wayneguow opened a new pull request #34853:
URL: https://github.com/apache/spark/pull/34853
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### What changes were proposed in this pull request?
Fix the bug that null values are saved as quoted empty strings "" (as the
same as empty strings) rather than nothing by default csv settings since Spark
2.4.
### Why are the changes needed?
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This is an unexpected bug, if don't fix it, we still can't distinguish null
values and empty strings in saved csv files.
As mentioned in [spark sql migration
guide](https://spark.apache.org/docs/latest/sql-migration-guide.html#upgrading-from-spark-sql-23-to-24)(2.3=>2.4),
empty strings are saved as quoted empty string "", null values as saved as
nothing since Spark 2.4.
> Since Spark 2.4, empty strings are saved as quoted empty strings "". In
version 2.3 and earlier, empty strings are equal to null values and do not
reflect to any characters in saved CSV files. For example, the row of "a",
null, "", 1 was written as a,,,1. Since Spark 2.4, the same row is saved as
a,,"",1. To restore the previous behavior, set the CSV option emptyValue to
empty (not quoted) string.
But actually, we found that null values are also saved as quoted empty
strings "" as the same as empty strings.
For codes follows:
```scala
Seq(("Tesla", null.asInstanceOf[String], ""))
.toDF("make", "comment", "blank")
.coalesce(1)
.write.csv(path)
```
actual results:
>Tesla,"",""
expected results:
>Tesla,,""
### Does this PR introduce _any_ user-facing change?
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Yes, if this bug has been fixed, the output of null values would been
changed to nothing rather than quoted empty strings "".
But, users can set nullValue to "\\"\\""(same as emptyValueInWrite's default
value) to restore the previous behavior since 2.4.
### How was this patch tested?
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Adding a test case.
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