Re: Online enabling of checksums

On Fri, Apr 6, 2018 at 8:59 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> This is PROPARALLEL_RESTRICTED. That doesn't strike me right, shouldn't
> they be PROPARALLEL_UNSAFE? It might be fine, but I'd not want to rely
> on it.

Just a fine-grained note on this particular point:

It's totally fine for parallel-restricted operations to write WAL,
write to the filesystem, or launch nukes at ${ENEMY_NATION}. Well, I
mean, the last one might be a bad idea for geopolitical reasons, but
it's not a problem for parallel query. It is a problem to insert or
update heap tuples because it might extend the relation; mutual
exclusion doesn't work properly there yet (there was a patch to fix
that, but you had some concerns and it didn't go in). It is a problem
to update or delete heap tuples which might create new combo CIDs; not
all workers will have the same view (there's no patch for this yet
AFAIK, but the fix probably doesn't look that different from
cc5f81366c36b3dd8f02bd9be1cf75b2cc8482bd and could probably use most
of the same infrastructure).

TL;DR: Writing pages (e.g. to set a checksum) doesn't make something
non-parallel-safe. Writing heap tuples makes it parallel-unsafe.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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