Problem with UNIQUE constraint on two columns

Hello.

I'm having a strange problem with a UNIQUE constraint on this set
of tables.

CREATE TABLE keys_public (
  key_row         INTEGER GENERATED ALWAYS AS IDENTITY NOT NULL PRIMARY KEY,
  key_id          VARCHAR(4)                           NOT NULL,
  key_fingerprint VARCHAR(4)                           NOT NULL,
  key_data        VARCHAR(4)                           NOT NULL
);

CREATE TABLE user_ids (
  user_id VARCHAR(4) NOT NULL PRIMARY KEY
);

CREATE TABLE user_public_keys (
  user_id VARCHAR(4) NOT NULL PRIMARY KEY,
  key_row INTEGER    NOT NULL,

  UNIQUE (user_id, key_row),
  FOREIGN KEY (user_id) REFERENCES user_ids    (user_id),
  FOREIGN KEY (key_row) REFERENCES keys_public (key_row)
);

INSERT INTO keys_public VALUES (DEFAULT, 'kid0', 'kfp0', '');
INSERT INTO user_ids VALUES ('u0');
INSERT INTO user_public_keys VALUES ('u0', 0);

SELECT * FROM keys_public;
SELECT * FROM user_ids;
SELECT * FROM user_public_keys;

At this point, everything is fine. However...

INSERT INTO keys_public VALUES (DEFAULT, 'kid1', 'kfp1', '');
SELECT * FROM keys_public;
INSERT INTO user_public_keys VALUES ('u0', 1);
-- integrity constraint violation: unique constraint or index violation;
SYS_PK_10124 table: USER_PUBLIC_KEYS

The keys_public table does contain a key_row with value 1. The
user_public_keys table contains one row ('u0', 0). There is no foreign
key or unique constraint violation that I can see.  What's going
on here?



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