Cyask! info: MySQL Query Error

Time: 2012-1-28 7:26am
Script: /index.php

SQL: select qid,title,sid1,sid2,sid3,score from cyask_ques where status=1 and (select count(*) from cyask_answer where qid=cyask_ques.qid)=0 order by qid desc limit 9
Error: Error writing file '/var/tmp/MYxTBIrR' (Errcode: 28)
Errno.: 3