[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.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -421,6 +421,9 @@ object FunctionRegistry {
     expression[BitwiseOr]("|"),
     expression[BitwiseXor]("^"),
 
+    // json
+    expression[StructToJson]("to_json"),
+
     // Cast aliases (SPARK-16730)
     castAlias("boolean", BooleanType),
     castAlias("tinyint", ByteType),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/jsonExpressions.scala

@@ -23,11 +23,12 @@ import scala.util.parsing.combinator.RegexParsers
 
 import com.fasterxml.jackson.core._
 
+import org.apache.spark.sql.AnalysisException
 import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
 import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.json._
-import org.apache.spark.sql.catalyst.util.{GenericArrayData, ParseModes}
+import org.apache.spark.sql.catalyst.util.{ArrayBasedMapData, GenericArrayData, ParseModes}
 import org.apache.spark.sql.types._
 import org.apache.spark.unsafe.types.UTF8String
 import org.apache.spark.util.Utils
@@ -330,7 +331,7 @@ case class GetJsonObject(json: Expression, path: Expression)
 
 // scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "_FUNC_(jsonStr, p1, p2, ..., pn) - Return a tuple like the function get_json_object, but it takes multiple names. All the input parameters and output column types are string.",
+  usage = "_FUNC_(jsonStr, p1, p2, ..., pn) - Returns a tuple like the function get_json_object, but it takes multiple names. All the input parameters and output column types are string.",
   extended = """
     Examples:
       > SELECT _FUNC_('{"a":1, "b":2}', 'a', 'b');
@@ -564,6 +565,17 @@ case class JsonToStruct(
 /**
  * Converts a [[StructType]] to a json output string.
  */
+// scalastyle:off line.size.limit
+@ExpressionDescription(
+  usage = "_FUNC_(expr[, options]) - Returns a json string with a given struct value",
+  extended = """
+    Examples:
+      > SELECT _FUNC_(named_struct('a', 1, 'b', 2));
+       {"a":1,"b":2}
+      > SELECT _FUNC_(named_struct('time', to_timestamp('2015-08-26', 'yyyy-MM-dd')), map('timestampFormat', 'dd/MM/yyyy'));
+       {"time":"26/08/2015"}
+  """)
+// scalastyle:on line.size.limit
 case class StructToJson(
     options: Map[String, String],
     child: Expression,
@@ -573,6 +585,14 @@ case class StructToJson(
 
   def this(options: Map[String, String], child: Expression) = this(options, child, None)
 
+  // Used in `FunctionRegistry`
+  def this(child: Expression) = this(Map.empty, child, None)
+  def this(child: Expression, options: Expression) =
+    this(
+      options = StructToJson.convertToMapData(options),
+      child = child,
+      timeZoneId = None)
+
   @transient
   lazy val writer = new CharArrayWriter()
 
@@ -613,3 +633,20 @@ case class StructToJson(
 
   override def inputTypes: Seq[AbstractDataType] = StructType :: Nil
 }
+
+object StructToJson {
+
+  def convertToMapData(exp: Expression): Map[String, String] = exp match {
+    case m: CreateMap
+        if m.dataType.acceptsType(MapType(StringType, StringType, valueContainsNull = false)) =>
+      val arrayMap = m.eval().asInstanceOf[ArrayBasedMapData]
+      ArrayBasedMapData.toScalaMap(arrayMap).map { case (key, value) =>
+        key.toString -> value.toString
+      }
+    case m: CreateMap =>
+      throw new AnalysisException(
+        s"A type of keys and values in map() must be string, but got ${m.dataType}")
+    case _ =>
+      throw new AnalysisException("Must use a map() function for options")
+  }
+}

sql/core/src/test/resources/sql-tests/inputs/json-functions.sql

+-- to_json
+describe function to_json;
+describe function extended to_json;
+select to_json(named_struct('a', 1, 'b', 2));
+select to_json(named_struct('time', to_timestamp('2015-08-26', 'yyyy-MM-dd')), map('timestampFormat', 'dd/MM/yyyy'));
+-- Check if errors handled
+select to_json(named_struct('a', 1, 'b', 2), named_struct('mode', 'PERMISSIVE'));
+select to_json();

sql/core/src/test/resources/sql-tests/results/json-functions.sql.out

+-- 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

sql/core/src/test/scala/org/apache/spark/sql/JsonFunctionsSuite.scala

@@ -197,4 +197,27 @@ class JsonFunctionsSuite extends QueryTest with SharedSQLContext {
       .select(to_json($"struct").as("json"))
     checkAnswer(dfTwo, readBackTwo)
   }
+
+  test("SPARK-19637 Support to_json in SQL") {
+    val df1 = Seq(Tuple1(Tuple1(1))).toDF("a")
+    checkAnswer(
+      df1.selectExpr("to_json(a)"),
+      Row("""{"_1":1}""") :: Nil)
+
+    val df2 = Seq(Tuple1(Tuple1(java.sql.Timestamp.valueOf("2015-08-26 18:00:00.0")))).toDF("a")
+    checkAnswer(
+      df2.selectExpr("to_json(a, map('timestampFormat', 'dd/MM/yyyy HH:mm'))"),
+      Row("""{"_1":"26/08/2015 18:00"}""") :: Nil)
+
+    val errMsg1 = intercept[AnalysisException] {
+      df2.selectExpr("to_json(a, named_struct('a', 1))")
+    }
+    assert(errMsg1.getMessage.startsWith("Must use a map() function for options"))
+
+    val errMsg2 = intercept[AnalysisException] {
+      df2.selectExpr("to_json(a, map('a', 1))")
+    }
+    assert(errMsg2.getMessage.startsWith(
+      "A type of keys and values in map() must be string, but got"))
+  }
 }