On 18 November 2012 08:52, Simon Riggs <si...@2ndquadrant.com> wrote:

> The difference here is that we still need to check visibility of each
> tuple, but that can be a very cheap check and never involves clog, nor
> does it dirty the page. Tuple access is reasonably expensive in
> comparison with a clog-less check on tuple xmin against snapshot xmin,
> so the extra work is negligible.

The attached *partial* patch shows how the tuple checks would work.

This should fit in neatly with the vismap skip code you've got already.

Looks to me possible to skip the all-vis hint completely, as you
suggest, plus avoid repeated checks of VM or clog.

 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

Attachment: all_visible_cheap_check.v1.patch
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