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TRUNCATE(7) PostgreSQL 9.2.24 Documentation TRUNCATE(7)
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 truncate. If 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.
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, "Caveats", in the documentation 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.
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.
PostgreSQL 9.2.24 2017-11-06 TRUNCATE(7)
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