Details
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Bug
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Status: Resolved
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Major
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Resolution: Won't Fix
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2.3.0
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None
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None
Description
This produces the expected answer:
df.select(from_utc_timestamp(lit("2018-03-13T06:18:23"), "GMT+1" ).as("dt")).show
+-------------------+
| dt|
+-------------------+
|2018-03-13 07:18:23|
+-------------------+
However, the equivalent UTC input (but with an explicit timezone) produces a wrong answer:
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|
+-------------------+
Additionally, the equivalent Unix time (1520921903, which is also "2018-03-13T06:18:23" in the UTC time zone) produces the same wrong answer:
df.select(from_utc_timestamp(to_timestamp(lit(1520921903)), "GMT+1" ).as("dt")).show
+-------------------+
| dt|
+-------------------+
|2018-03-13 00:18:23|
+-------------------+
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:
df.select(from_utc_timestamp(lit("2018-03-13T06:18:23"), "GMT+1" ).as("dt")).show
FromUTCTimestamp gets as input a Timestamp (long) value representing
2018-03-13T06:18:23-07:00 (long value 1520947103000000)
What FromUTCTimestamp needs instead is
2018-03-13T06:18:23+00:00 (long value 1520921903000000)
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.
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).
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.
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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