TRUNCATE — empty a table or set of tables
TRUNCATE [ TABLE ] [ ONLY ] name [ * ] [, ... ]
    [ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]   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 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 IDENTITYAutomatically restart sequences owned by columns of the truncated table(s).
CONTINUE IDENTITYDo 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.
     
RESTRICTRefuse 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.  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 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.
  
   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.
  
   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
   .  The clauses
   tablenameCONTINUE 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.