[SPARK-22973][SQL] Fix incorrect results of Casting Map to String

[maropu]

What changes were proposed in this pull request?

This pr fixed the issue when casting maps into strings;

scala> Seq(Map(1 -> "a", 2 -> "b")).toDF("a").write.saveAsTable("t")
scala> sql("SELECT cast(a as String) FROM t").show(false)
+----------------------------------------------------------------+
|a                                                               |
+----------------------------------------------------------------+
|org.apache.spark.sql.catalyst.expressions.UnsafeMapData@38bdd75d|
+----------------------------------------------------------------+

This pr modified the result into;

+----------------+
|a               |
+----------------+
|[1 -> a, 2 -> b]|
+----------------+

How was this patch tested?

Added tests in CastSuite.

[SparkQA]

Test build #85726 has finished for PR 20166 at commit efc28e3.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #85727 has finished for PR 20166 at commit be04e64.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

@cloud-fan

[cloud-fan]

map.keyArray() and map.valueArray appear many times, we can create 2 local variables for them.

[maropu]

ok

[cloud-fan]

LGTM except one minor comment

[maropu]

BTW, the current Datset.showString prints rows thru RowEncoder deserializers like;

scala> Seq(Seq(Seq(1, 2), Seq(3), Seq(4, 5, 6))).toDF("a").show(false)
+------------------------------------------------------------+
|a                                                           |
+------------------------------------------------------------+
|[WrappedArray(1, 2), WrappedArray(3), WrappedArray(4, 5, 6)]|
+------------------------------------------------------------+

If we cast them before prints, we could get more simpler forms like;

scala> Seq(Seq(Seq(1, 2), Seq(3), Seq(4, 5, 6))).toDF("a").show(false)
+------------------------+
|a                       |
+------------------------+
|[[1, 2], [3], [4, 5, 6]]|
+------------------------+

I'm not sure through, is this acceptable? (Probably, we might need to add a option to keep the old behaviour)

[SparkQA]

Test build #85751 has finished for PR 20166 at commit fb11796.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

I think this is a bug, in Dataset.showString I see code like

case seq: Seq[_] => seq.mkString("[", ", ", "]")

Which means we do want to show strings like [[1, 2], [3], [4, 5, 6]]

Anyway let's fix in another PR, I'm merging this PR first

[cloud-fan]

merging to master/2.3!

[maropu]

ok, I'll fix struct in a next following pr first.