Re: Conjunction junction

"Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message news:RFvhd.16$J%6.187_at_news.oracle.com...
> "Alan" <alan_at_erols.com> wrote in message
> news:2unbuhF2cbaliU1_at_uni-berlin.de...
> > UNION. Of course this can be done procedureally, but the XOR I described
> > would have allowed it in straight SQL. Maybe call it an IOR (Immediate
OR)
> > if XOR is confusing.
>
> There are 2 separate concepts:
> 1. what connectors between the boolean expressions are allowed, and
> 2. row limit "predicate".
>
> Admittedly item #2 would be different in different SQL dialects. For
example
>
> SELECT stock_location
> FROM pennants
> WHERE (color = 'red' OR color = 'green')
> and rownum <=1
> ^^^^^^^^^^^^
>
> Alternatively, it might be the client that controls how many rows are
> fetched.
>
> Performance of the above query is completely different topic. For example,
> optimizer might decide to convert this query into a concatenation. It
> executes the first branch, and the row satisfies the condition color =
> 'red', then it goes to the output, and execution stops immediately.
> Otherwise, it would follow to the second branch. Alternative execution
plan
> is what you suggested: it iterates the relation tuples and checks the
> [complex] condition color = 'red' OR color = 'green' and stops as soon as
a
> tuple is found.
>

Hmm... good points. Received on Mon Nov 01 2004 - 22:25:16 CET