Re: fool-toleranced optimizer

On Wed, 9 Mar 2005, Simon Riggs wrote:

> Oleg, this idea doesn't seem destine for greatness, so it might be worth
> adding that you can avoid the general case problem of incorrectly-
> specified-but-long-running query by using statement_timeout...

I have no problem with that ! I just wanted to take a note of such
"could be" mistaken errors.

>
> On Wed, 2005-03-09 at 22:38 +1100, Neil Conway wrote:
>> Simon Riggs wrote:
>>> Oleg is saying that the optimizer doesn't protect against foolish SQL
>>> requests. His query is an example of a foolishly written query.
>>
>> IMHO calling this a "foolishly written query" is completely arbitrary.
>
> Well, in this case "foolish" is defined by the person that wrote the
> query, as an expression of regret.
>
>> I
>> can imagine plenty of applications for which a cartesian join makes
>> sense.
>
> Yes, which is why I discussed using a GUC, set only by those people who
> want to be protected *from themselves*. It's a safety harness that you
> could choose to put on if you wished.
>
>> In this case the user didn't write the query they meant to write
>> -- but it is surely hopeless to prevent that in the general case :)
>>
>>> It seems a reasonable that there might be a GUC such as
>>> enable_cartesian = on (by default)
>>
>> I think the bar for adding a new GUC ought to be significantly higher
>> than that.
>
> Well, the point is moot until somebody writes the rest of the code
> anyhow. So, add it to the ideas shelf...
>
>> In any case, when this problem does occur, it is obvious to the user
>> that something is wrong, and no harm is done. Given a complex SQL query,
>> it might take a bit of examination to determine which join clause is
>> missing -- but the proper way to fix that is better query visualization
>> tools (perhaps similar RH's Visual Explain, for example). This would
>> solve the general problem: "the user didn't write the query they
>> intended to write", rather than a very narrow subset ("the user forgot a
>> join clause and accidentally computed a cartesian product").
>
> This issue only occurs when using SQL as the user interface language,
> which is common when using a database in iterative or exploratory mode
> e.g. Data Warehousing. If you are using more advanced BI tools then they
> seldom get the SQL wrong.
>
> This is not useful in a situation where people are writing SQL for a
> more static application.
>
> Best Regards, Simon Riggs
>

Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83