From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Kevin Grittner <kgrittn(at)ymail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Allow "snapshot too old" error, to prevent bloat |
Date: | 2015-02-16 21:35:46 |
Message-ID: | 20150216213546.GB24298@momjian.us |
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Lists: | pgsql-hackers |
On Sun, Feb 15, 2015 at 11:36:50AM -0500, Tom Lane wrote:
> I wonder if we couldn't achieve largely the same positive effects through
> adding a simple transaction-level timeout option. That would be far
> easier for users to understand and manage, it would be trivial to allow
> specific high-value transactions to run with a laxer setting, it does not
> create any implementation-detail-dependent behavior that we'd be having to
> explain to users forevermore, and (not incidentally) it would be a lot
> simpler and more trustworthy to implement. There's no well-defined
> correlation between your setting and the net effect on database bloat,
> but that's true with the "snapshot too old" approach as well.
While we have prided ourselves on not generating query cancel messages
due to snapshot-too-old, there is the downside of bloat. I understand
the need to allow users to make the trade-off between bloat and query
cancellation.
It seems we already have a mechanism in place that allows tuning of
query cancel on standbys vs. preventing standby queries from seeing old
data, specifically
max_standby_streaming_delay/max_standby_archive_delay. We obsessed
about how users were going to react to these odd variables, but there
has been little negative feedback.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
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