Re: regr_slope returning NULL

Steve Baldwin <steve(dot)baldwin(at)gmail(dot)com> writes:
> Consider the following:
> ...
> log=# select id, regr_slope(elapsed, ts) as trend from sb1 group by id;
> id | trend
> ------+----------------------
> c742 |
> 317e |
> 5fe6 | 5.78750952760444e-06
> 3441 |
> (4 rows)

Hm, I get

regression=# select id, regr_slope(elapsed, ts) as trend from sb1 group by id;
id | trend
------+-----------------------
c742 | 19.607858781290517
317e | -1.0838511987808963
5fe6 | 5.787509483586743e-06
3441 | -3.828395463097356
(4 rows)

What platform are you doing this on, and what exactly is the PG version?

> If pg is correctly returning NULL, I'd be interested to understand the
> circumstances under which this can occur.

The source code shows two cases in which NULL would be returned:

/* if N is 0 we should return NULL */
if (N < 1.0)
PG_RETURN_NULL();

/* per spec, return NULL for a vertical line */
if (Sxx == 0)
PG_RETURN_NULL();

Maybe the cases you're looking at are sufficiently numerically
ill-conditioned that you could get Sxx == 0 depending on platform-
specific roundoff error, but it seems fishy.

regards, tom lane