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
operation. This is most useful on large tables.
The name (optionally schema-qualified) of a table to truncate. If
ONLYis specified before the table name, only that table is truncated. If
ONLYis 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.
Automatically restart sequences owned by columns of the truncated table(s).
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
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. 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
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
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
triggers that might exist for the tables. But it will fire
ON TRUNCATE triggers.
ON TRUNCATE triggers are defined for any of
the tables, then all
BEFORE TRUNCATE triggers are
fired before any truncation happens, and all
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 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.
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
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
a failed transaction.
TRUNCATE is not currently supported for foreign tables.
This implies that if a specified table has any descendant tables that are
foreign, the command will fail.
Truncate the tables
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
othertable via foreign-key
TRUNCATE othertable CASCADE;
The SQL:2008 standard includes a
with the syntax
. The clauses
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.