Provided by: postgresql-client-8.4_8.4.11-1_i386
TRUNCATE - empty a table or set of tables
TRUNCATE [ TABLE ] [ ONLY ] name [, ... ]
[ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]
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.
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.
Automatically restart sequences owned by columns of the
Do not change the values of sequences. This is the default.
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.
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.
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
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. 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
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.
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
TRUNCATE TABLE foo RESTART IDENTITY;
COPY foo FROM ...;
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.
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;
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.