Search notes:

Oracle: Global Temporary Table (GTT)

create global temporary table tq84_temp (a number);
insert into tq84_temp values (42);

-- One record in user_objects:
select * from user_objects  where  object_name = 'TQ84_TEMP';

-- One record also in user_tables (with the value of TEMPORARY = Y):
select
   temporary,
   tablespace_name,
   duration
from
   user_tables
where
   table_name = 'TQ84_TEMP';

-- No record in user_segments:
select * from user_segments where segment_name = 'TQ84_TEMP';

Data dictionary

select
   tab.owner,
   tab.table_name,
   tab.duration
from
   dba_tables tab
where
   tab.temporary = 'Y'
;

ROWID

Global temporary tables have rowids which is demonstrated below.
We need a GTT with some data… 
create global temporary table tq84_gtt (
   num number,
   txt varchar2(10)
);

begin
   insert into tq84_gtt values (1, 'one'  );
   insert into tq84_gtt values (2, 'two'  );
   insert into tq84_gtt values (3, 'three');
   insert into tq84_gtt values (4, 'four' );
end;
/
The data is updated… 
begin

  for r in (select rowid, txt from tq84_gtt) loop
      update tq84_gtt set txt = upper(r.txt) where rowid = r.rowid;
  end loop;

end;
/
Show the updated values: 
select * from tq84_gtt;
       NUM TXT       
---------- ----------
         1 ONE       
         2 TWO       
         3 THREE     
         4 FOUR
Cleaning up: 
drop table tq84_gtt purge;

See also

temporary undo
The /*+ materialize */ SQL hint forces the data that is returned by a with clause to be stored in a global temporary table.
Pitfall when using GTTs in parallel execution.
The fixed table x$kxttstets stores session private stats for GTTs.
ORA-14452: attempt to create, alter or drop an index on temporary table already in use
Temporary tables.

Index

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 8 attempt to write a readonly database in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php:78 Stack trace: #0 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(78): PDOStatement->execute(Array) #1 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(30): insert_webrequest_('/notes/developm...', 1759777487, '216.73.216.149', 'Mozilla/5.0 App...', NULL) #2 /home/httpd/vhosts/renenyffenegger.ch/httpsdocs/notes/development/databases/Oracle/objects/tables/types/global-temporary/index(124): insert_webrequest() #3 {main} thrown in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php on line 78