The sequence jump and any other good solution

Question

Hi everyone,
One existing system apply for sequence number as report number. But sometime the report number will jump. Before create each report, the system will get the sequence number. Then the sequence number will plus one automatially. but how come the sequence number will jump from 32 to 250 itc. Seems it is impossible that user failed to create few hundreds report in a quite short term.
Any oralce bug caused the problem or good solution to replace current report number design?
Thanks in advance,
Hawk

Answer 1

Some users do like gap free serial numbers. To them it seems like a no-brainer, an easy thing to do. This is just one of the ways in which a real-world metaphor fails in RDBMS, because people don't understand database architecture. More importantly people have forgotten (or, increasingly, are too young to remember) how difficult this sort of thing used to be.
In the days before computers serial numbers ("sequences") were tracked by using ledger books or self-incrementing stampers. This was usually foolproof but: [list]
[*]If someone made a mistake - stamped a docket in error or entered an invalid application form - then we had a gap, there was no way to re-use that number.
[*]Only one person at a time could get the next number: only one person at a time can write in book. Which is fine if we just have a single accounts clerk, but a nightmare if we need to share those resources. If you lock the stamper in your drawer (for safekeeping) and go off to tea, I can't stamp any dockets until you come back.
[list]
So you can see that computers are actually quite like real life. The SEQUENCE object is geared up to be a shareable source of unique IDs. The trade-off for shareability is gaps. The only way to guarantee no gaps is the solution you have already, a code control table. But that introduces a bottleneck - you need to be careful to ensure that the table does not remained locked for a long period of time. What happens if someone starts to run a report and then goes off to tea? Will they prevent anybody else using the report? You could use AUTONOMOUS TRANSCATION to acheive that, but if the surrounding transaction is rolled back for any reason...you get a gap!
Ultimately this comes down to how often the report is run. If you've got hundreds of users running each day then you have a serious potential for resource contention. If it's only one guy running it once a week then it does not matter.
Cheers, APC