Update entry:

< * Allow heap reuse of UPDATEd rows if old and new versions are on the
<   same heap page?
> * Allow heap reuse of UPDATEd rows if no indexed columns are changed,
>   and old and new versions are on the same heap page?
<   This is possible for same-page updates because a single index row
<   can point to both old and new values.
>   While vacuum handles DELETEs fine, updating of non-indexed columns, like
>   counters, are difficult for VACUUM to handle efficiently.  This method
>   is possible for same-page updates because a single index row can be
>   used to point to both old and new values.