Re: index-only-scan when there is an index on all columns

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> If one runs vacuum on a table (small or otherwise) that is currently
> choosing an index scan as its best plan how likely is it that post-vacuum
> an index-only plan would be chosen if the index type and column presence
> conditions are met?

Offhand I think it would always prefer IOS over regular indexscan if the
table is mostly all-visible.  The problem in this example was that other
choices dominate both.

> Also, I recall discussion that select statements will touch the visibility
> map (hence causing write I/O even in a read-only query) but [1] indicates
> that only vacuum will set them ddl will clear them.

Hm, I don't recall that, but I've not been involved in the last few rounds
of hacking on that mechanism.

            regards, tom lane