[SPARK-13866] [SQL] Handle decimal type in CSV inference at CSV data source.

## What changes were proposed in this pull request?

https://issues.apache.org/jira/browse/SPARK-13866

This PR adds the support to infer `DecimalType`.
Here are the rules between `IntegerType`, `LongType` and `DecimalType`.

#### Infering Types

1. `IntegerType` and then `LongType`are tried first.

  ```scala
  Int.MaxValue => IntegerType
  Long.MaxValue => LongType
  ```

2. If it fails, try `DecimalType`.

  ```scala
  (Long.MaxValue + 1) => DecimalType(20, 0)
  ```
  This does not try to infer this as `DecimalType` when scale is less than 0.

3. if it fails, try `DoubleType`
  ```scala
  0.1 => DoubleType // This is failed to be inferred as `DecimalType` because it has the scale, 1.
  ```

#### Compatible Types (Merging Types)

For merging types, this is the same with JSON data source. If `DecimalType` is not capable, then it becomes `DoubleType`

## How was this patch tested?

Unit tests were used and `./dev/run_tests` for code style test.

Author: hyukjinkwon <gurwls223@gmail.com>
Author: Hyukjin Kwon <gurwls223@gmail.com>

Closes #11724 from HyukjinKwon/SPARK-13866.

1 files changed, 7 insertions, 0 deletions

diff --git a/sql/core/src/test/resources/decimal.csv b/sql/core/src/test/resources/decimal.csv
new file mode 100644
index 0000000000..870f6aaf1b
--- /dev/null
+++ b/sql/core/src/test/resources/decimal.csv
@@ -0,0 +1,7 @@
+~ decimal field has integer, integer and decimal values. The last value cannot fit to a long
+~ long field has integer, long and integer values.
+~ double field has double, double and decimal values.
+decimal,long,double
+1,1,0.1
+1,9223372036854775807,1.0
+92233720368547758070,1,92233720368547758070