DSXchange

Hi,

I'm having this job.

OCI stage--> Transformer --> OCI stage

The first OCI stage reads from a table, the second one updates the same table. The problem is one the volum of data is small, everything is OK. Now the job is trying to update nearly 20 millon records, it aborted because of following error 'snapshot too old: rollback segment number 2 with name "RBS_1" too small'. I asked DBA, he told me that is because I tried to select and upate the same table.

I am wondering if I dump the result of first query into seqential file, then update directly from seqential file, would that solve the problem? Also what's the best approach for such situations?

Thanks,

Carol

yiminghu wrote:Now the job is trying to update nearly 20 millon records, it aborted because of following error 'snapshot too old: rollback segment number 2 with name "RBS_1" too small'. I asked DBA, he told me that is because I tried to select and upate the same table.

That is contributing to the problem, but it's not the literal meaning of that particular error.

Yes, your approach would be a good way to work around the problem. It would also give you the ability to 'parallelize' the work if a single stream wasn't fast enough. Ken will counsel about MOD functions, bulk loads, use of the MERGE command, etc, etc. All of these can be considered.

You could also just do intermediate commits, assuming you have the Transaction Size set to zero right now. It would help in either approach but 'complicate' your restart/recovery plans.

Craigs suggestions are all good. But you have to realize that your SELECT in the source OCI is also consuming rollback segment because you're holding all of that source data as well. With 20+ million rows held, you're running the risk that other activity in the database will require rollback and they will fail, or your query will die because of the ongoing needs.

You need to get that data out of the source database as quickly as possible. You will NOT want to wait for the transformation and loading into the target table. The best choice is to write to a sequential text file. In the next job, read that file and it from there.

Here is the best link I have ever seen on Snapshot Too Old http://www.ixora.com.au/tips/admin/ora-1555.htm. As you can see, there are a couple of possible causes, but the PROBABLE cause in your case is that the SELECT is trying to perform a consistent point-in-time read on the data, which you are changing as it reads.

chullet wrote:You could also just do intermediate commits, assuming you have the Transaction Size set to zero right now. It would help in either approach but 'complicate' your restart/recovery plans.

There is a first time for everything: I disagree with Craig (here's where I need a "pensive" smiley).

More frequent COMMITs will not help. To get a consistent point-in-time read from your SELECT, it will still have to go to the rollback segments to see what the data used to look like. As the rollback segments get recycled, you may not be able to get them (hence Too Old error). In fact, you WILL NOT get a STO error if the Transaction Size in 0, because the writer OCI will not release the rollback segment, ensuring that they will not be recycled. Of course, 20M transactions is a lot with no commits.

Since you can get a Snapshot Too Old error when ANY user is changing the data you are reading, Ken's advice is spot-on ("relieved" smiley). Get the data out quick-as-you-can into a sequential file and run from there.

rleishman wrote:There is a first time for everything: I disagree with Craig (here's where I need a "pensive" smiley).

No problem. Looking back over it, you are right - I was getting my scenarios mixed up. At least I can blame it on it being 2 o'clock in the morning for me.

I didn't specifically state it as it's been covered here before and is a database issue not a DataStage issue - it *is* about Oracle attempting to maintain a point-in-time snapshot of data you need and it not being able to before your process completes. I didn't go into great detail on the 'get it out as quickly as possible' part as I knew Hurricane Ken would be along shortly to... what is the expression? 'Do the needful'.

Thanks for picking up on that.