On Fri, Oct 27, 2000 at 09:21:23PM +1000, Philip Warner wrote:
>
> [To hackers this time]
>
> At 20:59 26/10/00 -0400, Tom Lane wrote:
> >Hiroshi Inoue <Inoue@tpf.co.jp> writes:
> >> Yes I want to give optimizer a hint "return first rows fast".
> >> When Jan implemented LIMIT first,there was an option
> >> "LIMIT ALL" and it was exactly designed for the purpose.
> >
> >Well, we could make that work that way again, I think.
>
> I think that would be a *bad* idea. ISTM that the syntax is obtuse for the
> meaning it is being given. The (mild) confusion in this thread is evidence
> of that, at least.
>
Syncronicity, man. I didn't see the beginning of this thread (not on
COMMITERS) so I may be repeating things from there.
I was recently cleaning out a stack of old trade-rags lying around, and
snipped an article out of a DB2 mag I've been getting. Very technical,
and discusses the uses (and abuses) of OPTIMIZE FOR N ROWS, where N is
an actual number. Discusses how the DB2 optimizer will use this hint to
decide if it should use an index to get the right order, even if it's a
full scan, and the total cost might be higher. I'll see if I can find it
online, if anyones interested.
The original article is all in the context of cursors (and multi-gig
tables), but I think LIMIT brings in many of the same optimization
considerations.
ISTM that the most common use of LIMIT right now is to simulate a cursor
to provide some state over the stateless HTTP protocol, no? So the LIMIT
is not 'fast start' vs 'total cost': the webpage often allows the enduser
to select the batchsize. At some batchsize, 'total cost' wins over a
simplistic 'fast start' approach. And only the optimizer has any hope of
figuring out where that might be, as it will change with the exact query
structure.
Ross
--
Open source code is like a natural resource, it's the result of providing
food and sunshine to programmers, and then staying out of their way.
[...] [It] is not going away because it has utility for both the developers
and users independent of economic motivations. Jim Flynn, Sunnyvale, Calif.