Name
TRUNCATE -- empty a table or set of tablesSynopsis
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. IfONLY
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 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.
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.
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
. The clauses tablename
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.
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