util: Fix interval division with float rounds differently from Postgr…

…eSQL

Before this commit, the nanos will be rounded in `MakeDuration`.
This commit round down nanos before `rounded` is called in the div of duration.

Fixes cockroachdb#66118

Release note (bug fix): Fixed a bug with PostgreSQL compatibility where
dividing an interval by a number would round to the nearest Microsecond
instead of always rounding down.

pkg/sql/logictest/testdata/logic_test/interval

@@ -367,3 +367,12 @@ subtest regression_62369
 
 query error "10000000000000000000000000000000000": value out of range
 SELECT INTERVAL '10000000000000000000000000000000000 year'
+
+query T
+SELECT i / 2 FROM ( VALUES
+  ('0 days 0.253000 seconds'::interval),
+  (INTERVAL '0.000001'::interval)
+) regression_66118(i)
+----
+00:00:00.126
+00:00:00

pkg/util/duration/duration.go

@@ -619,9 +619,12 @@ func (d Duration) MulFloat(x float64) Duration {
 func (d Duration) DivFloat(x float64) Duration {
 	monthInt, monthFrac := math.Modf(float64(d.Months) / x)
 	dayInt, dayFrac := math.Modf((float64(d.Days) / x) + (monthFrac * DaysPerMonth))
+	// We ensure we round down nanoseconds when doing division to
+	// match PostgreSQL.
+	retNanos := time.Duration(float64(d.nanos)/x).Truncate(time.Millisecond)
 
 	return MakeDuration(
-		int64((float64(d.nanos)/x)+(dayFrac*float64(nanosInDay))),
+		int64(float64(retNanos.Nanoseconds())+(dayFrac*float64(nanosInDay))),
 		int64(dayInt),
 		int64(monthInt),
 	)

pkg/util/duration/duration_test.go

@@ -498,6 +498,18 @@ func TestFloatMath(t *testing.T) {
 			Duration{Months: 2, Days: 34, nanos: nanosInHour * 4},
 			Duration{Days: 23, nanos: nanosInHour * 13},
 		},
+		{
+			Duration{Months: 0, Days: 0, nanos: nanosInSecond * 0.253000},
+			3.2,
+			Duration{Months: 0, Days: 0, nanos: nanosInSecond * 0.8096},
+			Duration{Months: 0, Days: 0, nanos: nanosInSecond * 0.079062},
+		},
+		{
+			Duration{Months: 0, Days: 0, nanos: nanosInSecond * 0.000001},
+			2.0,
+			Duration{Months: 0, Days: 0, nanos: nanosInSecond * 0.000002},
+			Duration{Months: 0, Days: 0, nanos: nanosInSecond * 0},
+		},
 	}
 
 	for i, test := range tests {