Re: [GENERAL] Negating the list of selected rows of a join

Clark Evans <clark(dot)evans(at)manhattanproject(dot)com>/Ulf Mehlig
<umehlig(at)pandora3(dot)uni-bremen(dot)de> wrote:

> > > Hope this will do the trick.
> >
> > Maybe not -- doesn't that mean, that the query won't return a
> > single row in case there is *any* pair of equal names in both
> > tables?!
>
> No. It's a correlated sub-query. It's actually much more efficient
> with large tables than the other approach (which has to put
> the entire result set of table_b on the heap before it can
> process table_a). Your approach is, however, much better (by a
> large factor) if table_a is very large and table_b is very small,
> since you don't have to hit the index for table_b on every
> row of table_a...

Yes, it *will* do the trick!! Sorry, Clark, I misinterpreted your
sub-query ... have to read more carefully ...

Thanks for your correction!
Ulf

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Ulf Mehlig <umehlig(at)zmt(dot)uni-bremen(dot)de>
Center for Tropical Marine Ecology/ZMT, Bremen, Germany
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