Re: GetOldestXmin going backwards is dangerous after all

On 2 February 2013 00:24, Tom Lane <tgl@sss.pgh.pa.us> wrote:

>  * if allDbs is FALSE and there are no transactions running in the current
>  * database, GetOldestXmin() returns latestCompletedXid. If a transaction
>  * begins after that, its xmin will include in-progress transactions in other
>  * databases that started earlier, so another call will return a lower value.
>
> The reason this is a problem is that GetOldestXmin ignores XIDs of
> processes that are connected to other DBs.  It now seems to me that this
> is a flat-out bug.  It can ignore their xmins, but it should include
> their XIDs, because the point of considering those XIDs is that they may
> contribute to the xmins of snapshots computed in the future by processes
> in our own DB.  And snapshots never exclude any XIDs on the basis of
> which DB they're in.  (They can't really, since we can't know when the
> snap is taken whether it might be used to examine shared catalogs.)

Agree thats a bug.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services