Materialized view problem

Hi,

we have created materialized view with the below query .It is taking too much time,about 4 hrs for complete and 2 hrs for fast refresh.

select TIMESTAMP,CIRCLEID,decode(substr(charging,instr(charging,',',1,3)+1,
length(charging)),null,'Amount '||replace(substr(charging,instr(charging,',',-1,2)+1),',',' '),
case when instr(charging,':',1,1)>0 then substr(charging,(instr(charging,':',1,1)+1),length(charging))
else substr(charging,(instr(charging,',',1,3)+1),length(charging)) end ) CHARGING,
FROMADDRESS as shortcode,
nvl(sum(decode(RESULT,'0',WEIGHT)),0) as delivered,
nvl(sum(decode(RESULT,'2',WEIGHT,'1',WEIGHT)),0) as undelivered,