Re: Deferred uniqueness versus foreign keys

[Dean Rasheed <dean(dot)a(dot)rasheed(at)googlemail(dot)com>]

2009/7/28 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> [sigh, forgot to cc hackers the first time ]
>
> Foreign key behavior is only sane if the referenced column(s) are
> unique.  With the proposed patch, it is possible that the uniqueness
> check on the referenced columns is deferred, which means it might not
> occur till after an FK check does.  Discuss.
>
>                        regards, tom lane
>

Hmm, yes, looking in the SQL spec, I've just noticed this under 11.8,
referential constraint definition:

"The table constraint descriptor describing the <unique constraint
definition> whose <unique column list> identifies the referenced
columns shall indicate that the unique constraint is not deferrable."

which seems like a sensible policy now that I think about it.

 - Dean