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https://issues.apache.org/jira/browse/SPARK-23715?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16621395#comment-16621395
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Reynold Xin commented on SPARK-23715:
-------------------------------------
[~bersprockets] i think we should revert the change while we still can (before
2.4 release).
Spark treats timestamps very differently from Impala. IIUC, Impala basically
implements the ansi sql timestamp without timezone, whereas Spark implements
internally just absolute UTC time. These two functions make a lot of sense for
the ansi sql timestamp, but not Spark's timestamp. In Spark's timestamp, there
is no need to convert something from UTC time to a local time, because it's
when a timestamp is displayed, the time is interpreted using the session local
timezone already. These two functions in hindsight were added by mistake,
unless we overhaul Spark's timestamp semantics.
You identified one example of the result being different in Spark vs Impala,
but there are tons of other differences as a result of how timestamp is
implemented. These fundamental differences cannot be reconciled by fixing the
differences one by one.
I think we should deprecate these two functions in Spark 3.0. And for now, we
should document them more clearly, by explaining what the actual semantics are,
which is to take a timestamp, and actually shift the underlying utc time so the
timestamp would be displayed in string form to be the time in the target
timezone. We are literally shifting the absolute utc time here.
> from_utc_timestamp returns incorrect results for some UTC date/time values
> --------------------------------------------------------------------------
>
> Key: SPARK-23715
> URL: https://issues.apache.org/jira/browse/SPARK-23715
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.3.0
> Reporter: Bruce Robbins
> Assignee: Wenchen Fan
> Priority: Major
> Fix For: 2.4.0
>
>
> This produces the expected answer:
> {noformat}
> df.select(from_utc_timestamp(lit("2018-03-13T06:18:23"), "GMT+1"
> ).as("dt")).show
> +-------------------+
> | dt|
> +-------------------+
> |2018-03-13 07:18:23|
> +-------------------+
> {noformat}
> However, the equivalent UTC input (but with an explicit timezone) produces a
> wrong answer:
> {noformat}
> df.select(from_utc_timestamp(lit("2018-03-13T06:18:23+00:00"), "GMT+1"
> ).as("dt")).show
> +-------------------+
> | dt|
> +-------------------+
> |2018-03-13 00:18:23|
> +-------------------+
> {noformat}
> Additionally, the equivalent Unix time (1520921903, which is also
> "2018-03-13T06:18:23" in the UTC time zone) produces the same wrong answer:
> {noformat}
> df.select(from_utc_timestamp(to_timestamp(lit(1520921903)), "GMT+1"
> ).as("dt")).show
> +-------------------+
> | dt|
> +-------------------+
> |2018-03-13 00:18:23|
> +-------------------+
> {noformat}
> These issues stem from the fact that the FromUTCTimestamp expression, despite
> its name, expects the input to be in the user's local timezone. There is some
> magic under the covers to make things work (mostly) as the user expects.
> As an example, let's say a user in Los Angeles issues the following:
> {noformat}
> df.select(from_utc_timestamp(lit("2018-03-13T06:18:23"), "GMT+1"
> ).as("dt")).show
> {noformat}
> FromUTCTimestamp gets as input a Timestamp (long) value representing
> {noformat}
> 2018-03-13T06:18:23-07:00 (long value 1520947103000000)
> {noformat}
> What FromUTCTimestamp needs instead is
> {noformat}
> 2018-03-13T06:18:23+00:00 (long value 1520921903000000)
> {noformat}
> So, it applies the local timezone's offset to the input timestamp to get the
> correct value (1520947103000000 minus 7 hours is 1520921903000000). Then it
> can process the value and produce the expected output.
> When the user explicitly specifies a time zone, FromUTCTimestamp's
> assumptions break down. The input is no longer in the local time zone.
> Because of the way input data is implicitly casted, FromUTCTimestamp never
> knows whether the input data had an explicit timezone.
> Here are some gory details:
> There is sometimes a mismatch in expectations between the (string =>
> timestamp) cast and FromUTCTimestamp. Also, since the FromUTCTimestamp
> expression never sees the actual input string (the cast "intercepts" the
> input and converts it to a long timestamp before FromUTCTimestamp uses the
> value), FromUTCTimestamp cannot reject any input value that would exercise
> this mismatch in expectations.
> There is a similar mismatch in expectations in the (integer => timestamp)
> cast and FromUTCTimestamp. As a result, Unix time input almost always
> produces incorrect output.
> h3. When things work as expected for String input:
> When from_utc_timestamp is passed a string time value with no time zone,
> DateTimeUtils.stringToTimestamp (called from a Cast expression) treats the
> datetime string as though it's in the user's local time zone. Because
> DateTimeUtils.stringToTimestamp is a general function, this is reasonable.
> As a result, FromUTCTimestamp's input is a timestamp shifted by the local
> time zone's offset. FromUTCTimestamp assumes this (or more accurately, a
> utility function called by FromUTCTimestamp assumes this), so the first thing
> it does is reverse-shift to get it back the correct value. Now that the long
> value has been shifted back to the correct timestamp value, it can now
> process it (by shifting it again based on the specified time zone).
> h3. When things go wrong with String input:
> When from_utc_timestamp is passed a string datetime value with an explicit
> time zone, stringToTimestamp honors that timezone and ignores the local time
> zone. stringToTimestamp does not shift the timestamp by the local timezone's
> offset, but by the timezone specified on the datetime string.
> Unfortunately, FromUTCTimestamp, which has no insight into the actual input
> or the conversion, still assumes the timestamp is shifted by the local time
> zone. So it reverse-shifts the long value by the local time zone's offset,
> which produces a incorrect timestamp (except in the case where the input
> datetime string just happened to have an explicit timezone that matches the
> local timezone). FromUTCTimestamp then uses this incorrect value for further
> processing.
> h3. When things go wrong for Unix time input:
> The cast in this case simply multiplies the integer by 1000000. The cast does
> not shift the resulting timestamp by the local time zone's offset.
> Again, because FromUTCTimestamp's evaluation assumes a shifted timestamp, the
> result is wrong.
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