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Re: the XID question

From: Andres Freund <andres(at)anarazel(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Cc: Chris Browne <cbbrowne(at)acm(dot)org>
Subject: Re: the XID question
Date: 2011-01-19 18:31:51
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Wednesday, January 19, 2011 07:06:58 PM Chris Browne wrote:
> Kevin(dot)Grittner(at)wicourts(dot)gov ("Kevin Grittner") writes:
> > 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.
> Erm, "not *necessarily* in recent versions of PostgreSQL."
> A read-only transaction won't consume XIDs, but if you don't expressly
> declare it read-only, they're still liable to get eaten...
No. The Xid is generally only allocated at the first place a real xid is 
needed. See GetCurrentTransactionId, AssignTransactionId in xact.c and the 
caller of the former.


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