| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: HeapTupleSatisfiesToast() busted? (was atomic pin/unpin causing errors) |
| Date: | 2016-05-10 22:55:15 |
| Message-ID: | 22704.1462920915@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2016-05-10 18:29:59 -0400, Tom Lane wrote:
>> Having said that, I still say that changing HeapTupleSatisfiesToast
>> is the wrong thing. It can't go deciding not to return toast values
>> because they're committed dead --- the parent tuple could easily be
>> committed dead as well, and yet still be visible to our query's
>> snapshot.
> Hm. Shouldn't a mvcc snapshot be able to differentiate between those
> cases?
HeapTupleSatisfiesToast doesn't have one. And changing things so that
toast tuples are checked using MVCC rules is the wrong thing anyway,
because it would require adding hint-bit update traffic for toast
tables.
> When are we looking up toast tuple that's *not* visible to the
> current snapshot?
Once again, it's the parent tuple where we should be doing the
visibility check; noplace else.
regards, tom lane
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