SQL

truncate table postgres

truncate my_table restart identity

truncate all tables in database postgresql

DO
$do$
BEGIN
   -- dangerous, test before you execute!
   RAISE NOTICE '%',  -- once confident, comment this line ...
   -- EXECUTE         -- ... and uncomment this one
   (SELECT 'TRUNCATE TABLE ' || string_agg(oid::regclass::text, ', ') || ' CASCADE'
    FROM   pg_class
    WHERE  relkind = 'r'  -- only tables
    AND    relnamespace = 'public'::regnamespace
   );
END
$do$;

truncate psql

TRUNCATE [ TABLE ] [ ONLY ] name [ * ] [, ... ]
    [ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]