add test case and bug fix for distinct code

The "seenThisRow" value was never getting cleared, which meant that
if the first container on a row didn't happen to contain any post-filter
bits, the rest of the row wouldn't get evaluated.

executor.go

@@ -1460,10 +1460,14 @@ func executeDistinctShardSet(ctx context.Context, qcx *Qcx, idx *Index, fieldNam
 	for fragData.Next() {
 		k, c := fragData.Value()
 		row := k >> shardVsContainerExponent
-		if row == prevRow && seenThisRow {
-			continue
+		if row == prevRow {
+			if seenThisRow {
+				continue
+			}
+		} else {
+			seenThisRow = false
+			prevRow = row
 		}
-		prevRow = row
 		if filterBitmap != nil {
 			if roaring.IntersectionAny(c, filter[k%(1<<shardVsContainerExponent)]) {
 				_, err = rows.Add(row)

executor_test.go

@@ -6435,19 +6435,25 @@ func TestExecutor_BareDistinct(t *testing.T) {
 		pilosa.OptFieldTypeInt(0, math.MaxInt64),
 	)
 	c.CreateField(t, "i", pilosa.IndexOptions{}, "set")
+	c.CreateField(t, "i", pilosa.IndexOptions{}, "filter")
 
 	// Populate integer data.
 	c.Query(t, "i", fmt.Sprintf(`
 			Set(0, ints=1)
 			Set(%d, ints=2)
 		`, ShardWidth))
-	c.Query(t, "i", `Set(0, set=1)
-			 Set(1, set=2)`)
+	c.Query(t, "i", fmt.Sprintf(`
+		         Set(0, set=1)
+			 Set(1, set=2)
+			 Set(%d, set=2)
+			 Set(0, filter=1)
+			 Set(%d, filter=1)
+	        `, 65537, 65537))
 
 	for _, pql := range []string{
 		`Distinct(field="ints")`,
 		`Distinct(index="i", field="ints")`,
-		`Distinct(field="set")`,
+		`Distinct(Row(filter=1), field="set")`,
 	} {
 		exp := []uint64{1, 2}
 		res := c.Query(t, "i", pql).Results[0].(pilosa.SignedRow)