Sequences CYCLEing -- was RE: How do you genrate primary keys?
Ah yes. The exception case when sequence numbers are not unique. Believe me, I've seen Sequences with low MAXVALUE [the guy decided that the the number would never exceed 4 digits and didn't want to waste resources and space]. And I do vaguely remember that I HAVE seen a Sequence CYCLE over and restart. Can't remember the details, though this was many years ago. It takes all kinds of developers and database designers to make Oracle interesting. Hemant At 03:29 PM 05-11-03 -0800, you wrote: In theory I suppose it's possible to have overlaps, but this has nothing to do with OPS/RAC. If you create the sequence to CYCLE (not the default) AND set MAXVALUE to something less than reasonable (the default is NOMAXVALUE which IIRC means 10 to the power 27) AND don't create a unique index on the column storing the sequence, then maybe you can end up with multiple rows having the same value? Never heard of anyone doing that, of course, but in theory ... Pete Controlling developers is like herding cats. Kevin Loney, Oracle DBA Handbook Oh no, it's not. It's much harder than that! Bruce Pihlamae, long-term Oracle DBA -Original Message- Millsap Sent: Thursday, November 06, 2003 7:34 AM To: Multiple recipients of list ORACLE-L I've never heard of an Oracle sequence not generating unique id's, OPS/RAC or not. Gaps, yes. Overlaps, never. Cary Millsap Hotsos Enterprises, Ltd. http://www.hotsos.com Upcoming events: - Performance Diagnosis 101: 11/19 Sydney - SQL Optimization 101: 12/8-12 Dallas - Hotsos Symposium 2004: March 7-10 Dallas - Visit www.hotsos.com for schedule details... -Original Message- Todd Boss Sent: Wednesday, November 05, 2003 1:09 PM To: Multiple recipients of list ORACLE-L There's six very good reasons listed below to NOT use SSN as your unique PK, and honestly I can't believe this is STILL an issue for any dba who deals w/ SSNs. These arguments are YEARS old. Isn't this Data Modelling 101? I know for sure this exact case is in every text i've read. How to deal with Natural keys: - Create a surrogate PK that the user never sees but guarantees uniqueness. - Create a separate (unique if you can) index on your natural key. - Go on with life. I'm a bit more concerned about what i'm hearing about Sequences. Is it true that sequences are NOT guaranteed to be unique?? After all this time listening to Oracle people scoff at the Sybase/Ms Sql identity feature and its inadequacies as compared to Sequences for generating sequential surrogate keys they're NOT guaranteed to be unique if you're working in a parallel processing environment?? Is this really true? Do Oracle developers have to depend on circa 1990 techniques to generate something as BASIC as a surrogate key by designing their own little lookup table systems? Or am I just reading this thread incorrectly? Todd I'm fully convinced. SSN should not be used as a PK. Can we also conclude that natural keys in general are only good if you sit in an ivory tower and do unrealistic lab test? Yong Huang --- Bellow, Bambi [EMAIL PROTECTED] wrote: Having worked for the government in a situation where we were actually tracking information BY Social Security Number, let me tell you the problems with it. 1) Social Security Numbers ARE NOT GUARANTEED TO BE UNIQUE 2) Individuals ARE NOT GUARANTEED TO HAVE ONE AND ONLY ONE Social Security Number 3) Not all Social Security Numbers are numeric 4) Not all Social Security Numbers which ARE numeric are 9 characters in length 5) Social Security Numbers can be changed by the holder 6) It is illegal to use the Social Security Number for any purpose other than that which the government specifically uses Social Security Numbers for (ie., the distribution of benefits). I'll bet *that* one is strictly enforced. HTH, Bambi. -Original Message- Sent: Wednesday, November 05, 2003 8:00 AM To: Multiple recipients of list ORACLE-L Tom, I think using a natural key such as Soc. Sec. # as the primary key is a good idea. You don't need to maintain the sequence so there's no performance issue associated with sequences. There's no issue of gaps. No index root block contention. It doesn't seem to be industry common practice though. In your college student case, changing primary keys is rare so it's not a big problem. Yong Huang --- Mercadante, Thomas F [EMAIL PROTECTED] wrote: Jonathan, I think your idea of a paper is a good one. But I think we need to back th question up to what the requirements are. First, to me, a primary key should not be something that a user would ever see or use. So the Soc. Sec. # is out. (A side issue - I used to work at a college. Want to know how many times we had to change the Soc. for an individual student because the parent filled the form out and used their soc, or the kid used the wrong one?). Any id entered by a user is subject to
Re: Sequences CYCLEing -- was RE: How do you genrate primary keys?
Being sort of DBA Doubting Tom, I have a bad habit of trying and testing stuff. Here is what happens with sequences: SQL create sequence test1 start with 1 maxvalue 4 cycle nocache; Sequence created. SQL select test1.nextval from dual 2 / NEXTVAL -- 1 SQL / NEXTVAL -- 2 SQL / NEXTVAL -- 3 SQL / NEXTVAL -- 4 SQL / NEXTVAL -- 1 SQL / NEXTVAL -- 2 SQL On 2003.11.08 10:54, Hemant K Chitale wrote: Ah yes. The exception case when sequence numbers are not unique. Believe me, I've seen Sequences with low MAXVALUE [the guy decided that the the number would never exceed 4 digits and didn't want to waste resources and space]. And I do vaguely remember that I HAVE seen a Sequence CYCLE over and restart. Can't remember the details, though this was many years ago. It takes all kinds of developers and database designers to make Oracle interesting. Hemant At 03:29 PM 05-11-03 -0800, you wrote: In theory I suppose it's possible to have overlaps, but this has nothing to do with OPS/RAC. If you create the sequence to CYCLE (not the default) AND set MAXVALUE to something less than reasonable (the default is NOMAXVALUE which IIRC means 10 to the power 27) AND don't create a unique index on the column storing the sequence, then maybe you can end up with multiple rows having the same value? Never heard of anyone doing that, of course, but in theory ... Pete Controlling developers is like herding cats. Kevin Loney, Oracle DBA Handbook Oh no, it's not. It's much harder than that! Bruce Pihlamae, long-term Oracle DBA -Original Message- Millsap Sent: Thursday, November 06, 2003 7:34 AM To: Multiple recipients of list ORACLE-L I've never heard of an Oracle sequence not generating unique id's, OPS/RAC or not. Gaps, yes. Overlaps, never. Cary Millsap Hotsos Enterprises, Ltd. http://www.hotsos.com Upcoming events: - Performance Diagnosis 101: 11/19 Sydney - SQL Optimization 101: 12/8-12 Dallas - Hotsos Symposium 2004: March 7-10 Dallas - Visit www.hotsos.com for schedule details... -Original Message- Todd Boss Sent: Wednesday, November 05, 2003 1:09 PM To: Multiple recipients of list ORACLE-L There's six very good reasons listed below to NOT use SSN as your unique PK, and honestly I can't believe this is STILL an issue for any dba who deals w/ SSNs. These arguments are YEARS old. Isn't this Data Modelling 101? I know for sure this exact case is in every text i've read. How to deal with Natural keys: - Create a surrogate PK that the user never sees but guarantees uniqueness. - Create a separate (unique if you can) index on your natural key. - Go on with life. I'm a bit more concerned about what i'm hearing about Sequences. Is it true that sequences are NOT guaranteed to be unique?? After all this time listening to Oracle people scoff at the Sybase/Ms Sql identity feature and its inadequacies as compared to Sequences for generating sequential surrogate keys they're NOT guaranteed to be unique if you're working in a parallel processing environment?? Is this really true? Do Oracle developers have to depend on circa 1990 techniques to generate something as BASIC as a surrogate key by designing their own little lookup table systems? Or am I just reading this thread incorrectly? Todd I'm fully convinced. SSN should not be used as a PK. Can we also conclude that natural keys in general are only good if you sit in an ivory tower and do unrealistic lab test? Yong Huang --- Bellow, Bambi [EMAIL PROTECTED] wrote: Having worked for the government in a situation where we were actually tracking information BY Social Security Number, let me tell you the problems with it. 1) Social Security Numbers ARE NOT GUARANTEED TO BE UNIQUE 2) Individuals ARE NOT GUARANTEED TO HAVE ONE AND ONLY ONE Social Security Number 3) Not all Social Security Numbers are numeric 4) Not all Social Security Numbers which ARE numeric are 9 characters in length 5) Social Security Numbers can be changed by the holder 6) It is illegal to use the Social Security Number for any purpose other than that which the government specifically uses Social Security Numbers for (ie., the distribution of benefits). I'll bet *that* one is strictly enforced. HTH, Bambi. -Original Message- Sent: Wednesday, November 05, 2003 8:00 AM To: Multiple recipients of list ORACLE-L Tom, I think using a natural key such as Soc. Sec. # as the primary key is a good idea. You don't need to maintain the sequence so there's no performance issue associated with sequences. There's no issue of gaps. No index root block contention. It doesn't seem to be industry common practice though. In your college student case, changing primary keys is rare so it's not a big problem. Yong Huang --- Mercadante, Thomas F [EMAIL PROTECTED] wrote: Jonathan,
