| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Toast issues with OldestXmin going backwards |
| Date: | 2018-04-26 19:09:01 |
| Message-ID: | 25213.1524769741@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I wonder if perhaps get_actual_variable_range() has a hazard of this
> type as well. If OldestXmin retreats, then the special snapshot type
> which it uses could see a row whose TOAST entry has been removed
> meanwhile.
Hm, yeah. I wonder if we could fix that by extracting the value from
the index rather than recomputing it from the heap tuple, as we do now.
We'd still have to visit the heap tuple to check liveness, but we'd
not need to extract anything from its data part.
Have we given up on the angle of "prevent OldestXmin from retreating"?
That seems like it'd be far more bulletproof than trying to work
around places that break one-by-one.
regards, tom lane
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