of Exadata is managed as an independent entity by a group traditionally used to manage that type of infrastructure. For instance, the system admins would manage the Linux OS, overseeing all aspects of it such as creation of users to applying the patches and RPMs. The storage and database would be managed likewise by the specialist teams.

The benefit of this solution is its seeming simplicity - components are managed by their respective specialists without a need for advanced training. The only need for training is for storage, where the Exadata Storage Server commands are new and specific to Exadata.

While this approach seems a nobrainer on surface, it may not be so in reality. Exadata is not just something patched up from these components; it is an engineered system. There is a huge meaning behind that qualifier. These components are not designed to act alone; they are put together to make the entire structure a better database machine. And, note the stress here - not an application server, not a fileserver, not a mail server; not a general purpose server - but a database machine alone. This means the individual components - the compute nodes, the storage servers, the disks, the flashdisk cards and more - are tuned to achieve that overriding objective. Any incremental tuning in any specific component has to  be within the framework of the entire frame; otherwise it may fail to produce the desired result, or worse, produce undesirable result.

For instance the disks where the database resides are attached to the storage cell servers; not the database compute nodes. The cell servers, or Cells run Oracle Enterprise Linux, which is very similar to Red Hat Linux. Under this model of administration, the system admins are responsible for managing the operating system. A system admin looks at the host and determines that it is under tuned since the filesystem cache is very low. In a normal Linux system, that would have been a correct observation; but in Exadata, the database is in ASM and a filesystem cache is less important. On the other hand, the Cells need the memory to place the Storage Indexes on the disk contents. Placing a large filesystem cache not only produce nothing to help the filesystem; but actually hurt the performance for the paging of Storage Indexes.

This is just one example of how the engineered systems are closely interrelated. Assuming they are separate and assigning multiple groups with different skillsets may not work effectively.

Database Machine Administrator

This is leads to the other approach - making a single group responsible for the entire frame from storage to the database. The single group would be able to understand the impact of the changes in one component to the overall effectiveness of the rack and will be in a better position to plan and manage. The single role that performs the management of Exadata is known as Database Machine Administrator (DMA).

I can almost hear the questions firing off inside your brain. The most likely question probably is whether it is even possible to have a single skillset that encompasses storage, system, database and network.

Yes, it definitely is. Remember, the advantages of an engineered system do not stop at being a carefully coordinated individual components. Another advantage is the lack of controls in those components. There are less knobs to turn on each component in an Exadata system. Take for instance the Operating System. There are two types of servers - the compute nodes and the cells. In the cells, the activity performed by a system admin is severely limited - almost to the point of being none. On the compute nodes, the activities are limited as well. The only allowable activities are - setting up users, setting up email relays, possibly setting up an NFS mount and handful of more. This can easily be done by a non-expert. One does not have to a System Admin to manage the servers.

Consider storage, the other important component. Traditionally storage administrators perform critical functions such as adding disks, carving out LUNs, managing replication for DR and so on. These functions are irrelevant in Exadata. For instance, the disks are preallocated in Exadata, the LUNs are created at installation time, there is no replication since the DR is by Data Guard which at the Oracle database level. One need not be a storage expert to the perform the tasks in Exadata. Additionally the Storage Admins are experts in the specific brand of storage, e.g. EMC VMax or IBM XiV. In Exadata, the storage is different from all the other brands your storage admins may be managing. They have to learn about the Exadata storage anyway; so why not have someone else, specifically the DMA learn?

Consider Network. In Exadata the network components are very limited since it is only for the components inside the rack. This reduces the flexibility of the configuration compared to a regular general purpose network configuration. the special kind of hardware used in Exadata - Infiniband - requires some special skills which the network ops folks may have to learn anyway. So, why not the DMAs instead of them? Besides, Oracle already provides a lot of tools to manage this layer.

That leaves the most visible component - the database which is, after all, the heart and soul of Exadata. This layer is amenable to a considerable degree of tuning and the depth of skills in this layer is vital to managing Exadata effectively. Transferring the skills needed here to a non-DBA group or individual is difficult, if not impossible. This makes the DBA group the most natural choice for evolving into the DMA role after absorbing the relevant other skills. The other skills are not necessarily at par with the administrator of the respective components. For instance the DMA does not need to be a full scale Linux system admin; but just needs to know a few relevant concepts, commands and tools to perform the job well. Network management is Exadata is a fraction of the skills expected from a network admin. The storage management in cell servers are new to any group; so the DMA will find that as easy as any other group, if not easier.

By understanding the available knobs on all the constituent components of Exadata, the DMA can be better prepared to be an effective administrator of the Exadata system; not by divvying up the activities to individual groups which are generally autonomous. The advantages are particularly seen when troubleshooting or patching Exadata. Hence, I submit here for your consideration - a new role called DMA (Database Machine Administrator) for the management of Exadata. The role should have the following skillsets:

60% Database Administration
20% Cell Administration
15% Linux Administration
5% Miscellaneous (Infiniband, network, etc.)

I have written an article series on Oracle Technology Network - Linux for Oracle DBAs. This 5-part article series has all the commands an concepts the Oracle DBA should understand about Linux. I have also written a 4 part article series - Commanding Exadata - for DBAs to learn the 20% cell administration. With these two , you will have everything you need to be a DMA. Scroll down to the bottom of this page and click on "Collection of Some of My Very Popular Web Articles" to locate all these articles and more.


In this blog entry, I argued for creating  a single role to manage the Exadata system instead of multiple groups managing individual parts. Here are the reasons in a nutshell:

  1. Exadata is an engineered system where all the components play collaboratively instead of as islands. Managing them separately may be ineffective and detrimental.
  2. The support organizations of components such as Systems, storage, DBA, etc. in an organizations are designed with a generic purpose in mind. Exadata is not generic. Its management needs unprecedented close coordination among various groups which may be new to the organization and perhaps difficult to implement.
  3. The needed skillsets are mostly database centric; other components have very little to manage.
  4. These other skills are easy to add to the DBA skills making the natural transition to the DMA role.

Best of luck in becoming a DMA and implementing Exadata.