From: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> |
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To: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-committers(at)lists(dot)postgresql(dot)org |
Subject: | Re: pgsql: Support partition pruning at execution time |
Date: | 2018-04-08 03:34:22 |
Message-ID: | 876052idlu.fsf@news-spur.riddles.org.uk |
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Lists: | pgsql-committers pgsql-hackers |
>>>>> "David" == David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
>> Setting autovacuum_naptime to 10 seconds makes it occur in 10 second
>> intervals...
David> Ok, I thought it might have been some concurrent vacuum on the
David> table but the only tables I see being vacuumed are system
David> tables.
It's not vacuum that tends to be the problem, but analyze (on any
table). Lazy-vacuum's snapshots are mostly ignored for computing global
xmin horizons by other vacuums, but analyze's snapshots are not.
David> I tried performing a manual vacuum of each of these and could
David> not get it to trigger, but then I did:
David> select * from pg_class;
David> from another session and then the script starts spitting out
David> some errors.
Obviously, because the select holds a snapshot and therefore also holds
back OldestXmin.
You can't ever assume that data you just inserted will become
all-visible just because you just vacuumed the table, unless you know
that there is NO concurrent activity that might have a snapshot (and no
other possible reason why OldestXmin might be older than your insert).
--
Andrew (irc:RhodiumToad)
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