15.3 CREATE SYNONYM

Purpose

Use the CREATE SYNONYM statement to create a synonym, which is an alternative name for a table, view, sequence, operator, procedure, stored function, package, materialized view, Java class schema object, user-defined object type, or another synonym. A synonym places a dependency on its target object and becomes invalid if the target object is changed or dropped.

Synonyms provide both data independence and location transparency. Synonyms permit applications to function without modification regardless of which user owns the table or view and regardless of which database holds the table or view. However, synonyms are not a substitute for privileges on database objects. Appropriate privileges must be granted to a user before the user can use the synonym.

You can refer to synonyms in the following DML statements: SELECT, INSERT, UPDATE, DELETE, FLASHBACK TABLE , EXPLAIN PLAN , LOCK TABLE, MERGE, and CALL.

You can refer to synonyms in the following DDL statements: AUDIT, NOAUDIT, GRANT, REVOKE, and COMMENT.

See Also:

Oracle Database Concepts for general information on synonyms

Prerequisites

To create a private synonym in your own schema, you must have the CREATE SYNONYM system privilege.

To create a private synonym in another user's schema, you must have the CREATE ANY SYNONYM system privilege.

To create a PUBLIC synonym, you must have the CREATE PUBLIC SYNONYM system privilege.

Syntax

Semantics

OR REPLACE

Specify OR REPLACE to re-create the synonym if it already exists. Use this clause to change the definition of an existing synonym without first dropping it.

Restriction on Replacing a Synonym

You cannot use the OR REPLACE clause for a type synonym that has any dependent tables or dependent valid user-defined object types.

[ EDITIONABLE | NONEDITIONABLE ]

Use these clauses to specify whether the synonym is an editioned or noneditioned object if editioning is enabled for the schema object type SYNONYM in schema. For private synonyms, the default is EDITIONABLE. For public synonyms, the default is NONEDITIONABLE. For information about editioned and noneditioned objects, see Oracle Database Development Guide.

PUBLIC

Specify PUBLIC to create a public synonym. Public synonyms are accessible to all users. However each user must have appropriate privileges on the underlying object in order to use the synonym.

When resolving references to an object, Oracle Database uses a public synonym only if the object is not prefaced by a schema and is not followed by a database link.

If you omit this clause, then the synonym is private. A private synonym name must be unique in its schema. A private synonym is accessible to users other than the owner only if those users have appropriate privileges on the underlying database object and specify the schema along with the synonym name.

Notes on Public Synonyms

The following notes apply to public synonyms:

  • If you create a public synonym and it subsequently has dependent tables or dependent valid user-defined object types, then you cannot create another database object of the same name as the synonym in the same schema as the dependent objects.

  • Take care not to create a public synonym with the same name as an existing schema. If you do so, then all PL/SQL units that use that name will be invalidated.

schema

Specify the schema to contain the synonym. If you omit schema, then Oracle Database creates the synonym in your own schema. You cannot specify a schema for the synonym if you have specified PUBLIC.

synonym

Specify the name of the synonym to be created. The name must satisfy the requirements listed in "Database Object Naming Rules".

Note:

The maximum length of a synonym name is subject to the following rules:

  • If the COMPATIBLE initialization parameter is set to a value of 12.2 or higher, then the maximum length of a synonym name is 128 bytes. The database will allow you to create and drop synonyms of length 129 to 4000 bytes. However, unless these longer synonym names represent a Java name they will not work in any other SQL command.

  • If the COMPATIBLE initialization parameter is set to a value lower than 12.2, then the maximum length of a synonym name is 30 bytes. The database will allow you to create and drop synonyms of length 31 to 128 bytes. However, unless these longer synonym names represent a Java name they will not work in any other SQL command.

The longer synonym names are transformed into obscure shorter strings for storage in the data dictionary.

SHARING

This clause applies only when creating a synonym in an application root. This type of synonym is called an application common object and it can be shared with the application PDBs that belong to the application root. To determine how the synonym is shared, specify one of the following sharing attributes:

  • METADATA - A metadata link shares the synonym’s metadata, but its data is unique to each container. This type of synonym is referred to as a metadata-linked application common object.

  • NONE - The synonym is not shared.

If you omit this clause, then the database uses the value of the DEFAULT_SHARING initialization parameter to determine the sharing attribute of the synonym. If the DEFAULT_SHARING initialization parameter does not have a value, then the default is METADATA.

You cannot change the sharing attribute of a synonym after it is created.

See Also:

FOR Clause

Specify the object for which the synonym is created. The schema object for which you are creating the synonym can be of the following types:

  • Table or object table

  • View or object view

  • Sequence

  • Stored procedure, function, or package

  • Materialized view

  • Java class schema object

  • User-defined object type

  • Synonym

The schema object need not currently exist and you need not have privileges to access the object.

Restriction on the FOR Clause

The schema object cannot be contained in a package.

schema

Specify the schema in which the object resides. If you do not qualify object with schema, then the database assumes that the schema object is in your own schema.

If you are creating a synonym for a procedure or function on a remote database, then you must specify schema in this CREATE statement. Alternatively, you can create a local public synonym on the database where the object resides. However, the database link must then be included in all subsequent calls to the procedure or function.

dblink

You can specify a complete or partial database link to create a synonym for a schema object on a remote database where the object is located. If you specify dblink and omit schema, then the synonym refers to an object in the schema specified by the database link. Oracle recommends that you specify the schema containing the object in the remote database.

If you omit dblink, then Oracle Database assumes the object is located on the local database.

Restriction on Database Links

You cannot specify dblink for a Java class synonym.

See Also:

Examples

CREATE SYNONYM: Examples

To define the synonym offices for the table locations in the schema hr, issue the following statement:

CREATE SYNONYM offices 
   FOR hr.locations;

To create a PUBLIC synonym for the employees table in the schema hr on the remote database, you could issue the following statement:

CREATE PUBLIC SYNONYM emp_table 
   FOR hr.employees@remote.us.example.com;

A synonym may have the same name as the underlying object, provided the underlying object is contained in another schema.

Oracle Database Resolution of Synonyms: Example

Oracle Database attempts to resolve references to objects at the schema level before resolving them at the PUBLIC synonym level. For example, the schemas oe and sh both contain tables named customers. In the next example, user SYSTEM creates a PUBLIC synonym named customers for oe.customers:

CREATE PUBLIC SYNONYM customers FOR oe.customers;

If the user sh then issues the following statement, then the database returns the count of rows from sh.customers:

SELECT COUNT(*) FROM customers;

To retrieve the count of rows from oe.customers, the user sh must preface customers with the schema name. (The user sh must have select permission on oe.customers as well.)

SELECT COUNT(*) FROM oe.customers;

If the user hr's schema does not contain an object named customers, and if hr has select permission on oe.customers, then hr can access the customers table in oe's schema by using the public synonym customers:

SELECT COUNT(*) FROM customers;