Pardon me if this has been discussed before.
I believe that PG back-end does not version index rows the way it does the data
rows. Assume that the app updates a row frequently (several times in a second).
For each update, PG will create a new version. However I believe the primary
key index pointing to that row will point to only the first row and the back
end has to work its way traversing the list until it finds the appropriate row
matching the xmin/xmax. Does frequent vaccum take care of this by removing the
dead rows and hence reducing the commute.
