Failed to execute the SQL query

 select i.title, i.id,  UNIX_TIMESTAMP( ifnull(i.pubdate, i.added)) as ts_,  year( ifnull(i.pubdate, i.added)) as y_,  month( ifnull(i.pubdate, i.added)) as m_,  dayofmonth( ifnull(i.pubdate, i.added)) as d_  from  Gre__item  i  where i.cid = 2   and not(i.unread & 4)  order by ts_ desc, i.id asc

Error 3: Error writing file '/tmp/MYDcHR3c' (Errcode: 28)