Re: Planning without reason.

Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> On Fri, Jun 23, 2006 at 03:57:19PM +0300, Tzahi Fadida wrote:
>> (SELECT * FROM R
>> WHERE a=3, b=6,. ...)
>> UNION
>> (SELECT * FROM R
>> WHERE b=5, d=2,. ...)
>> UNION
>> ....
>> And lots of unions.

> Do you need UNION, or do you actually mean UNION ALL?

> Also, couldn't you just do:

> SELECT * FROM R
> WHERE (a=3, b=6, ...)
> OR (b=5, d=2, ...)
> etc

That seems to be what Tzahi wants the system to do for him. But the OR
format is not in general logically equivalent to either UNION or UNION
ALL, because UNION would cause duplicate output rows to be suppressed
whereas UNION ALL could allow the same table row to be emitted multiple
times (if two different WHERE conditions could select the same row).

It's conceivable that the planner could prove that neither effect is
possible in a particular query and then make the transformation
automatically, but I'm not about to expend that kind of planning effort
on such an odd case --- checking for it would waste entirely too many
cycles in most cases.

regards, tom lane