Provided by: postgresql-client-9.1_9.1.3-2_i386 bug

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 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. 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).

           Warning
           TRUNCATE is not MVCC-safe (see Chapter 13, Concurrency Control, 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
       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.