尝试在 Postgres 9.1 中创建这个示例表结构：

CREATE TABLE foo (
    name        VARCHAR(256) PRIMARY KEY
);

CREATE TABLE bar (
    pkey        SERIAL PRIMARY KEY,
    foo_fk      VARCHAR(256) NOT NULL REFERENCES foo(name), 
    name        VARCHAR(256) NOT NULL, 
    UNIQUE (foo_fk,name)
);

CREATE TABLE baz(   
    pkey        SERIAL PRIMARY KEY,
    bar_fk      VARCHAR(256) NOT NULL REFERENCES bar(name),
    name        VARCHAR(256)
);

运行上面的代码会产生一个错误，这对我来说没有意义：

NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index

“foo_pkey” for table “foo”
NOTICE: CREATE TABLE will create implicit sequence “bar_pkey_seq” for
serial column “bar.pkey”
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
“bar_pkey” for table “bar”
NOTICE: CREATE TABLE / UNIQUE will create implicit index
“bar_foo_fk_name_key” for table “bar”
NOTICE: CREATE TABLE will create implicit sequence “baz_pkey_seq” for
serial column “baz.pkey”
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
“baz_pkey” for table “baz”
ERROR: there is no unique constraint matching given keys for referenced
table “bar”

********** Error **********

ERROR: there is no unique constraint matching given keys for referenced table "bar"
SQL state: 42830

谁能解释为什么会出现这个错误？