Rounding error when reading long from db and then writing it back.

Hi,

I am observing rounding errors on integer fields when reading from database and writing them back.

I have a measurement with an integer column that represents a timestamps with nanosecond resolution (note, this is not the default time column).

SHOW FIELD KEYS ON db
name: latency_1
fieldKey   fieldType
--------   ---------
latency_ms integer
msg_name   string
msg_type   integer
path_id    integer
seq_no     integer
ts_egr     integer  <- timestamp_ns
ts_ingr    integer

I am reading the data from database using influxdb-java. All integer values that I am reading are Doubles now (this I believe related to this issue).

String qstatement = String.format(
            "SELECT * FROM %s WHERE time > now() - %dms ORDER BY time ",
            table_name, how_far_back_in_time_ms);
Query query = new Query(qstatement, db_name);
influxDB.query(query, TimeUnit.NANOSECONDS);

For completeness, before writing values back to db, I am adding one more column where I take original value of the "ts_egr" column and convert it to Long before writing it back, i.e.,:

Double ts_egr = (Double)obj; //original "ts_egr" value of the column
Long long_ts = new Long(ts_egr.longValue());

Now I am reading a row from the original table and the new table in InfluxDB:

> select * from latency_1 where seq_no = 70
name: latency_1
time                latency_ms msg_name             msg_type path_id seq_no ts_egr              ts_ingr
----                ---------- --------             -------- ------- ------ ------              -------
1523264395509000000 72         abcdefgh12345678abcd 1        1       70     1523264395509072018 1523264395509000000
> select * from modified_latency_1 where seq_no = 70
name: modified_latency_1
time                latency_ms msg_name             msg_type path_id seq_no ts_egr              ts_egr_long         ts_ingr
----                ---------- --------             -------- ------- ------ ------              -----------         -------
1523264395508999936 72         abcdefgh12345678abcd 1        1       70     1523264395509072100 1523264395509072128 1523264395509000000

Note, the original value of "ts_ingr" 1523264395509072018 has been changed to 1523264395509072100 (just by reading and writing it back) and if we do conversion back to Long in Java we get another variant 1523264395509072128. Overall, I was observing rounding errors up to 100 nanoseconds.

Am I doing something wrong here, and is there a way to avoid the rounding errors?

Thank you