Re: Comparing txid_current() to xmin

On Thu, Nov 08, 2012 at 11:47:37AM +0200, Marko Kreen wrote:
> On Wed, Nov 7, 2012 at 10:21 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On Tue, Nov 06, 2012 at 02:55:40PM -0800, Mike Lewis wrote:
> >> I am trying to make a trigger that updates a row once and only once per
> >> transaction (even if this trigger gets fired multiple times). The general
> >> idea is that for a user we have a version number. When we modify the
> >> user's data, the version number is incremented then set on the object. We
> >> only need to increment the version number once.
> >>
> >> I am thinking about doing something like:
> >>
> >> update user
> >> set version=version+1
> >> where txid_current() != xmin and user_id = 352395;
> >>
> >>
> >> So I guess my questions are:
> >>
> >> How dirty is this?
> >> Will I run into issues?
> >
> > It won't work in the presenence of subtransactions and is a bit more
> > complicated if you inserted the row in the same transaction.
>
> This can be solved by storing txid_current() into row
> and using that in comparision instead xmin/xmax.

If there is sufficient demand for this it should be easy enough to add a
function that checks for stuff like this using the information already
available in the backends memory.
The hardest part seems to be to find a good name...

It would basically only need to wrap
TransactionIdIsCurrentTransactionId.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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