1-to-1 relationship in PostgreSQL

Today I received mail from one of my clients:

I am using MicroOLAP Database Designer to develop a ERD for a PostGIS database. In my opinion one drawback to PgMDD is the lack of an explicit cardinality determination on the relationship. It is not obvious how to declare two tables in a IS_A relationship (1:1) and there is no sufficient explanation in the help section.
Would you be so kind as to support me with some information how I can determine a 1:1 cardinality on a relation.

I don’t want to discuss the reasons why one may need 1-to-1 relationship design. I just want to propose solution for it. Trick is simple, I suppose. All we need is to add unique constraint (or index) to child table.

Let’s imagine we have two tables Users and UserProfiles:

CREATE TABLE "UserProfiles" (
        "UProfileID" SERIAL NOT NULL,
...
        PRIMARY KEY("UProfileID")
);

CREATE TABLE "Users" (
        "UID" SERIAL NOT NULL,
        "UProfileID" int4 DEFAULT 0,
...
        PRIMARY KEY("UID"),
        UNIQUE("UProfileID"),
        FOREIGN KEY ("UProfileID") REFERENCES "Users"("UProfileID")
);