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Re: the XID question

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Charles(dot)Hou" <givemeegn(at)gmail(dot)com>, Filip Rembia*kowski <plk(dot)zuber(at)gmail(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: the XID question
Date: 2011-01-19 14:39:47
Message-ID: 4D36A35302000025000397CC@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-performance
Filip Rembia*kowski<plk(dot)zuber(at)gmail(dot)com> wrote: 
> 2011/1/19 Charles.Hou <givemeegn(at)gmail(dot)com>:
 
>> " select * from mybook" SQL command also increase the XID ?
> 
> Yes. Single SELECT is a transaction. Hence, it needs a transaction
> ID.
 
No, not in recent versions of PostgreSQL.  There's virtual
transaction ID, too; which is all that's needed unless the
transaction writes something.
 
Also, as a fine point, if you use explicit database transactions
(with BEGIN or START TRANSACTION) then you normally get one XID for
the entire transaction, unless you use SAVEPOINTs.
 
-Kevin

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