Re: How to check whether the row was modified by this transaction before?

On 2012-12-06 15:08:51 -0300, Alvaro Herrera wrote:
> Tom Lane wrote:
> > Vlad Arkhipov <arhipov(at)dc(dot)baikal(dot)ru> writes:
> > > In a BEFORE UPDATE trigger I need to know whether the row was previously
> > > modified by this transaction. Is it safe to use xmin and txid_current()
> > > for this purpose (xmin is 32-bit txid type but txid_current() returns
> > > 64-bit bigint).

I wonder if we shouldn't have a function txid_is_current(xid); I could
have used that previously to avoid queuing multiple external
cache-invalidations if something gets repeatedly updated in the same
transaction. And I seem to remember some people here asking this
question before on the lists.

> >
> > > IF OLD.xmin = txid_current() THEN
> >
> > Comparing to txid_current() mod 2^32 would probably work,
>
> I think we should be setting the initial epoch to something other than
> zero. That way, some quick testing would have revealed this problem
> immediately.

+1, currently the difference of xid vs bigint is hard to spot.

Greetings,

Andres Freund

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