On 07/18/2018 10:58 AM, Heikki Linnakangas wrote:
On 18/07/18 16:29, Robert Haas wrote:
On Wed, Jul 18, 2018 at 9:06 AM, Michael Paquier
<mich...@paquier.xyz> wrote:
What's wrong with the approach proposed in
http://postgr.es/m/55afc302.1060...@iki.fi ?
For back-branches that's very invasive so that seems risky to me
particularly seeing the low number of complaints on the matter.
Hmm. I think that if you disable the optimization, you're betting that
people won't mind losing performance in this case in a maintenance
release. If you back-patch Heikki's approach, you're betting that the
committed version doesn't have any bugs that are worse than the status
quo. Personally, I'd rather take the latter bet. Maybe the patch
isn't all there yet, but that seems like something we can work
towards. If we just give up and disable the optimization, we won't
know how many people we ticked off or how badly until after we've done
it.
Yeah. I'm not happy about backpatching a big patch like what I
proposed, and Kyotaro developed further. But I think it's the least
bad option we have, the other options discussed seem even worse.
One way to review the patch is to look at what it changes, when
wal_level is *not* set to minimal, i.e. what risk or overhead does it
pose to users who are not affected by this bug? It seems pretty safe
to me.
The other aspect is, how confident are we that this actually fixes the
bug, with least impact to users using wal_level='minimal'? I think
it's the best shot we have so far. All the other proposals either
don't fully fix the bug, or hurt performance in some legit cases.
I'd suggest that we continue based on the patch that Kyotaro posted at
https://www.postgresql.org/message-id/20180330.100646.86008470.horiguchi.kyotaro%40lab.ntt.co.jp.
I have just spent some time reviewing Kyatoro's patch. I'm a bit
nervous, too, given the size. But I'm also nervous about leaving things
as they are. I suspect the reason we haven't heard more about this is
that these days use of "wal_level = minimal" is relatively rare.
I like the fact that this is closer to being a real fix rather than just
throwing out the optimization. Like Heikki I've come round to the view
that something like this is the least bad option.
The code looks good to me - some comments might be helpful in
heap_xlog_update()
Do we want to try this on HEAD and then backpatch it? Do we want to add
some testing along the lines Michael suggested?
cheers
andrew
--
Andrew Dunstan https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
