TRUNCATE is a SQL keyword that quickly deletes active rows from a table and the b-tree structures of its indexes. The active rows are deleted without dropping the table or its schema, access privileges, triggers, constraints, and other attributes.

1 Overview

The TRUNCATE table statement quickly removes all rows from a table and all corresponding b-tree index data. The space formerly occupied by data rows and indexes becomes available for other tables, or can optionally reserve the space for reuse in subsequent operations on the same table or index partition.

TRUNCATE is not equivalent to DROP TABLE. After TRUNCATE successfully executes, the specified table (and all its columns, and any synonyms, views, constraints, indexes, triggers, and access privileges) still exists in the database schema, but with no rows of data.

The TRUNCATE statement works on tables that contain any type of columns, including smart large objects.

The table that you truncate can be a raw, temporary, or standard table, synonym (no view name). It can also be a virtual table, or a table with a virtual-index interface, provided that the virtual table or the virtual-index interface has a valid am_truncate() purpose function that was explicitly registered in the system catalog table.

The Dynamic Server automatically updates the statistics and distributions for the table and its indexes. It is not necessary to run the UPDATE STATISTICS statement immediately after committing the TRUNCATE statement.

Dynamic Server always logs the TRUNCATE operation, even for a non-logging database.

2 Syntax

Copy 
>>-TRUNCATE [TABLE]-+--------+--+--------------+--+- table ---+-------->
                                '- 'owner' --.-'  '- synonym -'

   .- DROP STORAGE -.
>--+----------------+------------------------------------------>>
   '- REUSE STORAGE '

3 Advantages

Removing rows with the TRUNCATE TABLE statement is faster than removing them with the DELETE statement. It performs the removal of all rows and of the B-tree structures of every index on the table as a single operation, and writes a single entry in the logical log when the transaction that includes TRUNCATE is committed or rolled back.

TRUNCATE requires fewer updates to the system catalog than the equivalent DROP TABLE, CREATE TABLE, and any additional DDL statements to redefine any synonyms, views, constraints, triggers, privileges, fragmentation schemes, and other attributes and associated database objects of the table.

Remarks

If the table was fragmented, after the statement executes, each fragment has a space allocated for it that is the same size as that of the first extent size. The fragment size of any indexes also correspond to the size of the first extents.

Before you run the TRUNCATE statement:

Copy 
$ oncheck -pt demodb:customer2


TBLspace Report for demodb:informix.customer2

    Physical Address               13:5577
    Creation date                  10/11/2017 04:20:27
    TBLspace Flags                 902        Row Locking
                                              TBLspace contains VARCHARS
                                              TBLspace use 4 bit bit-maps
    Maximum row size               227
    Number of special columns      3
    Number of keys                 0
    Number of extents              5
    Current serial value           1
    Current SERIAL8 value          1
    Current BIGSERIAL value        1
    Current REFID value            1
    Pagesize (k)                   2
    First extent size              8
    Next extent size               2048
    Number of pages allocated      13371
    Number of pages used           13164
    Number of data pages           13160
    Number of rows                 150000
    Partition partnum              13631734
    Partition lockid               13631734

    Extents
         Logical Page     Physical Page        Size Physical Pages
                    0         13:817888         736        736
                  736         13:818660         133        133
                  869         13:821583        5337       5337
                 6206         13:818915         117        117
                 6323         13:826920        7048       7048

After you run the TRUNCATE statement:

Copy 
$ oncheck -pt demodb:customer2

TBLspace Report for demodb:informix.customer2

    Physical Address               13:5577
    Creation date                  10/11/2017 04:20:27
    TBLspace Flags                 902        Row Locking
                                              TBLspace contains VARCHARS
                                              TBLspace use 4 bit bit-maps
    Maximum row size               227
    Number of special columns      3
    Number of keys                 0
    Number of extents              1
    Current serial value           1
    Current SERIAL8 value          1
    Current BIGSERIAL value        1
    Current REFID value            1
    Pagesize (k)                   2
    First extent size              8
    Next extent size               2048
    Number of pages allocated      736
    Number of pages used           1
    Number of data pages           0
    Number of rows                 0
    Partition partnum              13631734
    Partition lockid               13631734

    Extents
         Logical Page     Physical Page        Size Physical Pages
                    0         13:817888         736        736

Warning

The triggered action of any enabled trigger on table is ignored.
Copy 
CREATE TABLE wic_dual (
    dummy varchar(1) default 'X' not null
);

INSERT INTO wic_dual VALUES ('X');

CREATE TRIGGER wic_dual_del
DELETE  ON wic_dual
REFERENCING OLD AS prv
  FOR EACH ROW
   WHEN (((SELECT COUNT(*) FROM wic_dual) = 0))
   (
     EXECUTE PROCEDURE wic_raise_msg(0, 'wic_dual_del: The [wic_dual] table must contain a single record.')
   );
Copy 
delete from wic_dual where 1=1;
#^
#  746: wic_dual_del: The [wic_dual] table must contain a single record.
#
Copy 
SELECT COUNT(*) FROM wic_dual;
      (count(*))

               1
               
TRUNCATE TABLE wic_dual;

SELECT COUNT(*) FROM wic_dual;
      (count(*))

               0

4 Restrictions

  • The table cannot be truncated if it has an open cursor or dirty readers.
  • To truncate a table that has an R-tree, you must first drop the index. After truncate the table re-create the R-tree index.
  • A view name can’t be use with truncate statement.
  • The TRUNCATE statement does not automatically reset the serial value of a column. To reset the serial value of a column, you must do so explicitly, either before or after you run the TRUNCATE statement.

5 Examples

Example

All following statements have the same effect, delete all rows and any related index data from the sales table:

Copy 
TRUNCATE TABLE wic_user_soaplogs;
TRUNCATE wic_user_soaplogs;
TRUNCATE wic_user_soaplogs DROP STORAGE;
Example

The following statement deletes all rows and any related index data from the sales table but prevent the space from being de-allocated::

Copy 
TRUNCATE TABLE wic_user_soaplogs REUSE STORAGE;