Re: [HACKERS] IN vs EXISTS equivalence

[Decibel!]

On Aug 11, 2008, at 3:40 PM, Gregory Stark wrote:

"Decibel!" <[EMAIL PROTECTED]> writes:

On Aug 8, 2008, at 3:23 PM, Tom Lane wrote:

* has no set operations (UNION etc), grouping, set-returning functions

in the SELECT list, LIMIT, or a few other funny cases

Couldn't union/union all be treated as

EXISTS(a)
OR EXISTS(b)

Kind of confused by what you mean here. Can you give an example?

The usual transformation to consider with UNION is to transform

SELECT ... WHERE x OR y

into

SELECT ...
 WHERE x
UNION ALL
SELECT ...
 WHERE y AND NOT x

It was a case of the union being inside the EXISTS subquery, ie:

WHERE EXISTS (SELECT * FROM a UNION SELECT * FROM b)

AFAIK that's identical to

WHERE EXISTS (SELECT * FROM a) OR EXISTS (SELECT * FROM b)


--
Decibel!, aka Jim C. Nasby, Database Architect  [EMAIL PROTECTED]
Give your computer some brain candy! www.distributed.net Team #1828

But as Tom mentioned, as soon as you have it you can't answer it with a simple join, so it doesn't matter...

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