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Re: cheaper snapshots redux

From: Amit kapila <amit(dot)kapila(at)huawei(dot)com>
To: "robertmhaas(at)gmail(dot)com" <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: cheaper snapshots redux
Date: 2011-09-10 12:04:36
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
>> 4. Won't it effect if we don't update xmin everytime and just noting the committed XIDs. The reason I am asking is that it is used in tuple visibility check so with new idea in some cases instead of just returning >>     from begining by checking xmin it has to go through the committed XID list.

>> I understand that there may be less cases or the improvement by your idea can supesede this minimal effect. However some cases can be defeated.

>The snapshot xmin has to be up to date.  I'm not planning to break that because it would be wrong.

   In the approach mentioned in your idea, it mentioned that once after taking snapshot, only committed XIDs will be updated and sometimes snapshot itself.

   So when the xmin will be updated according to your idea.

>RecentGlobalXmin doesn't need to be completely up to date, and in fact recomputing it on every snapshot becomes prohibitively expensive with this approach.  I'm still struggling with the best way to handle that.

   RecentGlobalXmin and RecentXmin are mostly updated with snapshots xmin and that too outside ProcArrayLock, so why it will be expensive if you have updated xmin.

With Regards,

Amit Kapila.

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