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Oracle Tips by Burleson

Rebuilding and Dropping Tables

To completely remove a table from the tablespace, use the DROP TABLE command. This command’s format follows:

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Oracle will drop the table regardless of its contents. The only time a drop will fail is when a table’s primary key is referenced by another table’s foreign key via a restraint clause. The DBA can check for this situation by looking at the DBA_CONSTRAINTS and the DBA_CONS_COLUMNS views. A view called USER_CROSS_ REFS provides this information on a user-by-user basis. Using the CASCADE CONSTRAINTS clause will force a CASCADE DELETE to occur in all child tables.

Rebuilding Tables

The DBA may have to rebuild a table or tables after maintenance, after a physical disk crash, or--the leading cause--due to operational stupidity. If the application designers were thoughtful enough to provide a build script, there is no problem. However, for legacy systems, systems that have been modified and not redocumented, or systems created on the fly, there may be no current build scripts, if there were any to begin with. In this case, the DBA is in trouble. How can this situation be prevented? Require build scripts for each application and keep them up to date. For existing, undocumented systems, the script TAB_RCT9i.sql from the Wiley Web site will create a build script for simple existing tables. For very complex tables, I suggest implementing a utility such as TOAD from Quest, Inc. In Oracle9i, the DBMS_METADATA can also be used to generate the DDL for any database object. Any rebuild script generator must be run before any loss has occurred. Due to the added complexity of Oracle9i, it is doubtful many systems created on the fly will that make use of the complex options and types. Over time, I will be revising TAB_RCT9i.SQL to handle the more complex Oracle9i structures, and newer versions will be made available at the Wiley Web site.