Re: dataset lock

On Apr 16, 2013, at 7:50 AM, Philipp Kraus <philipp.kraus@flashpixx.de> wrote:

> Hello,
>
> I use a PG database on a HPC system (cluster). My processes get a dataset from the database and change the row, each
processis independend. 
> My table shows something like: id, status, data
>
> id = PK a unqiue number
> status a enum value which "open", "waiting", "working", "done"
>
> So each process calls a SQL statement select * from where status = "waiting", so the process should get the next
waitingtask, after the process 
> gets the task, the status should be changed to "working", so no other process shouldn't get the task. My processes
areindepended, so it can 
> be, that 2 (or more) processes call the select statement at the same time and get in this case equal tasks, so I need
somelocking. How can 
> I do this with Postgres, that each row / task in my table is read / write by one process. On threads I would create a
mutexeg: 
>
> lock()
> row = select * from table where status = waiting
> update status = working from table where id = row.id
> unlock()
>
> do something with row
>
> Which is the best solution with postgres? should I create a procedure which takes the next job, change it and returns
theid, so each process 
> calls "select getNextJob()" ?

"select for update" might be the answer to what you're asking for - it'll lock the rows matched until the end of the
transaction,blocking any other select for update on the same rows. 

If performance is important then you might want to look at some of the off the shelf queuing systems instead - PgQ or
queue_classic,for instance. 

Cheers,
  Steve