Piotr Findeisen commented on HIVE-12192:

Few clarifying questions

bq. The type is not really an instant, even before HIVE-12192. Semantics from 
querying perspective are similar to SQL timestamp. 

How much similar? Is it really the same as SQL (standard) timestamp, i.e. a 
"Record" of year/month/day/hour/minute/second[/fraction] ?
Did this semantics change over time? ([~haozhun] and I work on Presto and we 
would like Presto's Hive connector to work appropriately with different 
versions of Hive)

bq. it uses java.sql.timestamp class to store the value

What about values that cannot be represented using {{java.sql.Timestamp}}? 
(i.e. whenever there's a forward offset change in JVM's zone)
Did you also consider using a different representation, like 
{{java.time.LocalDateTime}}? (if this representation is indeed applicable)

bq. How does ORC fix this? \[...]

Do you happen to know it is handled for other file types? Parquet, RC binary, 
RC text, textfile?

> Hive should carry out timestamp computations in UTC
> ---------------------------------------------------
>                 Key: HIVE-12192
>                 URL: https://issues.apache.org/jira/browse/HIVE-12192
>             Project: Hive
>          Issue Type: Sub-task
>          Components: Hive
>            Reporter: Ryan Blue
>            Assignee: Jesus Camacho Rodriguez
>            Priority: Major
>              Labels: timestamp
>         Attachments: HIVE-12192.patch
> Hive currently uses the "local" time of a java.sql.Timestamp to represent the 
> SQL data type TIMESTAMP WITHOUT TIME ZONE. The purpose is to be able to use 
> {{Timestamp#getYear()}} and similar methods to implement SQL functions like 
> {{year}}.
> When the SQL session's time zone is a DST zone, such as America/Los_Angeles 
> that alternates between PST and PDT, there are times that cannot be 
> represented because the effective zone skips them.
> {code}
> hive> select TIMESTAMP '2015-03-08 02:10:00.101';
> 2015-03-08 03:10:00.101
> {code}
> Using UTC instead of the SQL session time zone as the underlying zone for a 
> java.sql.Timestamp avoids this bug, while still returning correct values for 
> {{getYear}} etc. Using UTC as the convenience representation (timestamp 
> without time zone has no real zone) would make timestamp calculations more 
> consistent and avoid similar problems in the future.
> Notably, this would break the {{unix_timestamp}} UDF that specifies the 
> result is with respect to ["the default timezone and default 
> locale"|https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF#LanguageManualUDF-DateFunctions].
>  That function would need to be updated to use the 
> {{System.getProperty("user.timezone")}} zone.

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