Youwei Wang has posted comments on this change.

Change subject: IMPALA-3504: function for current timestamp in UTC, i.e. 
utc_timestamp()
......................................................................


Patch Set 1:

> As I mentioned in your previous review and/or the JIRA, this needs
 > to use the query-wide timestamp, i.e. what we do for now().

Greetings, Matthew.
If you don't mind, I want to propose an idea about this task.
Considering three fators of utc_timestamp, local_timestamp and timezone, I 
believe we have following deductions:
utc_timestamp + local_timestamp → timezone
utc_timestamp + timezone → local_timestamp 
local_timestamp + timezone → utc_timestamp 
PS: So far as I know, the UTC time is essentially the time without any timezone 
offset. If you don't mind, I want to take the Beijing time as example. Since 
Beijing timezone is of +8h offset, I can get corresponding UTC time by minusing 
8 from current Beijing time. I have done some calculation and calibration using 
online time service. And this approach works. If you find some mistakes in my 
approach, please feel free to point them out. Thank you. :)

And I have found the following code:
query_ctx->__set_now_string(TimestampValue::LocalTime().DebugString());
in be/src/service/impala-server.cc:841. 
So please allow me to assume we have local_timestamp of the host where Impala 
service runs. In this case, we can:
1. directly store the utc_timestamp in the context data structure. Then 
utc_timestamp() can be accessed like the function now(). 
2. save the timezone/offset in the context data structure. Then we can use the 
now() and such timezone/offset to calculate the utc_timestamp.

If possbile, would you please tell which solution you prefer? 
Thank you for reading this long post. :)

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Gerrit-MessageType: comment
Gerrit-Change-Id: I5ee6ee192aa469f77c711f27ad324696a42004d1
Gerrit-PatchSet: 1
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Youwei Wang <youwei.a.w...@intel.com>
Gerrit-Reviewer: Jim Apple <jbap...@cloudera.com>
Gerrit-Reviewer: Matthew Jacobs <m...@cloudera.com>
Gerrit-Reviewer: Youwei Wang <youwei.a.w...@intel.com>
Gerrit-HasComments: No

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