Re: Mini improvement: statement_cost_limit

On Mon, Aug 04, 2008 at 11:59:03AM -0700, Josh Berkus wrote:
> Greg,
>
> >Well that's going to depend on the application.... But I suppose there's
> >nothing wrong with having options which aren't always a good idea to use.
> >The
> >real question I guess is whether there's ever a situation where it would
> >be a
> >good idea to use this. I'm not 100% sure.
>
> I can think of *lots*. Primarily, simple web applications, where
> queries are never supposed to take more than 50ms. If a query turns up
> with an estimated cost of 10000000000, then you know something's wrong;
> in the statistics if not in the query. In either case, that query has a
> good chance of dragging down the whole system.
>
> In such a production application, it is better to have false positives
> and reject otherwise-OK queries becuase their costing is wrong, than to
> let a single cartesian join bog down an application serving 5000
> simultaneous users. Further, with a SQL error, this would allow the
> query rejection to be handled in a user-friendly way from the UI
> ("Search too complex. Try changing search terms.") rather than timing
> out, which is very difficult to handle well.
>
> The usefulness of this feature for interactive sessions is
> limited-to-nonexistant. It's for production applications.

Ok, that is a different use case where an error seems very useful. What
about slightly extending the proposal to have the severity of exceeding
the limit configurable too. Something like:

costestimate_limit = 1000000000 # default 0 to ignore limit
costestimate_limit_severity = error # debug, notice, warning, error

-dg

--
David Gould daveg(at)sonic(dot)net 510 536 1443 510 282 0869
If simplicity worked, the world would be overrun with insects.