[jira] [Created] (SPARK-33291) Inconsistent NULL conversions to strings redux

["Stuart White (Jira)" <ji...@apache.org>]

Stuart White created SPARK-33291:
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             Summary: Inconsistent NULL conversions to strings redux
                 Key: SPARK-33291
                 URL: https://issues.apache.org/jira/browse/SPARK-33291
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 3.1.0
            Reporter: Stuart White


The changes in [SPARK-32501 Inconsistent NULL conversions to strings|https://issues.apache.org/jira/browse/SPARK-32501] introduced some behavior that I'd like to clean up a bit.

Here's sample code to illustrate the behavior I'd like to clean up:

{noformat}
val rows = Seq[String](null)
  .toDF("value")
  .withColumn("struct1", struct('value as "value1"))
  .withColumn("struct2", struct('value as "value1", 'value as "value2"))
  .withColumn("array1", array('value))
  .withColumn("array2", array('value, 'value))
  .withColumn("map1", map(lit("value1"), 'value))
  .withColumn("map2", map(lit("value1"), 'value, lit("value2"), 'value))

// Show the DataFrame using the "first" codepath.        
rows.show(truncate=false)
+-----+-------+-------------+------+--------+----------------+--------------------------------+
|value|struct1|struct2      |array1|array2  |map1            |map2                            |
+-----+-------+-------------+------+--------+----------------+--------------------------------+
|null |{ null}|{ null, null}|[]    |[, null]|{value1 -> null}|{value1 -> null, value2 -> null}|
+-----+-------+-------------+------+--------+----------------+--------------------------------+

// Write the DataFrame to disk, then read it back and show it to trigger the "codegen" code path:
rows.write.parquet("rows")
spark.read.parquet("rows").show(truncate=false)

+-----+-------+-------------+-------+-------------+----------------+--------------------------------+
|value|struct1|struct2      |array1 |array2       |map1            |map2                            |
+-----+-------+-------------+-------+-------------+----------------+--------------------------------+
|null |{ null}|{ null, null}|[ null]|[ null, null]|{value1 -> null}|{value1 -> null, value2 -> null}|
+-----+-------+-------------+-------+-------------+----------------+--------------------------------+
{noformat}

Notice:

1. If the first element of a struct is null, it is printed with a leading space (e.g. "\{ null\}").  I think it's preferable to print it without the leading space (e.g. "\{null\}").  This is consistent with how non-null values are printed inside a struct.
2. If the first element of an array is null, it is not printed at all in the first code path, and the "codegen" code path prints it with a leading space.  I think both code paths should be consistent and print it without a leading space (e.g. "[null]").

The desired result of this ticket is to product the following output via both code paths:

{noformat}
+-----+-------+------------+------+------------+----------------+--------------------------------+
|value|struct1|struct2     |array1|array2      |map1            |map2                            |
+-----+-------+------------+------+------------+----------------+--------------------------------+
|null |{null} |{null, null}|[null]|[null, null]|{value1 -> null}|{value1 -> null, value2 -> null}|
+-----+-------+------------+------+------------+----------------+--------------------------------+
{noformat}



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