From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Thom Brown <thom(at)linux(dot)com> |
Cc: | Joachim Wieland <joe(at)mcknight(dot)de>, Marko Tiikkaja <marko(dot)tiikkaja(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: synchronized snapshots |
Date: | 2011-10-23 02:15:00 |
Message-ID: | 20263.1319336100@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Thom Brown <thom(at)linux(dot)com> writes:
> Can I ask why it doesn't return the same snapshot ID each time?
> Surely it can't change since you can only export the snapshot of a
> serializable or repeatable read transaction?
No, that's incorrect. You can export from a READ COMMITTED transaction;
indeed, you'd more or less have to, if you want the control transaction
to be able to see what the slaves do.
> A "SELECT
> count(pg_export_snapshot()) FROM generate_series(1,10000000);" would
> quickly bork the pg_snapshots directory which any user can run.
Shrug ... you can create a much more severe DOS problem by making
zillions of tables, if the filesystem doesn't handle lots-o-files
well.
regards, tom lane
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