Oracle Concepts - Managing Oracle Synonyms Oracle Tips by Burleson Consulting

Managing Oracle Synonyms

These are not your daddy’s synonyms, or your SAT’s synonyms; no, these are Oracle synonyms. A synonym basically allows you to create a pointer to an object that exists somewhere else. You need synonyms because when you are logged into Oracle, it looks for all objects you are querying in your schema (account). If they are not there, it will give you a big fat error telling you that they do not exist. For example, assume from the ROBERT schema that we issue a query like SELECT * FROM emp; and the EMP table is not there, we get this error:

And we can see why this is a problem because of the results of this query:

SQL> select table_name from user_tables where table_name=’EMP’;

no rows selected

Clearly there is no EMP table, which can be a problem. Well, the truth is that there is an EMP table in our database, it is just owned by a user called SCOTT as we can see in this query:

SQL> select owner, table_name from dba_tables where table_name='EMP';

OWNER                          TABLE_NAME

------------------------------ ------------------------

SCOTT                          EMP

Since we know the table is in the SCOTT schema, and assuming we have been granted SELECT privileges to that table, we can query the table this way:

Notice that we added SCOTT to the beginning of the EMP table reference. This indicates, of course, that we want to query the EMP table in the SCOTT schema, and sure enough there is the table.  A schema is another word for a user.  While a “user” is an account you can log into, every user also has a “schema,” which is a virtual space for the user to create their own objects.  So what does that make the words “Users” and “Schemas?”  Synonyms, of course!

However, it would be a bit of a pain to have to always prefix all SQL calls to the EMP table with SCOTT, there must be an easier way. There is, the way is called synonyms. In the following sections we will discuss the creation and removal of synonyms.

Creating Synonyms

A synonym is named, and points to a specific object. For example, in the ROBERT schema we can create a private synonym for SCOTT.EMP using the create synonym command:

SQL> CREATE SYNONYM emp FOR SCOTT.EMP;

Now, when we issue the query with just the EMP (removing the SCOTT.)  We will see the data from the SCOTT.EMP table because Oracle will follow the synonym to the correct place as seen here:

Note that we said that this was a private synonym. That means that only the ROBERT user can use the synonym. We can also create public synonyms using the create public synonym command as seen here:

SQL> CREATE PUBLIC SYNONYM emp FOR SCOTT.EMP;

Generally good DBA’s try to avoid public synonyms. They do make management of the database a bit easier, but they also have security and performance issues associated with them. Hence, try not to use public synonyms unless you have to.

You can have a public and private synonym of the same name. In fact, you can have a public and private synonym called EMP in the SCOTT schema and have a table called EMP in the same schema. In cases where you have multiple synonyms and/or a table present, it can get confusing which object you are using (this is another reason we hate public synonyms). There is an order of precedence with regards to the use of synonyms and local objects. This is:

1. Local objects will always be accessed first.

2. If a local object does not exist, the object with a private synonym will be accessed.

3. If a private synonym does not exist or the object does not exist, then the public synonym will be used.

Note that a synonym can be created for an object that does not exist, and an object with an associated synonym can be dropped without removing the synonym. This can cause all sorts of interesting problems for DBA’s, so be careful.

Removing Synonyms

The drop synonym command is used to drop public and private synonyms. Here is an example of dropping a private synonym and a public synonym with the drop synonym command:

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