Re: automatic unique key genration

DANGER!
The rowid is not guaranteed to be unique across EXPORT IMPORT or other table defragmentation schemes (most use exp / imp).

The short story here is DON'T REINVENT THE SEQUENCE.

EndUser (enduser_at_enduser.com) wrote:
: to use max(cn)+1 you need t lock the table, get the value,
: or insert the row, then go on.
 

: to get a unique key which does not depend on a sequence, but is
: unique is to use the ROWID of the just inserted row, you should
: also lock tables/rows to insure your work.
 

: --
 

: In article <49hr6b$d2o_at_lyra.csx.cam.ac.uk>, Charles Jardine
: <cj10_at_cam.ac.uk> wrote:
 

: > Ramesh K Meda <74053.36_at_CompuServe.COM> wrote:
: > >Yes, unique key could be generated for primary key. There are two
: > >different ways.
: > >
: > >1. Use sequence and set value for new.<column_name>.
: > >2. Use max(PrimaryColumnName) + 1 to set new value.
: > >
: > >Depending on the platform and oracle release you might be using,
: > >you may receive "Mutating" error. Hope, you know how to solve
: > >this.
: >
: > Solution 2 above is simply wrong, first because of the mutating
: > problem and secondly because two concurrent transactions may well
: > see the same value of max(PrimaryColumnName) + 1. Any way of cheating
: > to get round there restriction on selecting from mutating tables
: > will make the second effect more likely.
: >
: > The whole point of sequences is that they are outside the transaction
: > structure, and can deliver guaranteed uniquness without blocking locks.
: > The only cost is that sequence numbers can be 'lost' because NEXTVALs
: > given to transaction which roll back are never given to anyone else.

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