Can this SQL statement be Optimized?

Question

This seems to be taking a very long time on large data sets. Will combining the first and last 3 queries into 1 make it faster? Does anyone have any input on what might make it faster? I appreciate it.

update "detail" set bal = (units * amount) where code_type = 'AB'; 
update "detail" set bal = (units * amount) where code_type = 'CD';
update "detail" set bal = (units * amount) where code_type = 'EF';
update "detail" set bal = (units * amount * -1) where code_type = 'GH';
update "detail" set bal = (units * amount * -1) where code_type = 'IK';
update "detail" set bal = (units * amount * -1) where code_type = 'LM';
update "detail" set bal = 0 where code_type = 'NO';

Additionally -

update bill set pay = 
  (select round(sum(bd1.bal),2) from "detail" bd1 where 
  bd1.inv = bill.inv and 
  (bd1.code_type = 'AB' or bd1.code_type = 'CD')); 
update bill set pay = 0 where pay is null;
update bill set cost = 
  (select round(sum(bd2.bal),2) from "detail" bd2 where 
  bd2.inv = bill.inv and 
  (not (bd2.code_type = 'AB' or bd2.code_type = 'CD'))); 
update bill set cost = 0 where cost is null;
update bill set balance = round(cost + pay,2);

Thanks

Answer 1

Performance probably stinks because you are updating the entire table, and you are updating it twelve times. If the table is seriously big, that's going to take time. Also, those two embedded subqueries will each get run once per row. Ouch.

The following frankenquery rolls everything into a single statement. It still has to hit the entire table, but at least it only does it once. I can't check syntax or test it against data, but this or something very much like it should work.

EDITED, split this into two updates (thus, two table scans requried)

UPDATE Detail
 set
   bal = case
           when code_type in ('AB','CD','EF') then  bi.units * bi.amount
           when code_type in ('gh','ik','lm') then -bi.units * bi.amount
           when code_type = 'NO' then 0
           else bal  --  If none of the above, change nothing
         end

and

UPDATE Bill
 set
   payments = isnull(bd1.amount, payments)  --  This changes nothing if nothing was calculated
  ,pay = case
           when pay is null then 0
           else pay
         end
   --  Ok, problem with cost: what if calculated amount is 0 and current value is non-zero?
   --  I've insufficient data here to correctly resolve all the possible permutations
  ,cost = case
            when bd2.amount is not null then cost
            when cost is null then 0
            else cost
          end
  ,balance = round(charges + isnull(bd1.amount, bi.payments), 2)
 from Bill bi
  --  These subqueries could be combined into one using similar CASE logic,
  --  and it would probably perform better (one table scan vs. two).  I'm
  --  leaving it this way for demonstration, and to keep the overall query
  --  a bit simpler.
  left outer join (select
                      inv
                     ,round(sum(bd1.bal), 2) amount
                    from detail
                    where code_type = 'AB'
                     or code_type = 'CD'
                    group by inv) bd1
   on bd1.inv = bi.inv  --  ADDED in second edit
  left outer join (select 
                      inv  --  RENAMED in second edit
                     ,round(sum(bd2.bal), 2) amount
                    from detail
                    where code_type <> 'AB'
                     and code_type <> 'CD'
                    group by inv) bd2  --  RENAMED in second edit
   on bd2.invoive = bi.inv  --  ADDED in second edit

Moral: the CASE statement can be the SQL developer's best friend.