[CALCITE-6832] Redundant fields/nullable entries in STRUCT serializat…

…ion to JSON

The JSON serialization of a RelDatatypeField of type STRUCT contains redundant fields and nullable entries along with unnecessary nesting.

The redundancy leads to wasted space and convoluted representation that is hard to follow especially when there are nested STRUCTS.

core/src/main/java/org/apache/calcite/rel/externalize/RelJson.java

@@ -558,15 +558,7 @@ private Object toJson(RelDataType node) {
   }
 
   private Object toJson(RelDataTypeField node) {
-    final Map<String, @Nullable Object> map;
-    if (node.getType().isStruct()) {
-      map = jsonBuilder().map();
-      map.put("fields", toJson(node.getType()));
-      map.put("nullable", node.getType().isNullable());
-    } else {
-      //noinspection unchecked
-      map = (Map<String, @Nullable Object>) toJson(node.getType());
-    }
+    Map<String, Object> map = (Map<String, Object>) toJson(node.getType());
     map.put("name", node.getName());
     return map;
   }

core/src/test/java/org/apache/calcite/plan/RelWriterTest.java

@@ -489,16 +489,13 @@ private static Fixture relFn(Function<RelBuilder, RelNode> relFn) {
           + "      \"name\": \"v\"\n"
           + "    },\n"
           + "    {\n"
-          + "      \"fields\": {\n"
-          + "        \"fields\": [\n"
-          + "          {\n"
-          + "            \"type\": \"DATE\",\n"
-          + "            \"nullable\": false,\n"
-          + "            \"name\": \"d\"\n"
-          + "          }\n"
-          + "        ],\n"
-          + "        \"nullable\": false\n"
-          + "      },\n"
+          + "      \"fields\": [\n"
+          + "        {\n"
+          + "          \"type\": \"DATE\",\n"
+          + "          \"nullable\": false,\n"
+          + "          \"name\": \"d\"\n"
+          + "        }\n"
+          + "      ],\n"
           + "      \"nullable\": false,\n"
           + "      \"name\": \"r\"\n"
           + "    }\n"

core/src/test/java/org/apache/calcite/rel/externalize/RelJsonTest.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.calcite.rel.externalize;
+
+import org.apache.calcite.rel.type.RelDataType;
+import org.apache.calcite.rel.type.RelDataTypeFactory;
+import org.apache.calcite.rel.type.RelDataTypeField;
+import org.apache.calcite.rel.type.RelDataTypeFieldImpl;
+import org.apache.calcite.rel.type.RelDataTypeSystem;
+import org.apache.calcite.sql.type.SqlTypeFactoryImpl;
+import org.apache.calcite.sql.type.SqlTypeName;
+import org.apache.calcite.test.DiffRepository;
+import org.apache.calcite.util.JsonBuilder;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for @{@link RelJson}.
+ */
+public class RelJsonTest {
+
+  private static final DiffRepository REPO =  DiffRepository.lookup(RelJsonTest.class);
+
+  @Test void testToJsonWithStructRelDatatypeField() {
+    RelDataTypeFactory typeFactory = new SqlTypeFactoryImpl(RelDataTypeSystem.DEFAULT);
+    RelDataType type = typeFactory.builder()
+        .add("street", SqlTypeName.VARCHAR, 50)
+        .add("number", SqlTypeName.INTEGER)
+        .add("building", SqlTypeName.VARCHAR, 20).nullable(true)
+        .build();
+    RelDataTypeField address =
+        new RelDataTypeFieldImpl("address", 0, type);
+
+    JsonBuilder builder = new JsonBuilder();
+    Object jsonObj = RelJson.create().withJsonBuilder(builder).toJson(address);
+    REPO.assertEquals("content", "${content}", builder.toJsonString(jsonObj));
+  }
+
+}

core/src/test/resources/org/apache/calcite/rel/externalize/RelJsonTest.xml

@@ -0,0 +1,46 @@
+<?xml version="1.0" ?>
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one or more
+  ~ contributor license agreements.  See the NOTICE file distributed with
+  ~ this work for additional information regarding copyright ownership.
+  ~ The ASF licenses this file to you under the Apache License, Version 2.0
+  ~ (the "License"); you may not use this file except in compliance with
+  ~ the License.  You may obtain a copy of the License at
+  ~
+  ~ http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+<Root>
+  <TestCase name="testToJsonWithStructRelDatatypeField">
+    <Resource name="content">
+      <![CDATA[{
+  "fields": [
+    {
+      "type": "VARCHAR",
+      "nullable": false,
+      "precision": 50,
+      "name": "street"
+    },
+    {
+      "type": "INTEGER",
+      "nullable": false,
+      "name": "number"
+    },
+    {
+      "type": "VARCHAR",
+      "nullable": true,
+      "precision": 20,
+      "name": "building"
+    }
+  ],
+  "nullable": false,
+  "name": "address"
+}]]>
+    </Resource>
+  </TestCase>
+</Root>