Question

I have two concurrent processes and I have two queries, eg.:

select top 10 * into #tmp_member
from member
where status = 0
order by member_id

and then

update member
set process_status = 1
from member inner join #tmp_member m
on member.member_id=m.member_id

I’d like each process to select different rows, so if a row was already selected by the first process, then do not use that one in the second process’ result list.

Do I have to play around with locks? UPDLOCK, ROWLOCK, READPAST hints maybe? Or is there a more straightforward solution?

Any help is appreciated,

cheers,

b

Answer 1

You need hints.

See my answer here: SQL Server Process Queue Race Condition

However, you can shorten your query above into a single statement with the OUTPUT clause. Otherwise you’ll need a transaction too (asuming each process executes the 2 statements above one after the other)

update m
set process_status = 1
OUTPUT Inserted.member_id
from
  (
  SELECT top 10
      process_status, member_id
    from member WITH (ROWLOCK, READPAST, UPDLOCK)
    where status = 0
    order by member_id
  ) m

Summary: if you want multiple processes to

1. select 10 rows where status = 0
2. set process_status = 1
3. return a resultset in a safe, concurrent fashion

…then use this code.