Are unique / primary constraints without index possible?

 Hello,

I found some constraint definition scripts for our database without any index keyword.

They don't use the keyword "using index" or  don't use explicitly "create index".

And when I'm looking at dba_constraints then the column INDEX_NAME is empty (NULL) for these created constraints.

After the first search for "constraint without index" at Metalink and internet documents I though that is is possible but in most situations inperformant.

After a closer look at more documents I was really unsettled because now I was tols that this is not possible because Oracle implicitly defines a unique index whenever a primary or unique constraint is defined (except that an existing (non)unique index can satisfy the condition).