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.

TRUNCATE obtains an ACCESS EXCLUSIVE lock on each table it operates on, which blocks all other concurrent operations on that table. If concurrent access to the table is required, use the DELETE command instead.

Precautions

• 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 ];

• Truncate the data in a partition.

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ALTER TABLE [ IF EXISTS  ] { [ ONLY  ] [[database_name.]schema_name.]table_name  
                           | table_name *  
                           | ONLY ( table_name )  } 
    TRUNCATE PARTITION { partition_name  
                       | FOR (  partition_value  [, ...] )  } ;

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.
• partition_name

  Indicates the partition in the target partition table.

  Value range: An existing partition name.

• partition_value

  Specifies the value of the specified partition key.

  The value specified by PARTITION FOR can uniquely identify a partition.

  Value range: The partition key of the partition to be deleted.

  

  When the PARTITION FOR clause is used, the entire partition where partition_value is located is cleared.

Examples

Clear the p1 partition of the customer_address table.

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ALTER TABLE tpcds.customer_address TRUNCATE PARTITION p1;

Clear a partitioned table.

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TRUNCATE TABLE tpcds.customer_address;