Question

I use oracle 10.
I have update statement like that :

update table1 t1 
   set v_value=(select v_value 
                from table2 t2 
                where t2.user_id=t1.user_id  
                  and t2.item_id=t1.item_id  )

It works but takes too much time. How can I optimize it ?

Answer 1

You can try a merge statement:

merge into table1 t1
using
(
   select user_id,
          item_id,
          v_value
   from table2
) t2 ON (t1.user_id = t1.user_id and t1.item_id = t1.item_id)
when matched then update
  set v_value = t2.v_value;

(you might need to check the syntax, there were same changes to which parts in a MERGE are mandatory and which not between 10 and 11 – haven’t used 10g for a long time)