RE: How do you genrate primary keys?

"Hit a table that keeps a counter" will not scale (will not perform at high concurrency). It will cause you no end of "buffer busy waits" waits, "latch free" waits for a cache buffers chains latch (even if db_block_buffers, _db_block_hash_buckets, and _db_block_hash_latches could be set to infinity), lots of unnecessary CPU service consumption due to the spinning (especially if you try to tinker with _spin_count), and possibly a wide range of side effects including "write complete waits" waits and others.

Cary Millsap
Hotsos Enterprises, Ltd.
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-----Original Message-----
Hemant K Chitale
Sent: Wednesday, November 05, 2003 8:25 AM To: Multiple recipients of list ORACLE-L

My comments [probably off-the-cuff without spending much time thinking the issues through .....?]

1. Hit a table that keeps a counter. Used to be a mechanism in the Oracle5 days [If I remember correctly, Sequences came in Oracle6]. Issues were with locking the single record used as the generator or scanning for the max(value) of the key. Not quite sure I understand how you encountered concurrency issues, though.
2. Stored sequences. Although I prefer not to use a Sequence as a PK in itself [preferring natural column/s which are Unique keys, with the NOT NULL, of course], I have used a Sequence in an Advanced Replication implementation that had no Primary Key and I needed a PK for Conflict Resolution [this was years ago and, if you ask me, I can't remember all the details]
3. SYS_GUID SYS_GUID I've never used. It doesn't generate a NUMBER value so it is not really similar to a Sequence. Can user's key in a SYS_GUID-generated value ? Is it really "human readable" or "recallable" as a plain NUMBER, Security Security Number, ZIP Code ??
4. Similar to SYS_GUID .. You hit on a fortuitous combination of columns.

Hemant

At 05:19 AM 05-11-03 -0800, you wrote:
>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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Hemant K Chitale
Oracle 9i Database Administrator Certified Professional My personal web site is : http://hkchital.tripod.com

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