Thread: What is xmin ?
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
> 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 -- GPG key ID E4071346 @ wwwkeys.pgp.net E167 67FD A291 2BEA 73BD 4537 78B9 A9F9 E407 1346
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)
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> http://svana.org/kleptog/ > 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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In an attempt to throw the authorities off his trail, antonypaul24@gmail.com (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. http://www.faqs.org/docs/ppbook/x4636.htm 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. -- wm(X,Y):-write(X),write('@'),write(Y). wm('cbbrowne','acm.org'). http://www.ntlug.org/~cbbrowne/languages.html We are in fact well and truly doomed. -- Jamie Zawinski http://www.jwz.org/gruntle/nscpdorm.html
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)