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7.8 Synonyms
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You can use the CREATE SYNONYM command to create synonyms for tables or views. Once created, you can use synonyms in all your SQL commands instead of real table (and view) names. For example, you could use synonyms for tables with very long table names.
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Synonyms are especially useful if you are accessing tables from different schemas, not owned by yourself. Without synonyms, you must explicitly prefix those object names with the schema name and a period. The Oracle data dictionary is a perfect example of synonym usage. You can simply specify the data dictionary view names in your queries, without any prefix, although you obviously don t own those data dictionary objects. Synonyms are a convenience feature. They don t provide any additional privileges, and they don t create security risks. They just save you some typing, and they also allow you to make your applications schema-independent. Schema-independence is important. By using synonyms, your applications don t need to contain explicit schema names. This makes your applications more flexible and easier to maintain, because the mapping to physical schema and object names is in the synonym definitions, separated from the application code. Figure 7-17 shows the syntax diagram for the CREATE SYNONYM command.
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Figure 7-17. A CREATE SYNONYM command syntax diagram Oracle supports public and private synonyms, as you can see in Figure 7-17. By default, synonyms are private. You need to specify the PUBLIC keyword to create public synonyms. All database users can use public synonyms, but you need DBA privileges to be able to create them. The synonyms for the data dictionary objects are examples of public synonyms. Anyone can create private synonyms, but only their owners can use them.
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Caution Although synonyms are useful, they can also cause performance problems. In particular, public synonyms are known to cause such problems. For further details, go to Steve Adams s web site (http://www.ixora.com.au) and search for avoiding public synonyms.
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Listing 7-16 shows how you can create a synonym, how the synonym shows up in the data dictionary views CAT and USER_SYNONYMS, and how you can drop a synonym. Listing 7-16. Creating and Dropping a Synonym SQL> create synonym e for employees; Synonym created.
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SQL> describe e Name -----------------------EMPNO ENAME INIT JOB MGR BDATE MSAL COMM DEPTNO SQL> select * from cat; TABLE_NAME -------------------EMPLOYEES DEPARTMENTS SALGRADES COURSES OFFERINGS REGISTRATIONS HISTORY DEPTNO_SEQ E
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Null -------NOT NULL NOT NULL NOT NULL
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Type -----------NUMBER(4) VARCHAR2(8) VARCHAR2(5) VARCHAR2(8) NUMBER(4) NOT NULL DATE NOT NULL NUMBER(6,2) NUMBER(6,2) NUMBER(2)
TABLE_TYPE ----------TABLE TABLE TABLE TABLE TABLE TABLE TABLE SEQUENCE SYNONYM
SQL> select synonym_name, table_owner, table_name 2 from user_synonyms; SYNONYM_NAME TABLE_OWNER TABLE_NAME -------------------- ----------- ---------------E BOOK EMPLOYEES SQL> drop synonym e; Synonym dropped. SQL> Synonyms are often used in distributed database environments to implement full data independence. The user (or database application) does not need to know where (in which database) tables or views are located. Normally, you need to specify explicit database links using the at sign (@) in the object name, but synonyms can hide those database link references.
7.9 The CURRENT_SCHEMA Setting
The ALTER SESSION command provides another convenient way to save you the effort of prefixing object names with their schema name, but without using synonyms. This is another convenience feature, just like synonyms.
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Suppose the demo schema SCOTT (with the EMP and DEPT tables) is present in your database, and suppose you are currently connected as database user BOOK. In that situation, you can use the ALTER SESSION command as shown in Listing 7-17. Listing 7-17. The CURRENT_SCHEMA Setting SQL> alter session set current_schema=scott; Session altered. SQL> show user USER is "BOOK" SQL> select * from dept; DEPTNO -------10 20 30 40 DNAME -------------ACCOUNTING RESEARCH SALES OPERATIONS LOC ------------NEW YORK DALLAS CHICAGO BOSTON
SQL> alter session set current_schema=book; Session altered. SQL> You can compare the CURRENT_SCHEMA setting in the database with the change directory (cd) command at the operating system level. In a similar way, it allows you to address all objects locally. Again, this does not change anything with regard to security and privileges. If you really want to assume the identity of a schema owner, you must use the SQL*Plus CONNECT command, and provide the username/schema name and the corresponding password.
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