TRUNCATE(7)							   SQL Commands 						       TRUNCATE(7)

NAME

       TRUNCATE - empty a table or set of tables

SYNOPSIS

       TRUNCATE [ TABLE ] [ ONLY ] name [, ... ]
	   [ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]

DESCRIPTION

       TRUNCATE  quickly  removes  all rows from a set of tables. It has the same effect as an unqualified DELETE on each table, but since it does
       not actually scan the tables it is faster. Furthermore, it reclaims disk space immediately, rather than requiring a subsequent VACUUM oper-
       ation. This is most useful on large tables.

PARAMETERS

       name   The  name  (optionally  schema-qualified) of a table to be truncated. If ONLY is specified, only that table is truncated. If ONLY is
	      not specified, the table and all its descendant tables (if any) are truncated.

       RESTART IDENTITY
	      Automatically restart sequences owned by columns of the truncated table(s).

       CONTINUE IDENTITY
	      Do not change the values of sequences. This is the default.

       CASCADE
	      Automatically truncate 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
	      Refuse  to  truncate  if	any  of the tables have foreign-key references from tables that are not listed in the command. This is the
	      default.

NOTES

       You must have the TRUNCATE privilege on a table to truncate it.

       TRUNCATE acquires an ACCESS EXCLUSIVE lock on each table it operates on, which blocks all other concurrent operations on the table. If con-
       current access to a table is required, then the DELETE command should be used instead.

       TRUNCATE cannot be used on a table that has foreign-key references from other tables, unless all such tables are also truncated in the same
       command. Checking validity in such cases would require table scans, and the whole point is not to do one. The CASCADE option can be used to
       automatically  include  all  dependent tables -- but be very careful when using this option, or else you might lose data you did not intend
       to!

       TRUNCATE will not fire any ON DELETE triggers that might exist for the tables. But it will fire ON TRUNCATE triggers.  If ON TRUNCATE trig-
       gers  are  defined for any of the tables, then all BEFORE TRUNCATE triggers are fired before any truncation happens, and all AFTER TRUNCATE
       triggers are fired after the last truncation is performed. The triggers will fire in the order that the tables are to be  processed  (first
       those listed in the command, and then any that were added due to cascading).

	      Warning:	TRUNCATE  is not MVCC-safe (see in the documentation for general information about MVCC). After truncation, the table will
	      appear empty to all concurrent transactions, even if they are using a snapshot taken before the truncation occurred. This will  only
	      be  an  issue  for  a transaction that did not access the truncated table before the truncation happened -- any transaction that has
	      done so would hold at least an ACCESS SHARE lock, which would block TRUNCATE until that transaction completes.  So  truncation  will
	      not  cause  any  apparent  inconsistency	in the table contents for successive queries on the same table, but it could cause visible
	      inconsistency between the contents of the truncated table and other tables in the database.

       TRUNCATE is transaction-safe with respect to the data in the tables: the truncation will be safely rolled back if the surrounding  transac-
       tion does not commit.

	      Warning:	Any ALTER SEQUENCE RESTART operations performed as a consequence of using the RESTART IDENTITY option are nontransactional
	      and will not be rolled back on failure. To minimize the risk, these operations are performed only after all the rest  of	TRUNCATE's
	      work  is	done.  However,  there is still a risk if TRUNCATE is performed inside a transaction block that is aborted afterwards. For
	      example, consider

	      BEGIN;
	      TRUNCATE TABLE foo RESTART IDENTITY;
	      COPY foo FROM ...;
	      COMMIT;

	      If the COPY fails partway through, the table data rolls back correctly, but the sequences will be left with values that are probably
	      smaller than they had before, possibly leading to duplicate-key failures or other problems in later transactions.  If this is likely
	      to be a problem, it's best to avoid using RESTART IDENTITY, and accept that the new contents of the table will  have  higher  serial
	      numbers than the old.

EXAMPLES

       Truncate the tables bigtable and fattable:

       TRUNCATE bigtable, fattable;

       The same, and also reset any associated sequence generators:

       TRUNCATE bigtable, fattable RESTART IDENTITY;

       Truncate the table othertable, and cascade to any tables that reference othertable via foreign-key constraints:

       TRUNCATE othertable CASCADE;

COMPATIBILITY

       The SQL:2008 standard includes a TRUNCATE command with the syntax TRUNCATE TABLE tablename.  The clauses CONTINUE IDENTITY/RESTART IDENTITY
       also appear in that standard but have slightly different but related meanings.  Some of the concurrency behavior of this  command  is  left
       implementation-defined by the standard, so the above notes should be considered and compared with other implementations if necessary.

SQL - Language Statements					    2010-05-14							       TRUNCATE(7)