Re: Improve error message for duplicate labels in enum types

Hi Jim,

On Mon, 25 Aug 2025 10:08:23 +0200
Jim Jones <jim(dot)jones(at)uni-muenster(dot)de> wrote:

> Hi Yugo
>
> On 03.07.25 17:04, Yugo Nagata wrote:
> > Currently, when creating an enum type, duplicate labels are caught by a unique
> > index on pg_enum, resulting in a generic error message.
> >
> > postgres=# create type t as enum ('a','b','a');
> > ERROR: duplicate key value violates unique constraint "pg_enum_typid_label_index"
> > DETAIL: Key (enumtypid, enumlabel)=(16418, a) already exists.
> >
> > I propose adding an explicit check for duplicate labels during enum creation,
> > so that a more user-friendly and descriptive error message can be produced,
> > similar to what is already done in ALTER TYPE ... ADD VALUE
> > or ALTER TYPE ... RENAME VALUE .. TO ....
> >
> > With the attached patch applied, the error message becomes:
> >
> > ERROR: label "a" used more than once
> >
>
> The error message for already existing enum labels starts with "enum",
> e.g.  ERROR:  enum label "bar" already exists. So, perhaps this new
> error message should follow the same pattern?

Thank you for taking a look. That makes sense, so I updated the message to:

ERROR: enum label "a" used more than once

> I also wonder if we need to add tests for it, so that we make sure the
> new error is triggered prior to the generic one, e.g. in create_type.sql
>
> -- check for duplicate enum entries
> CREATE TYPE den AS ENUM ('foo','bar','foo');
> CREATE TYPE en AS ENUM ('foo','bar');
> ALTER TYPE en ADD VALUE 'foo';
> ALTER TYPE en RENAME VALUE 'foo' TO 'bar';
> DROP TYPE en;

I also added a test for duplicate enum entries to enum.sql,
since tests for existing entries are already there.

Regards,
Yugo Nagata

--
Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>