Re: A new proof of the superiority of set oriented approaches: numerical/time serie linear interpolation

"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.

> >
> >> I would bet the process would run much faster than your solution. In
> >> rare
> >> cases, cursors can actually increase performance.
> > I don't think one can establish that for a certainty.
Received on Sun Apr 29 2007 - 21:35:55 CEST