What is xmin ?

[Antony Paul]

Hi,
I read about xmin in a thread on optimistic concurrency control.
Can you guys tell me how to interpret this value. I want to convert
this to date format. Is it possible ?

rgds
Antony Paul

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[Karsten Hilbert]

> Can you guys tell me how to interpret this value. I want to convert

> this to date format. Is it possible ?

No.

XMIN holds the id of the transaction which did the most recent
change to a row as is visible from within the current
transaction. Unless it wraps around (is that possible ?) older
transactions should have XMIN values lower than younger
transactions.

Karsten
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[Alvaro Herrera]

On Tue, Jan 18, 2005 at 02:43:09PM +0100, Karsten Hilbert wrote:

> XMIN holds the id of the transaction which did the most recent
> change to a row as is visible from within the current
> transaction. Unless it wraps around (is that possible ?) older
> transactions should have XMIN values lower than younger
> transactions.

Yes, it wraps around, that's why modulo arithmetic is used internally to
handle it. Beyond that, it skips a few (3 or 4) special values just
above zero.

--
Alvaro Herrera (<alvherre[@]dcc.uchile.cl>)
"There was no reply" (Kernel Traffic)

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[Martijn van Oosterhout]

On Tue, Jan 18, 2005 at 02:43:09PM +0100, Karsten Hilbert wrote:

> > Can you guys tell me how to interpret this value. I want to convert
> > this to date format. Is it possible ?

> No.
>
> XMIN holds the id of the transaction which did the most recent
> change to a row as is visible from within the current
> transaction. Unless it wraps around (is that possible ?) older
> transactions should have XMIN values lower than younger
> transactions.

Correct. XMIN is a transaction ID and it can wraparound. To counter
this there is a special XMIN value which means "older than any
transaction". Which the reason you must run VACUUM at least once every
billion transactions or so. Hardly a difficult request :)

There is no way to convert it to a date, since it doesn't represent a
date or even a point in time. It represents a transaction and how it
relates to other transactions.

Hope this helps,
--
Martijn van Oosterhout <kleptog@svana.org> [url]http://svana.org/kleptog/[/url]

> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

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[Christopher Browne]

In an attempt to throw the authorities off his trail, [email]antonypaul24@gmail.com[/email] (Antony Paul) transmitted:

> I read about xmin in a thread on optimistic concurrency control.
> Can you guys tell me how to interpret this value. I want to convert
> this to date format. Is it possible ?

xmin is a transaction ID number. There is no such conversion
possible.

[url]http://www.faqs.org/docs/ppbook/x4636.htm[/url]

It would be kind of neat if there were some table into which
transactions were logged; in that case, there would presumably be a
couple of timestamps, namely the time at which the transaction began
and the time at which the transaction ended. There is no such table
by default.
--
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[Alvaro Herrera]

On Tue, Jan 18, 2005 at 01:16:55PM -0500, Christopher Browne wrote:

> It would be kind of neat if there were some table into which
> transactions were logged; in that case, there would presumably be a
> couple of timestamps, namely the time at which the transaction began
> and the time at which the transaction ended. There is no such table
> by default.

It would be useful in a savepoints' world only if accompanied by the
time a subtransaction began and ended, and the information about
parent/child relationship.

--
Alvaro Herrera (<alvherre[@]dcc.uchile.cl>)
"Tiene valor aquel que admite que es un cobarde" (Fernandel)

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