Re: Getting rid of cheap-startup-cost paths earlier

On May 22, 2012, at 9:57 AM, Simon Riggs wrote:

> On 22 May 2012 06:50, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Currently, the planner keeps paths that appear to win on the grounds of
>> either cheapest startup cost or cheapest total cost. It suddenly struck
>> me that in many simple cases (viz, those with no LIMIT, EXISTS, cursor
>> fast-start preference, etc) we could know a-priori that cheapest startup
>> cost is not going to be interesting, and hence immediately discard any
>> path that doesn't win on total cost.
>
> My experience is that most people don't provide a LIMIT explicitly
> even when they know that's the desired behaviour. That's because
> either they simply don't understand that SQL can return lots of rows,
> or SQL knowledge isn't enough, or worse that people don't even know
> that specifying it would alter query plans.
>
> Regrettably the current planning of LIMIT clauses causes more problems
> so in many cases these have been explicitly removed from SQL by
> developers that know how many rows they wish to see.
>
> I would have proposed a default-LIMIT parameter before now, but for
> that last point.

this sounds like a total disaster to me ...
why in the world should we have a default LIMIT parameter?
i guess if somebody is not able to use LIMIT he should better not touch the DB.
we clearly cannot fix incompetence by adding parameters.

regards,

hans

--
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt, Austria
Web: http://www.postgresql-support.de