## What changes were proposed in this pull request?
It seems we are using `SQLUtils.getSQLDataType` for type string in structField. It looks we can replace this with `CatalystSqlParser.parseDataType`.
They look similar DDL-like type definitions as below:
```scala
scala> Seq(Tuple1(Tuple1("a"))).toDF.show()
```
```
+---+
| _1|
+---+
|[a]|
+---+
```
```scala
scala> Seq(Tuple1(Tuple1("a"))).toDF.select($"_1".cast("struct<_1:string>")).show()
```
```
+---+
| _1|
+---+
|[a]|
+---+
```
Such type strings looks identical when R’s one as below:
```R
> write.df(sql("SELECT named_struct('_1', 'a') as struct"), "/tmp/aa", "parquet")
> collect(read.df("/tmp/aa", "parquet", structType(structField("struct", "struct<_1:string>"))))
struct
1 a
```
R’s one is stricter because we are checking the types via regular expressions in R side ahead.
Actual logics there look a bit different but as we check it ahead in R side, it looks replacing it would not introduce (I think) no behaviour changes. To make this sure, the tests dedicated for it were added in SPARK-20105. (It looks `structField` is the only place that calls this method).
## How was this patch tested?
Existing tests - https://github.com/apache/spark/blob/master/R/pkg/inst/tests/testthat/test_sparkSQL.R#L143-L194 should cover this.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#17785 from HyukjinKwon/SPARK-20493.
70f1bcd7bc