Oracle PL/SQL Loop to Update Fields with Periodic Commits

You got very lucky that you didn’t run into exception ‘ora-01555 snapshot too old’.
Best would be to update in one statement, second best to do it in a loop that is restartable.
The code above is not restartable, this will always update all records.
My advice would be:

begin
loop
update dps_user
set password = lower(password)
where password != lower(password)
and rowcount <= 10000;
exit when sql%notfound; — this will be true after last records have been updated
commit;
end loop
end;

Might anything crash during execution, than you happily submit this code again and it will continue from wherever it crashed.

Cheers

Hi Devon,

When an update starts, Oracle ‘queries’ what data it will update. This is the starting point and needs to be consistent untill the update has finished. This is often referred to as read consistent view of the data.
Now, what is the use of that?
Say, for example, that another user would start inserting data while you’re still busy updating, then this user wouldn’t be happy to find that his new inserted data would be included in the update statement.
So, Oracle needs to keep track of the original state of the data as it was when the update started.

Now, how does commiting affect this read consistency?
While updating, undo information is saved. After a commit, this undo information is no longer protected.
This doesn’t have to be a problem, becaust Oracle can still access the information in the undo segment to create a read consistent view of the data.
But, the moment a another transaction reuses this undo space, the undo is overwritten by other undo and there is no longer read consistency: you’ll get the ‘ORA-01555: snapshot too old’ error.

Be aware that it doesn’t need to be another user to reuse undo: it might be yourself updating the next set.
Large and many undo segments will make it less likely to hit that error, but are no garantee.
You can have lots of undo space, but still run into this error, because after a commit a rollback segment will not grow but reuse the freed up space.
If your next transaction happens to use the same rollback segment, you’re hit the ORA-01555

To do it in one large update, how big should your rollback segment be?
That is hard to predict how much you need, I would suggest to just go for it.
If it fails, no damage is done. You can enlarge your undo tablespace and/or rollback segment and retry it.
If it doesn’t, fail, you’re home free.

I tried this update in my own test environment:
update dps_user set password = lower(password);
It took about 5 minutes, updated 3.7 million records and used 560 MB (70K undo blocks * 8K (db_block_size)).

I’m happy enough with that.

Cheers,

Peter

Hi,
Using the code Peter suggested results in an SQL error:
ORA-06550: line 5, column 7:
PL/SQL: ORA-00904: “ROWCOUNT”: invalid identifier

I used something like this:

begin
loop
update foo
set bar = lower(bar)
where rowcount <= 10000;
exit when sql%notfound;
commit;
end loop;
end;
/

(note that your original post lacked a semi-colon at the end of the “end loop” line)

How can I overcome the problem?

Thanks a ton both of you, this is just what I needed! I’d been in a similar situation where I needed to anonymise user specific data in a database that needed to be exported, to preserve customer privacy and wanted to loop through all records and put in some random values.