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slow query on 64bit sql server?

my company get a chance to run a test on demo system itanium2 with sql2k 64bit installed.
i ran some queries. FOr update & delete, it run faster.
But in select queries, there are some queries that run extremely slow, about 3x-10x, even 50x slower.
i look carefully at the queries, and after i modified the filter "where not b.fibcno is null" with
"where isnull(b.fibcno,’0′)<>’0’" note:b.fibcno is the other table joined within the queries it ran at the same speed, even a little bit faster as the 32bit. My question is, what is it all about? is wrong using "is null" instead of replacing the NULL values first, and then compared later, just like the query above? i tried to installed the SP4, but nothing happens. the queries involved about 10tables.
My guess is that bad execution plan was used, then you made b.fibcno no sarg, because function and <> are used, so QO was forced to try alternative exec plan and it happend that it is better one. Take a look at both execution plans to confirm or discard my theory.

what do you mean with "no sarg"?
i’m sorry that the demo machine has been returned back, so i don;t know wether the exec.plan is different after i modified the query. (i quite agree with u tough) but, why this only happened to 64bit? is the execution plan is in a different approach between sql32bit and sql64bit??? i once compared the exec.plan between 32bit and 64bit on the same query. it is different.
if that mean 32bit and 64bit has different exec.plan? the noisy thing is i don’t know exactly what the cause of it, and is there another problem with the sql 64bit? i just found this 1 problem.
in 32bit everything run smooth and fast. this kinda thing make me worried migrating to 64bit. thx,
Hi Edo, not wanting to put the cats amongst the pigeons but, if everything is running smooth and fast on 32bit platform, then why are you migrating? Cheers

the new server not intend to replace the old 32bit,
but to support the new application. it’s already budgeted to buy a new server, but then to chooose among the 32 or 64bit,because the 64bit server is priced as nearly the same as 32. so we got a demo system, and tested here, and got some bug here(i dont know wether it is bug).
however if i modified the query, the problem is gone, run fast.
i worried if someday i found another bug(big bug), and at that time we already bought the itanium.