Re: [Gnumed-devel] Comments on 0.2

[Karsten Hilbert]

On Wed, Jun 21, 2006 at 07:44:36PM +1000, Ian Haywood wrote:


> Okay, let's think about this. We all accept the need for XMIN, it's explicit 
> locking
> we are querying.
yes

> Imagine two transactions running in parallel. Postgres forces them to 
> serialize,
If so configured which we do.

> (randomly if they hit the server at *exactly* the same time)
> Both do
> UPDATE foo SET A=B,C=D WHERE pk=123 and xmin=456;
> 
> If we are in read-committed mode, one query sees the result of the other 
> (made to wait if necessary),
> and so fails (Cursor.rowcount=0) as xmin doesn't match anymore.
It doesn't "fail", it just updates zero rows - which is just
a nitpick difference but needs to be checked for explicitely.

> In serialisation mode, they don't see each others results, instead one will 
> fail
> with a serialisation error, and forced to try again, whereupon it will fail
> (as it has now seen the results of the other)
yes

Karsten
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