Re: fool-toleranced optimizer

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...

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