Generally, in an Oracle migration, the schema and DDL are created in the target database, and then the rows are imported into the new database using the Oracle imp utility with the IGNORE=Y parameter. The IGNORE=Y parameters tells Oracle to use the new DDL definitions instead of the DDL inside the import data file.

Prior to Oracle9i, getting table and index DDL was a time-consuming and tricky process. You could run the export utility (exp) with ROWS=NO, but the output was hard to reuse because of quoted strings on each line and poor formatting. The only other option was to write complex dictionary scripts that might not work on complex objects such as IOT and nested tables.

After Oracle9i, you can use the dbms_metadata utility package will easily display DDL and stored procedures directly from the data dictionary. Using this powerful utility, you can punch individual objects or an entire Oracle schema. Best of all, it is easy to use. You simply execute dbms_metadata.get_ddl, specify the object names, and Oracle will extract ready-to-use DDL.

To punch off all table and indexes for the EMP table, execute dbms_metadata. get_ddl, select from DUAL, and provide all required parameters as shown in Listing A:

Listing A:

set heading off;
 set echo off;
 Set pages 999;
 set long 90000;
 
spool ddl_list.sql
 select dbms_metadata.get_ddl('TABLE','DEPT','SCOTT') from dual;
 select dbms_metadata.get_ddl('INDEX','DEPT_IDX','SCOTT') from dual;
 spool off;

The output is shown below.

The only thing missing is the ending semicolons after each statement. Just for illustration, I'm showing how a primary key can be punched as part of the table DDL or separately using the INDEX argument.

Note that you have complete table and index definitions, including storage parameters (e.g., pctfree, pctused, freelists, and freelist groups) as well as tablespace storage and buffer pool directives.

For large migrations, you can modify the dbms_metadata syntax to punch a whole schema. As you can see in Listing C, it is easily done by selecting dbms_metadata. get_ddl and specifying USER_TABLES and USER_INDEXES. This syntax will punch all table and index definitions for the entire schema, in this example, the scott schema.
 

Listing C:

set pagesize 0

 set long 90000

 set feedback off

 set echo off
 spool scott_schema.sql
 connect scott/tiger;
 SELECT DBMS_METADATA.GET_DDL('TABLE',u.table_name)
     FROM USER_TABLES u;
 SELECT DBMS_METADATA.GET_DDL('INDEX',u.index_name)
     FROM USER_INDEXES u;
 spool off;

Punching PL/SQL from Oracle

The dbms_metadata utility is also used for punching source code, PL/SQL, and Java that is stored inside Oracle packages. Stored procedures or functions can also easily be extracted using dbms_metadata.

In the example in Listing D, I extract all procedures associated with the fred schema.
 

Listing D:

connect fred/flintstone;


 spool procedures_punch.lst


 select
   DBMS_METADATA.GET_DDL('PROCEDURE',u.object_name)
 from
   user_objects u
 where
   object_type = 'PROCEDURE';
 

 spool off;

The dbms_metadata utility is especially good for extracting procedural code because all of the original formatting is retained. Listing E is an example of the output from dbms_metadata for stored procedures.

Issues with dbms_metadata

Like all new Oracle utilities, there are some shortcomings with dbms_metadata. When you have tables with foreign key referential integrity (e.g., Oracle’s references constraints), it would be nice to have dbms_metadata punch the table DDL in its proper order for reloading into another database. If you don't punch the tables in the proper order of foreign key referential integrity, a table may be added that references another table that has not yet been added. Making this change would involve adding a WHERE clause that queries dba_constraints and dba_cons_columns to determine the proper table order.

Despite the immediate shortcomings, the dbms_metadata utility is fantastic for extracting DDL and procedural code from Oracle. I have no doubt a future release of Oracle will make dbms_metadata even more powerful.