Performance penalty of visibility info in indexes?

Has anyone actually measured the performance overhead of storing
visibility info in indexes? I know the space overhead sounds
daunting, but even if it doubled the size of the index in many cases
that'd still be a huge win over having to scan the heap as well as
the index (esp. for things like count(*)). There would also be
overhead from having to update the old index tuple, but for the case
of updates you're likely to need that page for the new index tuple
anyway.

I know this wouldn't work for all cases, but ISTM there are many
cases where it would be a win.
--
Jim Nasby jim(at)nasby(dot)net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)