Re: [HACKERS] [PATCH] Incremental sort

I think solving this may be fairly straight-forward. Essentially, until
now we only had one way to do the sort, so it was OK to make the sort
implicit by checking if the path is sorted

    if (input not sorted)
    {
        ... add a Sort node ...
    }

But now we have multiple possible ways to do the sort, with different
startup/total costs. So the places that create the sorts need to
actually generate the Sort paths for each sort alternative, and store
the information in the Sort node (instead of relying on pathkeys).

Ultimately, this should simplify the createplan.c places making all the
make_sort calls unnecessary (i.e. the input should be already sorted
when needed). Otherwise it'd mean the decision needs to be done locally,
but I don't think that should be needed.

But it's surely a fairly invasive change to the patch ...