truncate

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 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. 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 cascading).

              Warning: TRUNCATE is not MVCC-safe (see in the documentation for
              general information about MVCC). After truncation, 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 IDENTITY/RESTART
       IDENTITY also appear in that standard but have slightly different but
       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.



SQL - Language Statements         2014-02-17                       TRUNCATE(7)