| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>, Kevin Grittner <kgrittn(at)postgresql(dot)org>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
| Date: | 2016-07-20 15:26:11 |
| Message-ID: | CA+TgmoZWk1NK=kkU=v10p3YHKF6qNydttSD1v+XL2azp5PusyA@mail.gmail.com |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-committers pgsql-hackers |
On Wed, Jul 20, 2016 at 3:39 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>I think Snapshot's members whenTaken and lsn are updated/initialized
>>only in GetSnapshotData(). So if GetSnapshotData() is not used, how
>>will you expect those fields to be updated. We need those fields to
>>be updated for TestForOldSnapshot().
>
> That's why I suggested copying them from the current mvcc snapshot.
And how do you obtain that? The functions that reference
SnapshotToast are toast_delete_datum, toastrel_value_exists, and
toast_fetch_datum, toast_fetch_datum_slice, but none of those take a
snapshot as an argument, nor is there any reasonable way to make them
do so. Those are indirectly called by things like bttextcmp, which
don't know what snapshot was used to fetch the datum that they are
detoasting and can't reasonably be made to know.
I mean, you could do something *approximately* correct by calling
GetActiveSnapshot() but that doesn't seem likely to be correct in
detail.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Bruce Momjian | 2016-07-20 16:13:18 | Re: pgsql: doc: mention dependency on collation libraries |
| Previous Message | Kevin Grittner | 2016-07-20 13:37:59 | Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Joshua D. Drake | 2016-07-20 15:39:44 | Re: Design for In-Core Logical Replication |
| Previous Message | Robert Haas | 2016-07-20 15:15:05 | Re: Password identifiers, protocol aging and SCRAM protocol |