Truncate (SQL)

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In SQL, the TRUNCATE TABLE statement is a Data Definition Language (DDL) operation that marks the extents of a table for deallocation (empty for reuse). The result of this operation quickly removes all data from a table, typically bypassing a number of integrity enforcing mechanisms. It was officially introduced in the SQL:2008 standard.

The TRUNCATE TABLE mytable statement is logically (though not physically) equivalent to the DELETE FROM mytable statement (without a WHERE clause). The following characteristics distinguish TRUNCATE TABLE from DELETE:

  • In the Oracle Database, TRUNCATE is implicitly preceded and followed by a commit operation. (This may also be the case in MySQL, when using a transactional storage engine.)
  • Typically, TRUNCATE TABLE quickly deletes all records in a table by deallocating the data pages used by the table. This reduces the resource overhead of logging the deletions, as well as the number of locks acquired. Records removed this way cannot be restored in a rollback operation. Two notable exceptions to this rule are the implementations found in PostgreSQL and Microsoft SQL Server, both of which allow TRUNCATE TABLE statements to be committed or rolled back transactionally.
  • You cannot specify a WHERE clause in a TRUNCATE TABLE statement—it is all or nothing.
  • TRUNCATE TABLE cannot be used when a foreign key references the table to be truncated, since TRUNCATE TABLE statements do not fire triggers. This could result in inconsistent data because ON DELETE/ON UPDATE triggers would not fire.
  • In some database systems, TRUNCATE TABLE resets the count of an Identity column back to the identity's seed.
  • In Microsoft SQL Server 2000 and beyond in full recovery mode, every change to the database is logged, so TRUNCATE TABLE statements can be used for tables involved in log shipping.[1]

References[edit]

  1. ^ "Description of the effects of nonlogged and minimally logged operations on transaction log backup and the restore process in SQL Server". Microsoft. December 2005. 

Notes[edit]