Re: Shortcut evaluation in OR or IN

On 2013-05-06, Tim Uckun <timuckun@gmail.com> wrote:
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> 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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