Michael Glaesemann <grzm@myrealbox.com> writes:
> I thought that the unique key would be checked at the end of the update
> statement:
No, it's never worked that way. Each row is checked as it is inserted.
With an example like this, you may or may not get a failure depending on
chance physical ordering of rows.
This is not per spec, and I believe we have a TODO to fix it, but a fix
is far from trivial.
regards, tom lane