[SPARK-19637][SQL] Add to_json in FunctionRegistry

## What changes were proposed in this pull request?
This pr added entries  in `FunctionRegistry` and supported `to_json` in SQL.

## How was this patch tested?
Added tests in `JsonFunctionsSuite`.

Author: Takeshi Yamamuro <yamamuro@apache.org>

Closes #16981 from maropu/SPARK-19637.

1 files changed, 63 insertions, 0 deletions

diff --git a/sql/core/src/test/resources/sql-tests/results/json-functions.sql.out b/sql/core/src/test/resources/sql-tests/results/json-functions.sql.out
new file mode 100644
index 0000000000..d8aa4fb9fa
--- /dev/null
+++ b/sql/core/src/test/resources/sql-tests/results/json-functions.sql.out
@@ -0,0 +1,63 @@
+-- Automatically generated by SQLQueryTestSuite
+-- Number of queries: 6
+
+
+-- !query 0
+describe function to_json
+-- !query 0 schema
+struct<function_desc:string>
+-- !query 0 output
+Class: org.apache.spark.sql.catalyst.expressions.StructToJson
+Function: to_json
+Usage: to_json(expr[, options]) - Returns a json string with a given struct value
+
+
+-- !query 1
+describe function extended to_json
+-- !query 1 schema
+struct<function_desc:string>
+-- !query 1 output
+Class: org.apache.spark.sql.catalyst.expressions.StructToJson
+Extended Usage:
+    Examples:
+      > SELECT to_json(named_struct('a', 1, 'b', 2));
+       {"a":1,"b":2}
+      > SELECT to_json(named_struct('time', to_timestamp('2015-08-26', 'yyyy-MM-dd')), map('timestampFormat', 'dd/MM/yyyy'));
+       {"time":"26/08/2015"}
+  
+Function: to_json
+Usage: to_json(expr[, options]) - Returns a json string with a given struct value
+
+
+-- !query 2
+select to_json(named_struct('a', 1, 'b', 2))
+-- !query 2 schema
+struct<structtojson(named_struct(a, 1, b, 2)):string>
+-- !query 2 output
+{"a":1,"b":2}
+
+
+-- !query 3
+select to_json(named_struct('time', to_timestamp('2015-08-26', 'yyyy-MM-dd')), map('timestampFormat', 'dd/MM/yyyy'))
+-- !query 3 schema
+struct<structtojson(named_struct(time, to_timestamp('2015-08-26', 'yyyy-MM-dd'))):string>
+-- !query 3 output
+{"time":"26/08/2015"}
+
+
+-- !query 4
+select to_json(named_struct('a', 1, 'b', 2), named_struct('mode', 'PERMISSIVE'))
+-- !query 4 schema
+struct<>
+-- !query 4 output
+org.apache.spark.sql.AnalysisException
+Must use a map() function for options;; line 1 pos 7
+
+
+-- !query 5
+select to_json()
+-- !query 5 schema
+struct<>
+-- !query 5 output
+org.apache.spark.sql.AnalysisException
+Invalid number of arguments for function to_json; line 1 pos 7