Am I missing something? Could we represent not( attribute > 3 ) as
attribute <= 3 ?
It would involve some logic to identify the appropriate inverse of our
filters, but it may be worth it.
Jody Garnett
On Mon, Feb 3, 2014 at 8:50 PM, Andrea Aime <andrea.a...@geo-solutions.it>wrote:
> Hi,
> I've recently stumbled into a fun problem with our sql encoding.
> Say you have the following filter (which, while odd, is legal):
>
> not(attribute > 3)
>
> our sql translator translates it to:
>
> not(attribute > 3)
>
> Ahh.. all good no? No! The behavior of our in memory filtering, and
> database filtering,
> is not the same when "attribute" own value is null, for us null > 3 is
> false, hence
> not(null > 3) is true.
> However, the databases work differently, any logic predicate handling a
> null will return false,
> no matter how complex it is, the difference is that we use two valued
> logic, whilst databases
> use three valued one: http://en.wikipedia.org/wiki/Three-valued_logic
>
> Soo.. the correct SQL translation of the above filter would be:
>
> not(attribute > 3) or attribute is null
>
> or probably easier to encode:
>
> not(attribute is not null and attribute > 3)
>
> (that is, every time we do a binary comparison with an attribute, we add a
> "att is not null" in the
> predicate)
>
> Anyone has better ideas? :-)
> We could also have a look at the attribute metadata and see if the column
> is nullable, or not.
>
> Cheers
> Andrea
>
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