Slow SQL Updates within a Cursor

Question

I've added a new column, packageNo, to my table:

 create table Packages(
  id varchar(20) primary key,  -- ok, I know :)
  orderNo uniqueIdentifier not null,
  orderlineNo int not null, 
  packageNo int not null default(0)
)

Now I want to generate the packageNo with the following rules:

reset it for each order

ascendantfor order, orderline

My problem is that the script I wrote uses 15 minutes for 26500 rows on my testServer. Here it is:

set NoCount ON
declare @Counter int
declare @handledCounter int
declare @currentorder uniqueIdentifier
declare @fetchedOrder uniqueIdentifier
declare @fetchedId varchar(20) -- will using PK speed up things?

declare PackageNo_Cursor cursor  for 
  select orderNo, id from packages order by orderNo, orderlineNo for update of packageNo

open PackageNo_Cursor
fetch next from PackageNo_Cursor into @fetchedOrder, @fetchedId 

set @currentOrder = @fetchedOrder
set @counter = 0
set @handledCounter = 0
while @@fetch_status = 0
 begin 
     if (@currentOrder <> @fetchedOrder)
       begin  -- reset counter for each order
        set @currentOrder = @fetchedOrder
        set @counter = 0
       end
     set @counter = @counter + 1
     set @handledCounter = @handledCounter +1
     if (@handledCounter % 50 = 0)
      begin
        print 'handled = ' + cast(@handledCounter as varchar) 
      end
   update packages set packageNo = @counter where current of PackageNo_Cursor    
     fetch next from PackageNo_Cursor into @fetchedOrder, @fetchedId 
  end

close PackageNo_Cursor
deallocate PackageNo_Cursor

This should result in:

id   - orderno - lineNo - packageNo (what I need to set)  
ean1 - guid1   - 1      - 1  
ean2 - guid1   - 2      - 2   
ean3 - guid2   - 1      - 1  
ean15- guid2   - 3      - 2  
ean15- guid2   - 4      - 3

Can I make this run any faster?

Answer 1

You don't want to use a cursor, you want to do this as a set, like so:

update p set
    packageNo = r.packageNo
from
    packages p
    inner join 
      (select orderNo, orderLineNo, 
       ROW_NUMBER() OVER(PARTITION BY orderNo ORDER BY orderLineNo) as packageNo
       from packages) r on
        p.orderNo = r.orderNo
        and p.orderLineNo = r.orderLineNo

This will leverage SQL Server's ROW_NUMBER function to get you the correct count by each line. Using UPDATE...FROM, we can create an inner join from which to update. This is vastly more efficient than the cursor.

See, cursors add an iterative ability to a procedural and set based language (SQL). The two don't play well together. That update statement is being called in order for each row (and opening/committing a transaction, to boot). If you do it all in one statement, SQL can parallelize this to make it much faster.

The standard rule is this: Use cursors as sparingly as possible. There are some fringe cases where they're necessary, but if you aren't doing massive amounts of SQL administration a day, it's doubtful you'll ever come across those cases.