Re: unique constraint error

Hi Daniel,

Effectively, by creating a unique index on a table your are implementing a unique constraint on a table. So I guess from that perspective there is no difference.

However, in my opinion there are (at least) two good reasons for not creating a unique index directly:

1. Even though the table effectively has a constraint on it and is therefore implementing a business rule, this is *not* documented in the DD in the constraint views. If you were to look in dba_constraints, it would appear that there is no such constraint on your unique column. This is a pain and a potential trap for the unwary.
2. You lose flexibility in the *type* of unique (or primary key) constraint you wish to implement. You can't implement a deferrable constraint if a unique index exists already on the column. This is a pain and a potential trap for the unwary.

By implementing a unique or primary key constraint directly, Oracle will automatically create an appropriate index (unique/non unique) AND Oracle will document all the necessary info in the DD.

I always question the creation of a unique index for these reasons.

Regards

Richard
"Daniel Morgan" <dmorgan_at_exesolutions.com> wrote in message news:3D3C9353.B151C88C_at_exesolutions.com...
> "Howard J. Rogers" wrote:
>
> > And Oracle's own documentation advises you not to create Unique indexes.
> >
> > Regards
> > HJR
> >
> > "TurkBear" <jgreco1_at_mn.rr.com> wrote in message
> > news:ps2ojuo8ru3p2kjnmqihgame31qu6pj8nb_at_4ax.com...
> > >
> > > In general it is a short way to create both an index and constraint..
> > > A Unique index is usually faster than a non-unique one the unique
> > constraint enforces business rules about your data and
> > > helps maintain 'normalized' data in a RDBMS by not allowing the
accidental
> > insertion or creation of duplicate keys..
> > >
> > >
> > >
> > > "Cel" <cefernan_at_capgemini.fr> wrote:
> > >
> > > >so what exactly is a unique index ?
> > > >what's it used for ?
> > > >
> > > >"TurkBear" <jgreco1_at_mn.rr.com> a écrit dans le message news:
> > > >8qigju41jlklidlreeh9f480tvgmfhfk2e_at_4ax.com...
> > > >>
> > > >> Your procedure should check for the record and then do either an
update
> > or
> > > >an Insert..
> > > >>
> > > >> This is not an error, it is doing exactly what a Unique constraint
is
> > > >supposed to do..
> > > >>
> > > >> (BTW, cross-posting is not usually a good idea)
> > > >>
> > > >>
> > > >> "Cel" <cefernan_at_capgemini.fr> wrote:
> > > >>
> > > >> >I have a table A :
> > > >> >
> > > >> >CREATE TABLE A (
> > > >> >column1 VARCHAR2 (2) NOT NULL, column2 VARCHAR2 (4) NOT NULL,
column3
> > > >> >VARCHAR2 (2) NOT NULL,
> > > >> >
> > > >> >column4 VARCHAR2 (6) NOT NULL, column5 CHAR (8) NOT NULL, column6
CHAR
> > > >(8),
> > > >> >
> > > >> >column7 CHAR (1), column8 CHAR (1))
> > > >> >
> > > >> >There are 5 index on the first 5 columns:
> > > >> >CREATE UNIQUE A_IU ON "SCHEMA".A(column1 , column2 , column3 ,
column4
> > ,
> > > >> >column5 )
> > > >> >
> > > >> >TABLESPACE IDX_APP_A PCTFREE 5 STORAGE(INITIAL 16384 NEXT 8192
> > > >PCTINCREASE
> > > >> >0 ) ;
> > > >> >
> > > >> >I have a stored procedure which does either an INSERT :
> > > >> >
> > > >> >INSERT INTO A ( column1 , column2 , column3 , column4 , column5
,
> > > >column6
> > > >> >, column7 , column8 )
> > > >> > VALUES ( val1, val2, val3, val4, val5, val6, NULL, NULL ,
NULL);
> > > >> >
> > > >> >or an UPDATE :
> > > >> >UPDATE A SET column6 = val11 AND column3 = val3 AND column4 =
val4;
> > > >> >
> > > >> >Of course if the record already exists in the table,
> > > >> >it raises a unique constraint error.
> > > >> >
> > > >> >How do i do to avoid this error ?
> > > >> >thanks for your help
> > > >> >
> > > >> >
> > > >> >
> > > >> >
> > > >>
> > > >>
> > > >>
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>
> Not disputing your statement ... but I'd be interested in the "Why"?
>
> Which sort of leads to the question of whether there is any difference
between a
> unique index and a unique constraint other than the syntax that creates
it.
>
> Daniel Morgan
>
Received on Mon Jul 22 2002 - 19:01:52 CDT