Re: Visibility map and freezing

Zdenek Kotala <Zdenek(dot)Kotala(at)Sun(dot)COM> writes:

> What's about add second bit which mark frozen page (all tuples have freeze
> XID)? It should avoid full scan, but extend size of map.

That would only really work if you have a very static table where entire pages
get frozen and stay frozen long before the freeze_max_age is reached. Even
that wouldn't really work because the partial vacuums would never see those
pages.

One option is to keep a frozenxid per page which would allow us to visit only
pages that need freezing.

A more complex scheme would be to have a bit which indicates that all
non-frozen xids are > relfrozenxid+100M. When we find all the bits set we can
clear them all and bump relfrozenxid by 100M. This would allow regular partial
vacuums to gradually move the frozenxid forward.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
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