Re: [GENERAL] Using xmin and xmax for optimistic locking

On Mon, Feb 20, 2017 at 03:44:49PM -0500, Tom Lane wrote:

> >    where table.*::text = (saved from select).
>
> > If the row was changed between the time it was first read and updated, the
> > update will do touch any rows as the ::text will be different.
>
> > Why can't we use xmin and xmax columns to achieve the same.
>
> Well, that doesn't do quite the same thing: the cookbook query will
> proceed if there was a no-op update in between (or maybe even two updates
> that canceled each other out).  If you look at xmin then you won't proceed
> in such cases.  I could imagine either behavior being "right" depending on
> your application needs.

Also a consideration: table.*::text may become quite unwieldy
if there's one or more BYTEA columns in the table.

Karsten
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