drill-issues mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Paul Rogers (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-5360) Timestamp type documented as UTC, implemented as local time
Date Thu, 16 Mar 2017 23:25:41 GMT

    [ https://issues.apache.org/jira/browse/DRILL-5360?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15929136#comment-15929136
] 

Paul Rogers commented on DRILL-5360:
------------------------------------

Talking with other engineers, it seems that it never was Drill's intent for Timestamp to be
UTC. Instead, it tried to follow the SQL standard which is to have relative times (Date, Time)
with no implied TZ, and the newer "TZ" types that have a date/time with a TZ offset.

The {{Timestamp}} type avoids the need for a timezone by declaring that all values are in
the same timezone. On Unix (and the internet and other modern systems), timestamps are always
UTC and are often relative to the Unix epoch.

Drill, however, did not implement the TZ-based date/time type. So, the assumption is that
{{Timestamp}} can't have a timezone. Since it has no timezone, it should be local time.

This misunderstands how Unix timezones work, but seems to be closer to how databases worked
(before they added time zones and absolute time.) So, the Drill {{Timestamp}} is the worst
of worlds: it tries to be absolute, but is actually local, and is converted wrong in JDBC.

Perhaps the best solution is to implement either the standard SQL TZ-based types *or* implement
a non-standard {{TimestampUTC}} value that is always in UTC. (When times are in UTC, they
have a time zone, but that zone is fixed and so need not appear in the value vector itself.
This is distinct from relative dates where the time zone is not specified but is also not
predefined.)


> Timestamp type documented as UTC, implemented as local time
> -----------------------------------------------------------
>
>                 Key: DRILL-5360
>                 URL: https://issues.apache.org/jira/browse/DRILL-5360
>             Project: Apache Drill
>          Issue Type: Bug
>    Affects Versions: 1.10.0
>            Reporter: Paul Rogers
>
> The Drill documentation implies that the {{Timestamp}} type is in UTC:
> bq. JDBC timestamp in year, month, date hour, minute, second, and optional milliseconds
format: yyyy-MM-dd HH:mm:ss.SSS. ... TIMESTAMP literals: Drill stores values in Coordinated
Universal Time (UTC). Drill supports time functions in the range 1971 to 2037. ... Drill does
not support TIMESTAMP with time zone.
> The above is ambiguous. The first part talks about JDBC timestamps. From the JDK Javadoc:
> bq. Timestamp: A thin wrapper around java.util.Date. ... Date class is intended to reflect
coordinated universal time (UTC)...
> So, a JDBC timestamp is intended to represent time in UTC. (The "indented to reflect"
statement leaves open the possibility of misusing {{Date}} to represent times in other time
zones. This was common practice in early Java development and was the reason for the eventual
development of the Joda, then Java 8 date/time classes.)
> The Drill documentation implies that timestamp *literals* are in UTC, but a careful read
of the documentation does allow an interpretation that the internal representation can be
other than UTC. If this is true, then we would also rely on a liberal reading of the Java
`Timestamp` class to also not be UTC. (Or, we rely on the Drill JDBC driver to convert from
the (unknown) server time zone to a UTC value returned by the Drill JDBC client.)
> Still, a superficial reading (and common practice) would suggest that a Drill Timestamp
should be in UTC.
> However, a test on a Mac, with an embedded Drillbit (run in the Pacific time zone, with
Daylight Savings Time in effect) shows that the Timestamp binary value is actual local time:
> {code}
>       long before = System.currentTimeMillis();
>       long value = getDateValue(client, "SELECT NOW() FROM (VALUES(1))" );
>       double hrsDiff = (value - before) / (1000.00 * 60 * 60);
>       System.out.println("Hours: " + hrsDiff);
> {code}
> The above gets the actual UTC time from Java. Then, it runs a query that gets Drill's
idea of the current time using the {{NOW()}} function. (The {{getDateValue}} function uses
the new test framework to access the actual {{long}} value from the returned value vector.)
Finally, we compute the difference between the two times, converted to hours. Output:
> {code}
> Hours: -6.9999975
> {code}
> As it turns out, this is the difference between UTC and PDT. So, the time is in local
time, not UTC.
> Since the documentation and implementation are both ambiguous, it is hard to know the
intent of the Drill Timestamp. Clearly, common practice is to use UTC. But, there is wiggle-room.
> If the Timestamp value is supposed to be local time, then Drill should provide a function
to return the server's time zone offset (in ms) from UTC so that the client can to the needed
local-to-UTC conversion to get a true timestamp.
> On the other hand, if the Timestamp is supposed to be UTC (per common practice), then
{{NOW()}} should not report local time, it should return UTC.
> Further, if {{NOW()}} returns local time, but Timestamp literals are UTC, then it is
hard to see how any query can be rationally written if one timestamp value is local, but a
literal is UTC.
> So, job #1 is to define the Timestamp semantics. Then, use that to figure out where the
bug lies to make implementation consistent with documentation (or visa-versa.)



--
This message was sent by Atlassian JIRA
(v6.3.15#6346)

Mime
View raw message