yaooqinn opened a new pull request #27906: [SPARK-31150][SQL] Parsing seconds fraction with variable length for timestamp URL: https://github.com/apache/spark/pull/27906 ### What changes were proposed in this pull request? This PR is to support parsing timestamp values with variable length second fraction parts. e.g. 'yyyy-MM-dd HH:mm:ss.SSSSSS[zzz]' can parse timestamp with 0~6 digit-length second fraction but fail >=7 ```sql select to_timestamp(v, 'yyyy-MM-dd HH:mm:ss.SSSSSS[zzz]') from values ('2019-10-06 10:11:12.'), ('2019-10-06 10:11:12.0'), ('2019-10-06 10:11:12.1'), ('2019-10-06 10:11:12.12'), ('2019-10-06 10:11:12.123UTC'), ('2019-10-06 10:11:12.1234'), ('2019-10-06 10:11:12.12345CST'), ('2019-10-06 10:11:12.123456PST') t(v) 2019-10-06 03:11:12.123 2019-10-06 08:11:12.12345 2019-10-06 10:11:12 2019-10-06 10:11:12 2019-10-06 10:11:12.1 2019-10-06 10:11:12.12 2019-10-06 10:11:12.1234 2019-10-06 10:11:12.123456 select to_timestamp('2019-10-06 10:11:12.1234567PST', 'yyyy-MM-dd HH:mm:ss.SSSSSS[zzz]') NULL ``` Since 3.0, we use java 8 time API to parse and format timestamp values. when we create the `DateTimeFormatter`, we use `appendPattern` to create the build first, where the 'S..S' part will be parsed to a fixed-length(= `'S..S'.length`). This fits the formatting part but too strict for the parsing part because the trailing zeros are very likely to be truncated. ### Why are the changes needed? <!-- Please clarify why the changes are needed. For instance, 1. If you propose a new API, clarify the use case for a new API. 2. If you fix a bug, you can clarify why it is a bug. --> improve timestamp parsing ### Does this PR introduce any user-facing change? <!-- If yes, please clarify the previous behavior and the change this PR proposes - provide the console output, description and/or an example to show the behavior difference if possible. If no, write 'No'. --> no, the related changes are newly added ### How was this patch tested? <!-- If tests were added, say they were added here. Please make sure to add some test cases that check the changes thoroughly including negative and positive cases if possible. If it was tested in a way different from regular unit tests, please clarify how you tested step by step, ideally copy and paste-able, so that other reviewers can test and check, and descendants can verify in the future. If tests were not added, please describe why they were not added and/or why it was difficult to add. --> add uts
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