TRUNCATE TABLE

Purpose

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

• Deallocates all space used by the removed rows except that specified by the MINEXTENTS storage parameter

• Sets the NEXT storage parameter to the size of the last extent removed from the segment by the truncation process

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.

Prerequisites

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

Syntax

truncate_table::=

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 cluster.

• You can truncate index-organized tables and temporary tables. When you truncate a temporary table, only the rows created during the current session are removed.

• Oracle Database changes the NEXT storage parameter of table to be the size of the last extent deleted from the segment in the process of truncation.

• Oracle Database also automatically truncates and resets any existing UNUSABLE indicators for the following indexes on table: range and hash partitions of local indexes and subpartitions of local indexes.

• If table is not empty, then the database marks UNUSABLE all nonpartitioned indexes and all partitions of global partitioned indexes on the table. However, when the table is truncated, the index is also truncated, and a new high water mark is calculated for the index segment. This operation is equivalent to creating a new segment for the index. Therefore, at the end of the truncate operation, the indexes are once again USABLE.

• For a domain index, this statement invokes the appropriate truncate routine to truncate the domain index data.

  See Also:

  Oracle Database Data Cartridge Developer's Guide for more information on domain indexes

• If a regular or index-organized table contains LOB columns, then all LOB data and LOB index segments are truncated.

• If table is partitioned, then all partitions or subpartitions, as well as the LOB data and LOB index segments for each partition or subpartition, are truncated.

  Note:

  When you truncate a table, Oracle Database automatically removes all data in the table's indexes and any materialized view direct-path INSERT information held in association with the table. This information is independent of any materialized view log. If this direct-path INSERT information is removed, then an incremental refresh of the materialized view may lose data.

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

• You cannot individually truncate a table that is part of a cluster. You must either truncate the cluster, delete all rows from the table, or drop and re-create the table.

• You cannot truncate the parent table of an enabled foreign key constraint. You must disable the constraint before truncating the table. An exception is that you can truncate the table if the integrity constraint is self-referential.

• If a domain index is defined on table, then neither the index nor any index partitions can be marked IN_PROGRESS.

• You cannot truncate the parent table of a reference-partitioned table. You must first drop the reference-partitioned child table.

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.

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.

STORAGE Clauses

The STORAGE clauses let you determine what happens to the space freed by the truncated rows. The DROP 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 or cluster. 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.

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 or cluster was created. This space can subsequently be used only by new data in the table or cluster 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:

• You cannot roll back a TRUNCATE TABLE statement.

• All cursors are invalidated.

• You cannot flash back to the state of the table before the truncate operation.

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;