Question

Trying to create this example table structure in Postgres 9.1:

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)
);

Running the above code produces an error, which does not make sense to me:

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

Can anyone explain why this error arises?

Answer 1

It’s because the name column on the bar table does not have the UNIQUE constraint.

So imagine you have 2 rows on the bar table that contain the name 'ams' and you insert a row on baz with 'ams' on bar_fk, which row on bar would it be referring since there are two rows matching?