Sync behavior of var_samp and stddev_samp for single NaN inputs.

var_samp(numeric) and stddev_samp(numeric) disagreed with their float
cousins about what to do for a single non-null input value that is NaN.
The float versions return NULL on the grounds that the calculation is
only defined for more than one non-null input, which seems like the
right answer.  But the numeric versions returned NaN, as a result of
dealing with edge cases in the wrong order.  Fix that.  The patch
also gets rid of an insignificant memory leak in such cases.

This inconsistency is of long standing, but on the whole it seems best
not to back-patch the change into stable branches; nobody's complained
and it's such an obscure point that nobody's likely to complain.
(Note that v13 and v12 now contain test cases that will notice if we
accidentally back-patch this behavior change in future.)

Report and patch by me; thanks to Dean Rasheed for review.

Discussion: https://postgr.es/m/353062.1591898766@sss.pgh.pa.us

src/backend/utils/adt/numeric.c

@@ -5172,11 +5172,21 @@ numeric_stddev_internal(NumericAggState *state,
 				vsumX,
 				vsumX2,
 				vNminus1;
-	const NumericVar *comp;
+	int64		totCount;
 	int			rscale;
 
-	/* Deal with empty input and NaN-input cases */
-	if (state == NULL || (state->N + state->NaNcount) == 0)
+	/*
+	 * Sample stddev and variance are undefined when N <= 1; population stddev
+	 * is undefined when N == 0.  Return NULL in either case (note that NaNs
+	 * count as normal inputs for this purpose).
+	 */
+	if (state == NULL || (totCount = state->N + state->NaNcount) == 0)
+	{
+		*is_null = true;
+		return NULL;
+	}
+
+	if (sample && totCount <= 1)
 	{
 		*is_null = true;
 		return NULL;
@@ -5184,9 +5194,13 @@ numeric_stddev_internal(NumericAggState *state,
 
 	*is_null = false;
 
+	/*
+	 * Deal with NaN inputs.
+	 */
 	if (state->NaNcount > 0)
 		return make_result(&const_nan);
 
+	/* OK, normal calculation applies */
 	init_var(&vN);
 	init_var(&vsumX);
 	init_var(&vsumX2);
@@ -5195,21 +5209,6 @@ numeric_stddev_internal(NumericAggState *state,
 	accum_sum_final(&(state->sumX), &vsumX);
 	accum_sum_final(&(state->sumX2), &vsumX2);
 
-	/*
-	 * Sample stddev and variance are undefined when N <= 1; population stddev
-	 * is undefined when N == 0. Return NULL in either case.
-	 */
-	if (sample)
-		comp = &const_one;
-	else
-		comp = &const_zero;
-
-	if (cmp_var(&vN, comp) <= 0)
-	{
-		*is_null = true;
-		return NULL;
-	}
-
 	init_var(&vNminus1);
 	sub_var(&vN, &const_one, &vNminus1);
 

src/test/regress/expected/aggregates.out

@@ -214,13 +214,13 @@ SELECT stddev_pop(3.0::numeric), stddev_samp(4.0::numeric);
 SELECT var_pop('nan'::numeric), var_samp('nan'::numeric);
  var_pop | var_samp 
 ---------+----------
-     NaN |      NaN
+     NaN |         
 (1 row)
 
 SELECT stddev_pop('nan'::numeric), stddev_samp('nan'::numeric);
  stddev_pop | stddev_samp 
 ------------+-------------
-        NaN |         NaN
+        NaN |            
 (1 row)
 
 -- verify correct results for null and NaN inputs