Managing extent deallocation with the TRUNCATE keyword

TRUNCATE is an SQL keyword that quickly deletes active rows from a table and the b-tree structures of its indexes, without dropping the table or its schema, access privileges, triggers, constraints, and other attributes. With this SQL data-definition language statement, you can depopulate a local table and reuse the table without re-creating it, or you can release the storage space that formerly held its data rows and b-tree structures.

Two implementations of TRUNCATE exist:
  • The first implementation, called "fast truncate," operates on most tables.
  • The second implementation, called "slow truncate," operates on tables that include opaque or smart large object data types, or inherited indexes that are defined on ROW types within data type hierarchies.

The performance advantages of using the TRUNCATE TABLE statement instead of the DELETE statement are much better for the fast truncate implementation, because this implementation does not examine or run all of the rows in a table. Slow truncation implementation occurs on tables that include opaque or smart large object data types or inherited indexes that are defined on ROW types within data types, because the truncate operation examines each row containing these items.

For more information about using TRUNCATE, see the HCL Informix® Guide to SQL: Syntax.


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