Re: Shortcut evaluation in OR or IN

Thanks for the explanation.

On Mon, May 6, 2013 at 8:43 PM, Jasen Betts <jasen(at)xnet(dot)co(dot)nz> wrote:

> On 2013-05-06, Tim Uckun <timuckun(at)gmail(dot)com> wrote:
> > --047d7b2e4ea07402b004dc034a3b
> > Content-Type: text/plain; charset=UTF-8
> >
> > Say I have a select like this.
> >
> > SELECT * FROM table where field = X OR field = Y limit 1
> >
> > And I have two records one that matches X and one that matches Y will I
> > always get X because the evaluation will stop after the first clause in
> the
> > OR matches?
>
> no. there is no guarantee which matching row you will get. Testing may
> suggest that one answer is preferred but udating the table can change
> which one. also you may get a different row without updating the table.
>
> > What about for IN (X, Y)
>
> same deal.
>
> > how about if I am doing an update
> >
> > UPDATE table1 set x=table2.y where table1.field1 = table2.field1 OR
> > table1.field2=table2.field2
> >
> > Will it update based on field1 if both fields match?
>
> what difference does that make to the result?
>
> > Basically I want to know if and how OR shortcuts the evaluation.
>
> In a word. "unpredictably".
>
> The planner will try to do the cheapest, most useful side first.
>
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