Plan scans even if destination is nil

fixes #810

rows.go

@@ -204,10 +204,7 @@ func (rows *connRows) Scan(dest ...interface{}) error {
 
 	if rows.scanPlans == nil {
 		rows.scanPlans = make([]pgtype.ScanPlan, len(values))
-		for i, dst := range dest {
-			if dst == nil {
-				continue
-			}
+		for i := range dest {
 			rows.scanPlans[i] = ci.PlanScan(fieldDescriptions[i].DataTypeOID, fieldDescriptions[i].Format, dest[i])
 		}
 	}

values_test.go

@@ -950,3 +950,36 @@ func TestRowDecodeBinary(t *testing.T) {
 		ensureConnValid(t, conn)
 	}
 }
+
+// https://github.com/jackc/pgx/issues/810
+func TestRowsScanNilThenScanValue(t *testing.T) {
+	t.Parallel()
+
+	testWithAndWithoutPreferSimpleProtocol(t, func(t *testing.T, conn *pgx.Conn) {
+		sql := `select null as a, null as b
+union
+select 1, 2
+order by a nulls first
+`
+		rows, err := conn.Query(context.Background(), sql)
+		require.NoError(t, err)
+
+		require.True(t, rows.Next())
+
+		err = rows.Scan(nil, nil)
+		require.NoError(t, err)
+
+		require.True(t, rows.Next())
+
+		var a int
+		var b int
+		err = rows.Scan(&a, &b)
+		require.NoError(t, err)
+
+		require.EqualValues(t, 1, a)
+		require.EqualValues(t, 2, b)
+
+		rows.Close()
+		require.NoError(t, rows.Err())
+	})
+}