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       TRUNCATE - empty a table or set of tables


       TRUNCATE [ TABLE ] [ ONLY ] name [, ... ]


       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.


       name   The   name  (optionally  schema-qualified)  of  a  table  to  be
              truncated. If ONLY is specified, only that table  is  truncated.
              If  ONLY  is  not  specified,  the  table and all its descendant
              tables (if any) are truncated.

              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 CASCADE.

              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 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

              COPY foo FROM ...;

              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.


       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;


       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.