| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Kevin Grittner <kgrittn(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Kevin Grittner <kgrittn(at)postgresql(dot)org>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, "pgsql-committers(at)postgresql(dot)org" <pgsql-committers(at)postgresql(dot)org> |
| Subject: | Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
| Date: | 2016-06-16 16:02:39 |
| Message-ID: | CA+TgmobTQ=71y-c5P9VjWxOREVYWvkKd2ajQ5-J5HRkebdZ8DQ@mail.gmail.com |
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| Lists: | pgsql-committers pgsql-hackers |
On Thu, Jun 16, 2016 at 11:37 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> If that were really true, why would we not have the problem in
>> current production versions that the toast table could be vacuumed
>> before the heap, leading to exactly the issue you are talking
>> about?
>
> The issue isn't there without the feature, because we (should) never
> access a tuple/detoast a column when it's invisible enough for the
> corresponding toast tuple to be vacuumed away. But with
> old_snapshot_timeout that's obviously (intentionally) not the case
> anymore. Due to old_snapshot_threshold we'll prune tuples which,
> without it, would still be considered HEAPTUPLE_RECENTLY_DEAD.
Is there really an assumption that the heap and the TOAST heap are
only ever vacuumed with the same OldestXmin value? Because that seems
like it would be massively flaky.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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