TRUNCATE(7)                                               SQL Commands                                               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 be truncated. If ONLY is specified, only that table is trun-
              cated. If ONLY is not specified, the table and all its descendant tables (if any) are truncated.

       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 com-
              mand. 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. 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. 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 cascad-
       ing).

              Warning: TRUNCATE is not MVCC-safe (see in the documentation for general information about  MVCC).  After  trunca-
              tion,  the  table will appear empty to all concurrent transactions, even if they are using a snapshot taken before
              the truncation occurred. This will only be an issue for a transaction that did  not  access  the  truncated  table
              before the truncation happened -- any transaction that has done so would hold at least an ACCESS SHARE lock, which
              would block TRUNCATE until that transaction completes. So truncation will not cause any apparent inconsistency  in
              the  table contents for successive queries on the same table, but it could cause visible inconsistency between the
              contents of the truncated table and other tables in the database.


       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.

              Warning: Any ALTER SEQUENCE RESTART operations performed as a consequence of using the RESTART IDENTITY option are
              nontransactional and will not be rolled back on failure. To minimize the risk, these operations are performed only
              after  all  the  rest of TRUNCATE's work is done. However, there is still a risk if TRUNCATE is performed inside a
              transaction block that is aborted afterwards. For example, consider

              BEGIN;
              TRUNCATE TABLE foo RESTART IDENTITY;
              COPY foo FROM ...;
              COMMIT;

              If the COPY fails partway through, the table data rolls back correctly, but the sequences will be left with values
              that  are  probably  smaller than they had before, possibly leading to duplicate-key failures or other problems in
              later transactions.  If this is likely to be a problem, it's best to avoid using RESTART IDENTITY, and accept that
              the new contents of the table will have higher serial numbers than the old.


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 IDEN-
       TITY/RESTART IDENTITY also appear in that standard but have slightly different but related meanings.  Some of the concur-
       rency  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.



SQL - Language Statements                                  2011-09-22                                                TRUNCATE(7)