From: | "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> |
---|---|
To: | "Simon Riggs" <simon(at)2ndquadrant(dot)com> |
Cc: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Neil Conway" <neilc(at)samurai(dot)com>, "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>, <pgsql-hackers(at)postgreSQL(dot)org> |
Subject: | Re: [PATCHES] Proposed patch: synchronized_scanning GUCvariable |
Date: | 2008-01-30 20:55:22 |
Message-ID: | 47A0E43A.7050106@enterprisedb.com |
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Lists: | pgsql-hackers pgsql-patches |
Simon Riggs wrote:
> On Wed, 2008-01-30 at 18:42 +0000, Heikki Linnakangas wrote:
>> It's even worse than that. Elsewhere in this thread Simon mentioned a
>> partitioned table, where each partition on its own is smaller than the
>> threshold, but you're seq scanning several partitions and the total size
>> of the seq scans is larger than memory size. In that scenario, you would
>> want BAS and synchronized scans, but even a per-table setting wouldn't
>> cut it.
>
>> For synchronized scans to help in the partitioned situation, I guess
>> you'd want to synchronize across partitions. If someone is already
>> scanning partition 5, you'd want to start from that partition and join
>> the pack, instead of starting from partition 1.
>
> You're right, but in practice its not quite that bad with the
> multi-table route. When you have partitions you generally exclude most
> of them, with typically 1-2 per query, usually different ones.
Yep. And in that case, you *don't'* want BAS or sync scans to kick in,
because you're only accessing a relatively small chunk of data, and it's
worthwhile to cache it.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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