Github user aa8y commented on a diff in the pull request: https://github.com/apache/spark/pull/20068#discussion_r159160508 --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVOptions.scala --- @@ -152,7 +152,11 @@ class CSVOptions( writerSettings.setIgnoreLeadingWhitespaces(ignoreLeadingWhiteSpaceFlagInWrite) writerSettings.setIgnoreTrailingWhitespaces(ignoreTrailingWhiteSpaceFlagInWrite) writerSettings.setNullValue(nullValue) - writerSettings.setEmptyValue(nullValue) + // The Univocity parser parses empty strings as `null` by default. This is the default behavior + // for Spark too, since `nullValue` defaults to an empty string and has a higher precedence to + // setEmptyValue(). But when `nullValue` is set to a different value, that would mean that the + // empty string should be parsed not as `null` but as an empty string. + writerSettings.setEmptyValue("") --- End diff -- I talked about this with Hyukjin Kwon before. I think the previous behavior should _not_ be exposed as an option as the previous behavior was a bug. All it did was that it _always_ coerced empty values to `null`s. If the `nullValue` was not set, then the it was set to `""` by default which coerced `""` to `null`. The empty value being set to `""` had no affect in this case. If it was set to something else, say `\N`, then the empty value was also set to `\N` which resulted in parsing both `\N` and `""` to `null`, as `""` was no longer considered as an empty value and the `""` being coerced to null is the Univocity parser's default. Setting empty value explicitly to the `""` literal would ensure that an empty string is always parsed as empty string, unless `nullValue` is not set or it is set to `""`, which is what people would do if they want `""` to be parsed as `null`, which would be the old behavior.
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