Re: Strange behaviour updating primary key column.

Luis Neves wrote:
> In the above table why does the query:
>
> UPDATE "story"
> SET id = (id + 1500);
>
> fails with:
> "ERROR: duplicate key violates unique constraint 'story_pkey'"
>
> (I have 6000 records in the table with "id" spanning from 1 to 6000)
>
> I find this behaviour strange, SQL is a set based language, but
> PostgreSQL is acting like is changing the values in sequence.

Quite right - it shouldn't happen. The issue is that the unique check
isn't deferred, but is made whenever a value changes. So 1=>1501 of
course gives the error.

The work-around usually given is to do two updates:
UPDATE story SET id = -id;
UPDATE story SET id = -id + 1500;

The real solution would be to check unique constraints at the end of
statement, but I assume this is a tricky change or it would have been
done by now.

--
Richard Huxton
Archonet Ltd