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From Niclas Hedhman <nic...@hedhman.org>
Subject Re: [ESQL] Improvement....
Date Tue, 28 Jan 2003 05:02:36 GMT
On Tuesday 28 January 2003 01:32, Hunsberger, Peter wrote:
> >> if (exist <esql:more-results>)
> >>    LIMIT 5+1
> >> else
> >>    LIMIT 5
> >>
> >> I think this is an elegant solution. What you think?
> >
> > Sure - but the problem is that we also would need to adjust the length
> > of the resultset inside the helper class. (you always want to see only 5
> > rows in your page)
> >
> > The length of the resultset inside the class would sometimes be one more
> > and sometimes *exact* - depending on whether there is a <more-results>
> > tag or not. Which would be very confusing!
> >
> > Give me a day to think about this...
>
> Torsten,
>
> You're chasing a non-existent problem.  There is never a real life case
> that will have both good performance for N records and bad performance for
> N+1 records.  The only way you can guarantee having good performance for N
> records is if you can build an index.  If you can build an index then the
> search will always terminate after looking at N+1 records.

I give you a "real world" case;

In natural language;
Give me the first COLOR (3D point) which resides not further than dE units 
from COLOR [L1, a1, b1].

In SQL database, I can only place the L, a, b columns, individually, and the 
spherical search would be;

WHERE SQRT( SQ( L - L1 ) + SQ( a - a1) + SQ( a - a2 ) ) < dE

where L1, a1, b1 and dE are parameters given at the invocation of the Select 
statement.

<exclusion>
I could go into a long discussion how this could be optimized, both in SQL 
(multi-levelled subqueries and regeneration of indexes) and internal to the 
DB engine (3D indecies), but I won't.
</exclusion>

If I have a well populated database, evenly and randomly spread out, I will on 
the average have a 100% penalty on LIMIT N+1, as the search will go through 
roughly the same number of records to find the "next one", which I don't care 
about.

Niclas

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