Tom Lane wrote:
The main problem with the levels proposed by Takahiro-san is that any
transition from FROZEN to not-FROZEN *must* be exactly recovered,
because vacuum will never visit an allegedly frozen page at all.  This
appears to require WAL-logging DSM state changes, which is a pretty
serious performance hit.

I doubt it would be a big performance hit. AFAICS, all the information needed to recover the DSM is already written to WAL, so it wouldn't need any new WAL records.

 I'd be happier if the DSM content could be
treated as just a hint.  I think that means not trusting it for whether
a page is frozen to the extent of not needing vacuum even for
wraparound.  So I'm inclined to propose that there be only two states
(hence only one DSM bit per page): page needs vacuum for space recovery,
or not.  Vacuum for XID wraparound would have to hit every page

If we don't have a frozen state, we can't use the DSM to implement index-only scans. Index-only scans will obviously require a lot more work than just the DSM, but I'd like to have a solution that enables it in the future.

  Heikki Linnakangas

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