| From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: MultiXactId error after upgrade to 9.3.4 |
| Date: | 2014-03-31 14:02:45 |
| Message-ID: | 20140331140245.GX9567@eldon.alvh.no-ip.org |
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| Thread: | |
| Lists: | pgsql-hackers |
Andres Freund wrote:
> On 2014-03-31 08:54:53 -0300, Alvaro Herrera wrote:
> > My conclusion here is that some part of the code is failing to examine
> > XMAX_INVALID before looking at the value stored in xmax itself. There
> > ought to be a short-circuit. Fortunately, this bug should be pretty
> > harmless.
> >
> > .. and after looking, I'm fairly sure the bug is in
> > heap_tuple_needs_freeze.
>
> heap_tuple_needs_freeze() isn't *allowed* to look at
> XMAX_INVALID. Otherwise it could miss freezing something still visible
> on a standby or after an eventual crash.
I think this rule is wrong. I think the rule ought to be something like
"if the XMAX_INVALID bit is set, then reset whatever is there if there
is something; if the bit is not set, proceed as today". Otherwise we
risk reading garbage, which is what is happening in this case.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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