Re: Ramifications of turning off Nested Loops for slow queries

On 3/4/08, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> writes:
> > On Tue, Mar 4, 2008 at 8:42 AM, in message
> > <483ACAF5-A485-40D9-9D7E-7008EF12F909(at)vistashare(dot)com>, Chris Kratz
> > <chris(dot)kratz(at)vistashare(dot)com> wrote:
> >> So, I've now been asked to ping the list as to whether turning off
> >> nested loops system wide is a bad idea, and why or why not.
>
> > In our environment, the fastest plan for a lot of queries involve
> > nested loops. Of course, it's possible that these never provide the
> > fasted plan in your environment, but it seems very unlikely --
> > you're just not noticing the queries where it's doing fine.
>
>
> Yeah, I seem to recall similar queries from other people who were
> considering the opposite, ie disabling the other join types :-(
>
> The rule of thumb is that nestloop with an inner indexscan will beat
> anything else for pulling a few rows out of a large table. But on
> the other hand it loses big for selecting lots of rows. I don't think
> that a global disable in either direction would be a smart move, unless
> you run only a very small number of query types and have checked them
> all.
>
> regards, tom lane
>

So, if we can't find another way to solve the problem, probably our best bet
is to turn off nested loops on particularly bad queries by prepending them
w/ set enable_nested_loop=off? But, leave them on for the remainder of the
system?

Do you think it's worth testing on 8.3 to see if the estimator is able to
make a better estimate?

-Chris