| From: | Kevin Grittner <kgrittn(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Robert Haas <robertmhaas(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 18:16:35 |
| Message-ID: | CACjxUsOkMWZYFchAfjD1R4J40X5RYJ-YLo8xLPVmcHkBLSGspg@mail.gmail.com |
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| Lists: | pgsql-committers pgsql-hackers |
On Thu, Jun 16, 2016 at 1:01 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> The relevant part is the HeapTupleSatisfiesMVCC check, we're using
> SatisfiesToast for toast lookups.
>
> FWIW, I just tried to reproduce this with old_snapshot_threshold = 0 -
> but ran into the problem that I couldn't get it to vacuum anything away
> (neither main nor toast rel). That appears to be
> if (old_snapshot_threshold == 0)
> {
> if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
> && TransactionIdFollows(latest_xmin, xlimit))
> xlimit = latest_xmin;
> because latest_xmin always is equal to MyPgXact->xmin, which is actually
> kinda unsurprising?
Sure -- the STO feature *never* advances the point for early
pruning past the earliest still-active transaction ID. If it did
we would have all sorts of weird problems.
--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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