update statement not working....

[lunas]

I have the following update statement bt it is not working, even after leaving it for 16 hrs it dint end and neighter any of the records got updated

update
allele_temp
set
allele_temp.Aut oMarkerID =
(
select
distinct(marker _temp.AutoMarke rID)
from
marker_temp
where
marker_temp.dbS NPrsID = allele_temp.dbS NPrsID
)
where
allele_temp.Aut oMarkerID is null

Can anyone tell me whts wrong in the query.
FYI
Allele_temp has 25.5 million records and
Marker_temp has 13.3 million records.

for a smaller dataset the query works fine

[debasisdas]

As the table contains more records i would like to know is there any indexes on the table.

[arr1]

one question
Is the inner query returning one row?
If not then will update work this way for multiple records being returned?

thanks,
ARR