[SPARK-33915] Allow json expression to be pushable column

Author: tedyu

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

@@ -139,7 +139,7 @@ case class GetJsonObject(json: Expression, path: Expression)
 
   @transient private lazy val parsedPath = parsePath(path.eval().asInstanceOf[UTF8String])
 
-  override def eval(input: InternalRow): Any = {
+  override def eval(input: InternalRow): UTF8String = {
     val jsonStr = json.eval(input).asInstanceOf[UTF8String]
     if (jsonStr == null) {
       return null

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSourceStrategy.scala

@@ -737,6 +737,8 @@ abstract class PushableColumnBase {
         }
       case s: GetStructField if nestedPredicatePushdownEnabled =>
         helper(s.child).map(_ :+ s.childSchema(s.ordinal).name)
+      case GetJsonObject(col, field) if nestedPredicatePushdownEnabled =>
+        Some(Seq("GetJsonObject(" + col + "," + field + ")"))
       case _ => None
     }
     helper(e).map(_.quoted)