TRUNCATE

Function

TRUNCATE quickly removes all rows from a database table.

It has the same effect as an unqualified DELETE on each table, but it is faster since it does not actually scan the tables. This is most useful on large tables.

Precautions

• Exercise caution when running the TRUNCATE TABLE statement. Before running this statement, ensure that the table data can be deleted. After you run the TRUNCATE TABLE statement to delete table data, the data cannot be restored.

• In the storage-compute decoupling architecture, it is not possible to perform the TRUNCATE operation on common tables and temporary tables simultaneously.

• Avoid performing ALTER TABLE, ALTER TABLE PARTITION, DROP PARTITION, and TRUNCATE operations during peak hours to prevent long SQL statements from blocking these operations or SQL services.
• For more information about development and design specifications, see Development and Design Proposal.

TRUNCATE TABLE

• TRUNCATE TABLE has the same function as a DELETE statement with no WHERE clause, emptying a table.
• TRUNCATE TABLE uses less system and transaction log resources as compared with DELETE.
  • DELETE deletes a row each time, and records the deletion of each row in the transaction log.
  • TRUNCATE TABLE deletes all rows in a table by releasing the data page storing the table data, and records the releasing of the data page only in the transaction log.
• The differences between TRUNCATE, DELETE, and DROP are as follows:
  • TRUNCATE TABLE deletes content, releases space, but does not delete definitions.
  • DELETE TABLE deletes content, but does not delete definitions nor release space.
  • DROP TABLE deletes content and definitions and releases space.

Syntax

TRUNCATE empties a table or set of tables.

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TRUNCATE [ TABLE ] [ ONLY ] {[[database_name.]schema_name.]table_name [ * ]} [, ... ]
    [ CONTINUE IDENTITY ] [ CASCADE | RESTRICT ] ;

Parameter Description

• ONLY

  If ONLY is specified, only the specified table is cleared. Otherwise, the table and all its subtables (if any) are cleared.

• database_name

  Database name of the target table

• schema_name

  Schema name of the target table

• table_name

  Specifies the name (optionally schema-qualified) of a target table.

  Value range: an existing table name

• CONTINUE IDENTITY

  Does not change the values of sequences. This is the default.

• CASCADE | RESTRICT
  • CASCADE: automatically truncates all tables that have foreign-key references to any of the named tables, or to any tables added to the group due to CASCADE.
  • RESTRICT (default): refuses to truncate if any of the tables have foreign-key references from tables that are not listed in the command.

Examples

Clear a partitioned table:

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TRUNCATE TABLE tpcds.customer_address;