Re: brin index vacuum versus transaction snapshots

Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:

>> 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.

Please excuse my naiveté on the topic, but could you explain (or
point me to the documented explanation) of why we don't scan using
a non-MVCC snapshot and build the page range based on all non-dead
tuples? I understand that the range being scanned would need to be
locked, but we're OK with doing that for creation of other indexes.
(There is no mention of snapshots or locks in the BRIN README....)

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company