Steve Baldwin <steve.baldwin@xxxxxxxxx> 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
