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Oracle® Database SQL Language Reference
11g Release 2 (11.2)

Part Number E17118-03
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TRUNCATE TABLE

Purpose

Caution:

You cannot roll back a TRUNCATE TABLE statement, nor can you use a FLASHBACK TABLE statement to retrieve the contents of a table that has been truncated.

Use the TRUNCATE TABLE statement to remove all rows from a table. By default, Oracle Database also performs the following tasks:

Removing rows with the TRUNCATE TABLE statement can be more efficient than dropping and re-creating a table. Dropping and re-creating a table invalidates dependent objects of the table, requires you to regrant object privileges on the table, and requires you to re-create the indexes, integrity constraints, and triggers on the table and respecify its storage parameters. Truncating has none of these effects.

Removing rows with the TRUNCATE TABLE statement can be faster than removing all rows with the DELETE statement, especially if the table has numerous triggers, indexes, and other dependencies.

See Also:

Prerequisites

To truncate a table, the table must be in your schema or you must have DROP ANY TABLE system privilege.

Syntax

truncate_table::=

Description of truncate_table.gif follows
Description of the illustration truncate_table.gif

Semantics

TABLE Clause

Specify the schema and name of the table to be truncated. This table cannot be part of a cluster. If you omit schema, then Oracle Database assumes the table is in your own schema.

Restrictions on Truncating Tables This statement is subject to the following restrictions:

MATERIALIZED VIEW LOG Clause

The MATERIALIZED VIEW LOG clause lets you specify whether a materialized view log defined on the table is to be preserved or purged when the table is truncated. This clause permits materialized view master tables to be reorganized through export or import without affecting the ability of primary key materialized views defined on the master to be fast refreshed. To support continued fast refresh of primary key materialized views, the materialized view log must record primary key information.

Note:

The keyword SNAPSHOT is supported in place of MATERIALIZED VIEW for backward compatibility.

PRESERVE Specify PRESERVE if any materialized view log should be preserved when the master table is truncated. This is the default.

PURGE Specify PURGE if any materialized view log should be purged when the master table is truncated.

See Also:

Oracle Database Advanced Replication for more information about materialized view logs and the TRUNCATE statement

STORAGE Clauses

The STORAGE clauses let you determine what happens to the space freed by the truncated rows. The DROP STORAGE clause, DROP ALL STORAGE clause, and REUSE STORAGE clause also apply to the space freed by the data deleted from associated indexes.

DROP STORAGE Specify DROP STORAGE to deallocate all space from the deleted rows from the table except the space allocated by the MINEXTENTS parameter of the table. This space can subsequently be used by other objects in the tablespace. Oracle Database also sets the NEXT storage parameter to the size of the last extent removed from the segment in the truncation process. This setting, which is the default, is useful for small and medium-sized objects. The extent management in locally managed tablespace is very fast in these cases, so there is no need to reserve space.

DROP ALL STORAGE Specify DROP ALL STORAGE to deallocate all space from the deleted rows from the table, including the space allocated by the MINEXTENTS parameter. All segments for the table, as well as all segments for its dependent objects, will be deallocated.

Restrictions on DROP ALL STORAGE This clause is subject to the same restrictions as described in "Restrictions on Deferred Segment Creation".

Note:

The DROP ALL STORAGE clause is available starting with Oracle Database 11g Release 2 (11.2.0.2).

REUSE STORAGE Specify REUSE STORAGE to retain the space from the deleted rows allocated to the table. Storage values are not reset to the values when the table was created. This space can subsequently be used only by new data in the table resulting from insert or update operations. This clause leaves storage parameters at their current settings.

This setting is useful as an alternative to deleting all rows of a very large table—when the number of rows is very large, the table entails many thousands of extents, and when data is to be reinserted in the future. TRUNCATE TABLE with REUSE STORAGE performs several orders of magnitude faster than deleting all rows, but has the following drawbacks:

This clause is not valid for temporary tables. A session becomes unbound from the temporary table when the table is truncated, so the storage is automatically dropped.

If you have specified more than one free list for the object you are truncating, then the REUSE STORAGE clause also removes any mapping of free lists to instances and resets the high-water mark to the beginning of the first extent.

Examples

Truncating a Table: Example The following statement removes all rows from a hypothetical copy of the sample table hr.employees and returns the freed space to the tablespace containing employees:

TRUNCATE TABLE employees_demo; 

The preceding statement also removes all data from all indexes on employees and returns the freed space to the tablespaces containing them.

Preserving Materialized View Logs After Truncate: Example The following statements are examples of TRUNCATE statements that preserve materialized view logs:

TRUNCATE TABLE sales_demo PRESERVE MATERIALIZED VIEW LOG; 

TRUNCATE TABLE orders_demo;