I've previously noted upthread (along with several others), that I
don't see a good reason to limit this new capability to index only
scans. In addition to the reasons upthread, this also prevents using
the new feature on physical replicas since index only scans require
visibility map (IIRC) information that isn't safe to make assumptions
about on a replica.
That being said, it strikes me that this likely indicates an existing
architecture issue. I was discussing the problem at PGCon with Andres
and Heiki with respect to an index scan variation I've been working on
myself. In short, it's not clear to me why we want index only scans
and index scans to be entirely separate nodes, rather than optional
variations within a broader index scan node. The problem becomes even
more clear as we continue to add additional variants that lie on
different axis, since we end up with an ever multiplying number of
combinations.
In that discussion no one could remember why it'd been done that way,
but I'm planning to try to find the relevant threads in the archives
to see if there's anything in particular blocking combining them.
I generally dislike gating improvements like this on seemingly
tangentially related refactors, but I will make the observation that
adding the skip scan on top of such a refactored index scan node would
make this a much more obvious and complete win.
As I noted to Jesper at PGCon I'm happy to review the code in detail
also, but likely won't get to it until later this week or next week at
the earliest.
Jesper: Is there anything still on your list of things to change about
the patch? Or would now be a good time to look hard at the code?
James Coleman