Re: IN vs EXISTS equivalence

On Thu, Aug 14, 2008 at 06:50:09PM +0100, Simon Riggs wrote:
> 
> On Fri, 2008-08-08 at 16:23 -0400, Tom Lane wrote:
> 
> > NOT IN is a lot trickier,
> > condition: you must also assume that the comparison operator involved
> > never yields NULL for non-null inputs.  That might be okay for btree
> > comparison functions but it's not a very comfy assumption in general;
> > we certainly haven't got any explicit knowledge that any functions are
> > guaranteed to act that way.  So this case might be worth doing later
... 
> Just found this comment, after reading what you said on other thread
> about NOT IN.
> 
> NOT IN is a serious performance issue for most people. We simply can't
> say to people "you were told not to".
> 
> If we can fix it easily for the majority of cases, we should. We can't
> let the "it won't work in certain cases" reason prevent various

A suggestion: what about adding an attribute to functions to declare that
they never return null? 
  declare foo(int, int) returns int immutable not null as ...


-dg


-- 
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.