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You can drop a table by using the DROP TABLE table_name command. In order to be able to drop a table, the user must own the table (it must be in your schema), or the user must have the DROP ANY TABLE privilege.
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CHAPTER 5 SCHEMA MANAGEMENT
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When you use the DROP TABLE command, however, the table doesn t go away immediately Oracle simply renames the table and stores it in the recycle bin, which is in reality simply a data dictionary table. Thus, you can bring back a table you dropped accidentally by using the following command: SQL> FLASHBACK TABLE emp TO BEFORE DROP; The ability to bring back a dropped table is known as the Flashback Drop feature. 16 explains this feature in detail, and provides information about managing the recycle bin. If you are sure that you ll never need the table, you can get rid of it permanently by using the PURGE option with your DROP TABLE command, as shown here: SQL> DROP TABLE emp PURGE; When you use the preceding PURGE command, the emp table is dropped immediately, and you can t get it back! Again, you ll see a lot more about this command in 16.
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The DROP TABLE table_name PURGE command is equivalent to the old DROP TABLE table_name command.
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When you drop a table, all indexes you had defined on the table will be dropped as well. If the table you want to drop contains any primary or unique keys referenced by foreign keys of other tables, you must include the CASCADE clause in the DROP TABLE statement, in order to drop those constraints as well: SQL> DROP TABLE emp CASCADE CONSTRAINTS;
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The simple tables you saw in the previous sections satisfy most of the data needs of an application, but these aren t the only kind of tables Oracle allows you to create. You can create several kinds of specialized tables, such as temporary tables, external tables, and index-organized tables. In the following sections we ll examine these important types of tables.
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Oracle allows you to create temporary tables to hold data just for the duration of a session or even a transaction. After the session or the transaction ends, the table is truncated (the rows are automatically removed). Temporary tables are handy when you are dealing with complex queries or transactions that require transitory row information to be stored briefly before it is written to a permanent table. The data in temporary tables cannot be backed up like that in other permanent tables. No data or index segments are automatically allotted to temporary tables or indexes upon their creation, as is the case for permanent tables and indexes. Space is allocated in temporary segments for the temporary tables only after the first INSERT command is used for the tables. Temporary tables increase the performance of transactions that involve complex queries. One of the traditional responses to complex queries is to use a view to make the complex queries simpler to handle, but the view needs to execute each time you access it, thereby negating its benefits in many cases. Temporary tables are an excellent solution for cases like this, because they can be created as the product of complex SELECT statements used for the particular session or transaction, and they are automatically purged of data after the session.
CHAPTER 5 SCHEMA MANAGEMENT
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Although Oracle doesn t analyze the temporary table data to gather the data distribution, that s not a problem for efficient query processing, because the temporary tables can keep constantly accessed join and other information in one handy location. You can repeatedly access this table rather than having to repeatedly execute complex queries.
Temporary tables are created in the user s temporary tablespace and are assigned temporary segments only after the first INSERT statement is issued for the temporary table. They are deallocated after the completion of the transaction or the end of the session, depending on how the temporary tables were defined. Here are some attractive features of temporary tables from the Oracle DBA s point of view: Temporary tables drastically reduce the amount of redo activity generated by transactions. Redo logs don t fill up as quickly if temporary tables are used extensively during complex transactions. Temporary tables can be indexed to improve performance. Sessions can update, insert, and delete data in temporary tables just as in normal permanent tables. The data is automatically removed from the temporary table after a session or a transaction. Table constraints can be defined on temporary tables. Different users can access the same temporary table, with each user seeing only his or her session data. Temporary tables provide efficient data access because complex queries need not be executed repeatedly. The minimal amount of locking of temporary tables means more efficient query processing. The structure of the table persists after the data is removed, so future use is facilitated.
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