Re: UPDATE sql question

On Fri, 2003-08-01 at 10:16, Tom Lane wrote:
> Ron Johnson <ron(dot)l(dot)johnson(at)cox(dot)net> writes:
> > On Mon, 2003-07-28 at 03:24, Andrei Verovski wrote:
> >> What exactly will happen if UPDATE sql statement instructs to update
> >> some columns with the same values as already in the database? Will
> >> Postgres update only different values or it will simply modify all
> >> columns listed in UPDATE sql?
>
> > Looks like it does what you tell it to do...
>
> I think he was asking an implementation question, viz: does it skip the
> physical update if no values in a row actually change? The answer is
> no. I'd think that in most cases, the extra time spent checking to see
> whether the updated columns didn't change would be a net loss.

Would it always be a net loss, though?

If *none* of the fields were updated, then you could burn some CPU
(doing comparisons) to save a disk write.

CPUs are so fast, nowadays. How many microseconds *would* be spent?

Of course, one could always say, "Hey, application! Don't update
unchanged values!!!!".

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