sqlerr:-243 and isam err:-111 in esql/c

Dear All.. I've a esql/c ap running in RedHat Enterprise Server 5.x , the engine is 11.50.UC6 , also in RHEL 5.x ... the schema of the table is : create raw table "informix".ordertx ( ..... clordid integer, .... stkno char(6), .... ) extent size 200 next size 100 lock mode row; create index "informix".ordertx_4 on "informix".ordertx (stkno,clordid) using btree in dbspace1; The raw table with index in a buffered database !! The esql/c application encountered sql error:-243 with isam error : -111 sometimes , the ap runs in an endless loop everyday ,encountered this error about once for 3 months or longer , it seldom happens , but never stop happening!!! The following sql is the sql caused the -243 error , The data is about 150,000 rows , some other aps are doing delete and insert frequently , EXEC SQL SET ISOLATION TO DIRTY READ; EXEC SQL SET LOCK MODE TO WAIT 5 ; EXEC SQL WHENEVER ERROR STOP; ..... EXEC SQL select nvl(max(clordid),0) into $maxseqnox from ordertx where stkno = $stkid and function_code = "1" and errcode = "0000" ; I can say that , this sql executed for millions of times and then encountered sql error -243 once , don't know why ... Any suggestions ? sorry for my poor english ~~

This "may" be a good place for USELASTCOMMITTED set to all in the onconfig.