Bob Badour wrote:
> David Cressey wrote:
>
>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> news:jy5Zh.4247$uJ6.3542_at_newssvr17.news.prodigy.net...
>>
>>> "Cimode" <cimode_at_hotmail.com> wrote in message
>>> news:1177863186.258396.136950_at_y5g2000hsa.googlegroups.com...
>>>
>>>> On 29 avr, 16:21, "Brian Selzer" <b..._at_selzer-software.com> wrote:
>>>>
>>>>> I would have done it differently. I would have used a cursor to
>>
>>
>> generate
>>
>>>> [Snipped]
>>>> I respectfully think you miss the point of the entire thread.
>>>> Thinking procedurally (use of cursors) is inherently inferior to set
>>>> oriented approaches independently of a specific implementation and
>>>> regardless of how one defines performance as response time, ability to
>>>> run well in concurrent query environment and so forth.
>>>>
>>>
>>> You're right. As a rule, set-based thinking is superior to procedural
>>> thinking. But there are times when it becomes necessary to break the
>>
>> rules.
>>
>>> It is critical to think procedurally in order to understand the
>>> execution
>>> plans that an optimizer produces.
>>
>>
>> This is correct, but it largely misses the point. One thinks about
>> how the
>> optimizer does its job in the (hopefully rare) cases where the
>> optimizer's
>> output is materially suboptimal.
>>
>> My own experience some 15 years ago with DEC Rdb is a case in point. The
>> DEC optimizer was so good that, in most cases, it was simply not
>> necessary
>> to think about how it worked. That allowed the query designer (and
>> for that
>> matter the database designer) to focus on the other aspects of quality
>> without sacrificing much in the way of speed.
>>
>>
>>> Consider a merge join: both inputs are
>>> sorted, and then a single pass is made through each result--similar
>>> to an
>>> old COBOL program that updates a master file from a transaction
>>> file. If
>>> you don't understand what each step in an execution plan does and how it
>>> works, then how can you possibly optimize a query?
>>>
>>> When it becomes clear that an iterative approach could eliminate several
>>> self-joins, thus decreasing the duration that locks are held,
>>> reducing the
>>> impact on temporary storage and decreasing cpu utilization, then it
>>> might
>>
>>
>> be
>>
>>> a good idea to break the rules. You mentioned that switching from a
>>
>>
>> poorly
>>
>>> designed procedural solution to one that is set-based dropped the
>>> time it
>>> took for the process to run from 10 hours to 30 minutes. That's
>>> great! I
>>> merely suggest that reintroducing a cursor to eliminate the several
>>> self-joins in your solution might further decrease the time it would
>>> take
>>> for the process to run--without any negative impact on concurrency or
>>> overall performance.
>>
>>
>> All of these comments should lead to one conclusion: one engages in this
>> sort of analysis when one is intent on writing a better optimizer, or
>> when
>> coming up with a workaround in the case of a deficient optimizer. Not in
>> the case of using a good optimizer to good effect.
>
>
> David, please don't be too dismayed when I say: Lately, I have been
> liking your answers better and better.
I'd like to join that club. But as an old fan of serials and the
Mythical Man-Month", I can't help but think of silver bullets. My
favourite serial was "Captain Midnight and the Silver Dart". Years
after I watched that one, I saw the original set on television and was
dismayed. What looked modern and sophisticated to a 'fifties kid was in
fact silver-painted cardboard.
Years after that I stumbled upon a basic truth which has nothing to do
with theory but everything to do the end-users of theory. When
requirements are expressed in terms that the
customer/user/client/interested party or so-called "stakeholder" can
understand, all bets are off. With the right viewpoint, one can get the
big cheese to toss in the garbage a requirement that all the long-term
underlings will claim are essential and important. This even applies to
issues of human safety, such as airline cargoes. Having stalled while
travelling on more than a few UK trains after they were privatized (one
locomotive almost self-destructed), I would think that when it comes to
concurrency I'd be quite happy to dispense with concurrent access by
accounting bureaucrats in favour of the tensile/structural integrity
users. I don't see why a track sensor interpolation shouldn't be
isolated from the rest of the db while it is going on, for any train,
let alone the TGV. The biggest thing that has bugged me about most
consultants is that they usually don't question requirements.
p
Received on Sun Apr 29 2007 - 22:16:46 CDT
