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https://issues.apache.org/jira/browse/SPARK-31030?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Wenchen Fan reassigned SPARK-31030:
-----------------------------------
Assignee: Yuanjian Li
> Backward Compatibility for Parsing and Formatting Datetime
> ----------------------------------------------------------
>
> Key: SPARK-31030
> URL: https://issues.apache.org/jira/browse/SPARK-31030
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.0.0
> Reporter: Yuanjian Li
> Assignee: Yuanjian Li
> Priority: Major
> Attachments: image-2020-03-04-10-54-05-208.png,
> image-2020-03-04-10-54-13-238.png
>
>
> *Background*
> In Spark version 2.4 and earlier, datetime parsing, formatting and conversion
> are performed by using the hybrid calendar ([Julian +
> Gregorian|https://docs.oracle.com/javase/7/docs/api/java/util/GregorianCalendar.html]).
>
> Since the Proleptic Gregorian calendar is de-facto calendar worldwide, as
> well as the chosen one in ANSI SQL standard, Spark 3.0 switches to it by
> using Java 8 API classes (the java.time packages that are based on [ISO
> chronology|https://docs.oracle.com/javase/8/docs/api/java/time/chrono/IsoChronology.html]
> ).
> The switching job is completed in SPARK-26651.
>
> *Problem*
> Switching to Java 8 datetime API breaks the backward compatibility of Spark
> 2.4 and earlier when parsing datetime. Spark need its own patters definition
> on datetime parsing and formatting.
>
> *Solution*
> To avoid unexpected result changes after the underlying datetime API switch,
> we propose the following solution.
> * Introduce the fallback mechanism: when the Java 8-based parser fails, we
> need to detect these behavior differences by falling back to the legacy
> parser, and fail with a user-friendly error message to tell users what gets
> changed and how to fix the pattern.
> * Document the Spark’s datetime patterns: The date-time formatter of Spark
> is decoupled with the Java patterns. The Spark’s patterns are mainly based on
> the [Java 7’s
> pattern|https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html]
> (for better backward compatibility) with the customized logic (caused by the
> breaking changes between [Java
> 7|https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html]
> and [Java
> 8|https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html]
> pattern string). Below are the customized rules:
> ||Pattern||Java 7||Java 8|| Example||Rule||
> |u|Day number of week (1 = Monday, ..., 7 = Sunday)|Year (Different with y, u
> accept a negative value to represent BC, while y should be used together with
> G to do the same thing.)|!image-2020-03-04-10-54-05-208.png! |Substitute ‘u’
> to ‘e’ and use Java 8 parser to parse the string. If parsable, return the
> result; otherwise, fall back to ‘u’, and then use the legacy Java 7 parser to
> parse. When it is successfully parsed, throw an exception and ask users to
> change the pattern strings or turn on the legacy mode; otherwise, return NULL
> as what Spark 2.4 does.|
> | z| General time zone which also accepts
> [RFC 822 time zones|#rfc822timezone]]|Only accept time-zone name, e.g.
> Pacific Standard Time; PST|!image-2020-03-04-10-54-13-238.png! |The
> semantics of ‘z’ are different between Java 7 and Java 8. Here, Spark 3.0
> follows the semantics of Java 8.
> Use Java 8 to parse the string. If parsable, return the result; otherwise,
> use the legacy Java 7 parser to parse. When it is successfully parsed, throw
> an exception and ask users to change the pattern strings or turn on the
> legacy mode; otherwise, return NULL as what Spark 2.4 does.|
>
>
>
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