I wrote:
> Noah Misch <noah@leadboat.com> writes:
>> Just a suspicion ... when looking at the B-tree page reclamation algorithm, I
>> had a thought that the logic in _bt_page_recyclable() was obsolete as of the
>> introduction (in 8.3) of xid-free read-only transactions. A transaction
>> without a persistent xid does not hold back RecentXmin, so how could waiting
>> for a RecentXmin window to pass prove that no scan still holds a link to the
>> page? Similarly, running VACUUMs do not hold back RecentXmin.
> Uh, sure they do. It's their advertised snapshot xmin that counts, not
> their own xid (if any).
No, wait a second, I think you're right. The existing mechanism should
protect against transactions that might be updating the btree, so the
worst possible consequences can't happen; but it seems possible that a
read-only transaction in flight to the page could get confused and give
wrong answers. That would only explain transient failures not persistent
ones, though.
regards, tom lane
