Re: brin index vacuum versus transaction snapshots

> I think the real solution to this problem is to avoid use of
> GetTransactionSnapshot(), and instead use GetLatestSnapshot().  As far
> as I can see, that should completely close the hole.  This requires
> patching IndexBuildHeapRangeScan() to allow for that.

Actually I think there's another problem: if a transaction starts and
inserts a tuple into the page range, then goes to sleep, and then
another session does the summarization of the page range, session 1 is
seen as "in progress" by session 2 (so the scan does not see the new
tuple), but the placeholder tuple was not modified either because it was
inserted later than the snapshot.  So the update is lost.

I think the only way to close this hole is to have summarize_range()
sleep until all open snapshots are gone after inserting the placeholder
tuple and before acquiring the snapshot, similarly to how CREATE INDEX
CONCURRENTLY does it.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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