TRUNCATE(7)		PostgreSQL 9.2.24 Documentation		   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 operation.
       This is most useful on large tables.

PARAMETERS
       name
	   The name (optionally schema-qualified) of a table to truncate. If
	   ONLY is specified before the table name, only that table is
	   truncated. If ONLY is not specified, the table and all its
	   descendant tables (if any) are truncated. Optionally, * can be
	   specified after the table name to explicitly indicate that
	   descendant tables are included.

       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. When
       RESTART IDENTITY is specified, any sequences that are to be restarted
       are likewise locked exclusively. If concurrent 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 triggers
       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 and any
       sequences are reset. 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).

       TRUNCATE is not MVCC-safe. After truncation, the table will appear
       empty to concurrent transactions, if they are using a snapshot taken
       before the truncation occurred. See Section 13.5, "Caveats", in the
       documentation for more details.

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

       When RESTART IDENTITY is specified, the implied ALTER SEQUENCE RESTART
       operations are also done transactionally; that is, they will be rolled
       back if the surrounding transaction does not commit. This is unlike the
       normal behavior of ALTER SEQUENCE RESTART. Be aware that if any
       additional sequence operations are done on the restarted sequences
       before the transaction rolls back, the effects of these operations on
       the sequences will be rolled back, but not their effects on currval();
       that is, after the transaction currval() will continue to reflect the
       last sequence value obtained inside the failed transaction, even though
       the sequence itself may no longer be consistent with that. This is
       similar to the usual behavior of currval() after a failed transaction.

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
       though 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.



PostgreSQL 9.2.24		  2017-11-06			   TRUNCATE(7)