Re: How do you genrate primary keys?

The only other method that I've seen that hasn't been mentioned is to generate the primary key of a new row as max(primary_key)+1. Inefficient as all get out but I've seen it done on small tables with very low volatility where the business rules required absolute sequentiality. It worked but I'd only recommend it under very specific circumstances.

                                                                                                                                       
                      Jonathan Gennick                                                                                                 
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                      11/05/2003 08:19                                                                                                 
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The recent article that mentioned sequences got me to thinking. I might pitch a more detailed article on sequences to Builder.com. But a more interesting article might be one that explored various ways to automatically generate primary keys. So, in the name of research, let me throw out the following questions:

What mechanisms have you used to generate primary keys? Which ones worked well, and why? Which mechanisms worked poorly?

I've run up against the following approaches:

• Hit a table that keeps a counter. This is the "roll your own sequence method". The one time I recall encountering this approach, I helped convert it over to using stored sequences. This was because of concurrency problems: with careful timing, two users could end up with the same ID number for different records. Is there ever a case when this roll-your-own approach makes sense, and is workable?
• Stored sequences. I worked on one app that used a separate sequence for each automatically generated primary key. I worked on another app, a smaller one, that used the same sequence for more than one table. The only issue that I recall is that sometimes numbers would be skipped. But end users really didn't care, or even notice.
• The SYS_GUID approach. I've never used SYS_GUID as a primary key generator. I wonder, was that Oracle's motivation for creating the function? Has anyone used it for primary keys in a production app? What's the real reason Oracle created this function?
• Similar to SYS_GUID, I once worked on an obituary-tracking application that built up a primary key from, as best I can recall now: date of death, part of surname, part of first name, and a sequence number used only to resolve collisions, of which there were few. The approached worked well, actually, because whatever fields we munged together to generate a primary key gave us a unique key the vast majority of the time.

The SYS_GUID approach is interesting, but if you need an ID number that users will see, and that users might type in themselves (e.g. social security number), is SYS_GUID really all that viable?

Best regards,

Jonathan Gennick --- Brighten the corner where you are http://Gennick.com * 906.387.1698 * mailto:jonathan@gennick.com

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