Deferrable conditional unique constraints

I know I can create a deferrable constraint trigger to accomplish this but it 
would be cool if one could do it with regular unique constraint.   I'm trying 
to enforce that a company can only have *one* "is_preferred" business-field, 
where is_preferred is NOT NULL DEFAULT FALSE.   table company( id serial name 
varchar )   table businessfield_company( business_field_id FK businessfield 
company_id FK company )   I can then add a conditional UNIQUE index to enforce 
that a company can only have *one* preferred=true business-field:   create 
unique index company_bf_is_pref_idx on businessfield_company(business_field_id, 
company_id) where is_preferred;   However, I need it to be DEFERRABLE INITIALLY 
DEFERRED, which isn't possible (to my knowledge) with unique indexes.   It is 
possible to create deferrable unique constraints, but they cannot be 
conditional: alter table businessfield_company add constraint 
businessfield_company_is_pref_idx UNIQUE (company_id, is_preferred)where 
is_preferred deferrable initially deferred; ERROR:  syntax error at or near 
"where" Is this possible using "standard syntax" or do I have to use constraint 
triggers?   Thanks.   --
 Andreas Joseph Krogh <andreak@officenet.no>      mob: +47 909 56 963
 Senior Software Developer / CTO - OfficeNet AS - http://www.officenet.no
 Public key: http://home.officenet.no/~andreak/public_key.asc