## What changes were proposed in this pull request?
https://issues.apache.org/jira/browse/SPARK-14231
Currently, JSON data source supports to infer `DecimalType` for big numbers and `floatAsBigDecimal` option which reads floating-point values as `DecimalType`.
But there are few restrictions in Spark `DecimalType` below:
1. The precision cannot be bigger than 38.
2. scale cannot be bigger than precision.
Currently, both restrictions are not being handled.
This PR handles the cases by inferring them as `DoubleType`. Also, the option name was changed from `floatAsBigDecimal` to `prefersDecimal` as suggested [here](https://issues.apache.org/jira/browse/SPARK-14231?focusedCommentId=15215579&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15215579).
So, the codes below:
```scala
def doubleRecords: RDD[String] =
sqlContext.sparkContext.parallelize(
s"""{"a": 1${"0" * 38}, "b": 0.01}""" ::
s"""{"a": 2${"0" * 38}, "b": 0.02}""" :: Nil)
val jsonDF = sqlContext.read
.option("prefersDecimal", "true")
.json(doubleRecords)
jsonDF.printSchema()
```
produces below:
- **Before**
```scala
org.apache.spark.sql.AnalysisException: Decimal scale (2) cannot be greater than precision (1).;
at org.apache.spark.sql.types.DecimalType.<init>(DecimalType.scala:44)
at org.apache.spark.sql.execution.datasources.json.InferSchema$.org$apache$spark$sql$execution$datasources$json$InferSchema$$inferField(InferSchema.scala:144)
at org.apache.spark.sql.execution.datasources.json.InferSchema$.org$apache$spark$sql$execution$datasources$json$InferSchema$$inferField(InferSchema.scala:108)
at
...
```
- **After**
```scala
root
|-- a: double (nullable = true)
|-- b: double (nullable = true)
```
## How was this patch tested?
Unit tests were used and `./dev/run_tests` for coding style tests.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#12030 from HyukjinKwon/SPARK-14231.
2262a93358