Re: How to influence the planner

On 8/31/07, Richard Ray <rray(at)mstc(dot)state(dot)ms(dot)us> wrote:

> Changing to enable_seqscan = on does solve this problem, thanks
> Is there some method of crafting a query that will assert my wishes to the planner
> When is enable_seqscan = off appropriate

enable_xxx = off are troubleshooting tools. They override the query planner.

For instance, I had a query that was running slow, and using set
enable_nestloop=off allowed the query to run fast. However, using
explain analyze I could see that the estimated number of rows was off.
Analyze didn't fix it, so I increased the stats target for the column
I was working with, reanalyzed, and voila, the query ran fine with
nestloop=on.

So, enable_xxx=off is normally only appropriate when troubleshooting
an issue, not as a fix all. That's doubly true for
enable_seqscan=off.

If you do have a query that nothing else seems to work on it, you can
set one of the enable_xxx settings off for that connection only and
not worry about messing up all the other sessions connecting to your
db.