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Re: is cachedFetchXid ever invalidated?

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: is cachedFetchXid ever invalidated?
Date: 2010-12-02 06:15:31
Message-ID: 1291270531.18031.184.camel@jdavis (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, 2010-12-01 at 23:34 -0500, Tom Lane wrote:
> Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> > I can't see any place that "cachedFetchXid" is ever invalidated.
> > Shouldn't it be invalidated before transaction ID wraparound?
> 
> The assumption is that the value won't sit there (in a particular
> session), without ever being replaced, while more than 2G transactions
> elapse.  Actually you'd need a full 4G transactions to elapse, and then
> to wake up just in time to probe the doppelganger of the very same
> transaction number, in order to have any risk of a failure.

Yeah, it's pretty far-fetched.

> One comparable failure case is that starting a transaction
> that acquires an XID, and then going to sleep for ~2G transactions,
> will cause all kinds of trouble.

I think it's well-known that holding a transaction open indefinitely
causes problems. I had assumed that a session was different (for
instance, a connection pool might keep connections around for a long
time). I'll re-align that assumption with reality.

Regards,
	Jeff Davis


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