CodeSOD: A Unique Way to Primary Key

"This keeps giving me a primary key violation!" complained one of Nancy's co-workers. "Screw it, I'm dropping the primary key constraint!"

That was a terrifying thing to hear someone say out loud. Nancy decided to take a look at the table before anyone did anything they'd regret.

CREATE TYPE record_enum AS ENUM('parts');
CREATE TABLE IF NOT EXISTS parts (
    part_uuid VARCHAR(40) NOT NULL,
    record record_enum NOT NULL,
    ...
    ...
    ...
    PRIMARY KEY (part_uuid, record)
);

This table has a composite primary key. The first is a UUID, and the second is an enum with only one option in it- the name of the table. The latter column seems, well, useless, and certainly isn't going to make the primary key any more unique. But the UUID column should be unique. Universally unique, even.

Nancy writes:

Was the UUID not unique enough, or perhaps it was too unique?! They weren't able to explain why they had designed the table this way.

Nor were they able to explain why they kept violating the primary key constraint. It kept happening to them, for some reason until eventually it stopped happening, also for some reason.

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This post originally appeared on The Daily WTF.