focal (7) TRUNCATE.7.gz

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! Note in particular that when the table to be truncated is a partition, siblings partitions are
       left untouched, but cascading occurs to all referencing tables and all their partitions with no
       distinction.

       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. 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 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.

SEE ALSO

       DELETE(7)