Re: Disallow unique index on system columns

On 2016-04-15 13:26:35 +1200, David Rowley wrote:
> On 15 April 2016 at 13:02, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > David Rowley <david.rowley@2ndquadrant.com> writes:
> >> I proposed a fix over there, but it didn't go anywhere, probably
> >> because Tom and Andres discussed just disallowing unique indexes on
> >> system columns altogether. So, the attached patch does just that, and
> >> also fixes up the replica identity bugs too, as it's still possible
> >> that someone could create a unique index on a system column with an
> >> old version, upgrade, then try to set the replica identity to that
> >> index. We'd need to handle that correctly, so I fixed that too.
> >
> > AFAIR, what we were discussing was disallowing any index on a system
> > column (other than OID).  I do not see why only unique indexes are
> > problematic for them; the semantic issues are independent of that.
> 
> I have to admit that my thoughts only considered ctid, which I
> imagined would have been OK to have an index on. As for the other
> system columns (apart from OID), I agree.

What'd be the point of indexing ctid, and why would it be correct?
Wouldn't, hm, HOT break it?

Andres