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

• 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

• Delete data from a table.

  Truncate ::= TRUNCATE {[ TABLE ] [ ONLY ] {table_name [ * ]} [, ... ]
  [ CONTINUE IDENTITY ] [ CASCADE | RESTRICT][PURGE]};

• Truncate the data in a partition.

  TruncatePartition ::= ALTER TABLE [ IF EXISTS  ] { [ ONLY  ] table_name
                         | table_name *
                         | ONLY ( table_name )  }
  TRUNCATE PARTITION { partition_name
                     | FOR (  partition_value  [, ...] )  } [ UPDATE GLOBAL INDEX ];

Parameter Description

• ONLY

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

• table_name

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

  Value range: an existing table name

• CONTINUE IDENTITY

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

• CASCADE | RESTRICT

  • CASCADE: clears all tables that are added to a group.
  • RESTRICT (default value): clears all data.

• PURGE: purges table data in the recycle bin by default.

• partition_name

  Specifies the partition in the target partitioned table.

  Value range: an existing table name

• partition_value

  Specifies the value of the specified partition key.

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

  Value range: value range of the partition key for the partition to be renamed

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

• UPDATE GLOBAL INDEX

  If this parameter is used, all global indexes in the partitioned table are updated to ensure that data can be queried correctly using global indexes. If this parameter is not used, all global indexes in the partitioned table will become invalid.

Example

-- Create a table.
mogdb=# CREATE TABLE tpcds.reason_t1 AS TABLE tpcds.reason;

-- Truncate the tpcds.reason_t1 table.
mogdb=# TRUNCATE TABLE tpcds.reason_t1;

-- Delete the table.
mogdb=# DROP TABLE tpcds.reason_t1;
Copy Code-- Create a partitioned table.
mogdb=# CREATE TABLE tpcds.reason_p
(
  r_reason_sk integer,
  r_reason_id character(16),
  r_reason_desc character(100)
)PARTITION BY RANGE (r_reason_sk)
(
  partition p_05_before values less than (05),
  partition p_15 values less than (15),
  partition p_25 values less than (25),
  partition p_35 values less than (35),
  partition p_45_after values less than (MAXVALUE)
);

-- Insert data.
mogdb=# INSERT INTO tpcds.reason_p SELECT * FROM tpcds.reason;

-- Clear the p_05_before partition.
mogdb=# ALTER TABLE tpcds.reason_p TRUNCATE PARTITION p_05_before;

-- Clear the p_15 partition.
mogdb=# ALTER TABLE tpcds.reason_p TRUNCATE PARTITION for (13);

-- Clear the partitioned table.
mogdb=# TRUNCATE TABLE tpcds.reason_p;

-- Delete the table.
mogdb=# DROP TABLE tpcds.reason_p;