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.
The name (optionally schema-qualified) of a table to truncate.
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.
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
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
TRUNCATE will not fire any
ON DELETE triggers that might exist
for the tables. But it will fire
TRUNCATE triggers. If
TRUNCATE triggers are defined for any of the tables, then
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,
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
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.
Truncate the tables
TRUNCATE bigtable, fattable;
The same, and also reset any associated sequence generators:
TRUNCATE bigtable, fattable RESTART IDENTITY;
Truncate the table
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 . The clauses
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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