Only keep the first element of multivalue field response.

docs/experiment/ppl/general/datatypes.rst

@@ -406,3 +406,16 @@ PPL query::
     |-----------+-------------|
     | 1         | Seattle     |
     +-----------+-------------+
+
+Example 3: Selecting Field of Array Value
+-----------------------------------------
+
+Select deeper level for object fields of array value which returns the first element in the array. For example, because inner field ``accounts.id`` has three values instead of a tuple in this document, the first entry is returned.::
+
+    od> source = people | fields accounts, accounts.id;
+    fetched rows / total rows = 1/1
+    +------------+---------------+
+    | accounts   | accounts.id   |
+    |------------+---------------|
+    | {'id': 1}  | 1             |
+    +------------+---------------+

elasticsearch/src/main/java/com/amazon/opendistroforelasticsearch/sql/elasticsearch/data/utils/Content.java

@@ -0,0 +1,106 @@
+/*
+ *     Copyright 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ *     Licensed under the Apache License, Version 2.0 (the "License").
+ *     You may not use this file except in compliance with the License.
+ *     A copy of the License is located at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *     or in the "license" file accompanying this file. This file 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 com.amazon.opendistroforelasticsearch.sql.elasticsearch.data.utils;
+
+import java.util.Iterator;
+import java.util.Map;
+import org.apache.commons.lang3.tuple.Pair;
+
+/**
+ * Regardless the underling data format, the {@link Content} define the data in abstract manner.
+ * which could be parsed by ElasticsearchExprValueFactory.
+ * There are two major use cases:
+ * 1. Represent the JSON data retrieve from Elasticsearch search response.
+ * 2. Represent the Object data extract from the Elasticsearch aggregation response.
+ */
+public interface Content {
+
+  /**
+   * Is null value.
+   */
+  boolean isNull();
+
+  /**
+   * Is number value.
+   */
+  boolean isNumber();
+
+  /**
+   * Is string value.
+   */
+  boolean isString();
+
+  /**
+   * Get integer value.
+   */
+  Integer intValue();
+
+  /**
+   * Get long value.
+   */
+  Long longValue();
+
+  /**
+   * Get short value.
+   */
+  Short shortValue();
+
+  /**
+   * Get byte value.
+   */
+  Byte byteValue();
+
+  /**
+   * Get float value.
+   */
+  Float floatValue();
+
+  /**
+   * Get double value.
+   */
+  Double doubleValue();
+
+  /**
+   * Get string value.
+   */
+  String stringValue();
+
+  /**
+   * Get boolean value.
+   */
+  Boolean booleanValue();
+
+  /**
+   * Get map of {@link Content} value.
+   */
+  Iterator<Map.Entry<String, Content>> map();
+
+  /**
+   * Get array of {@link Content} value.
+   */
+  Iterator<? extends Content> array();
+
+  /**
+   * Get geo point value.
+   */
+  Pair<Double, Double> geoValue();
+
+  /**
+   * Get {@link Object} value.
+   */
+  Object objectValue();
+}

elasticsearch/src/main/java/com/amazon/opendistroforelasticsearch/sql/elasticsearch/data/utils/ElasticsearchJsonContent.java

@@ -0,0 +1,123 @@
+/*
+ *     Copyright 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ *     Licensed under the Apache License, Version 2.0 (the "License").
+ *     You may not use this file except in compliance with the License.
+ *     A copy of the License is located at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *     or in the "license" file accompanying this file. This file 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 com.amazon.opendistroforelasticsearch.sql.elasticsearch.data.utils;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.google.common.collect.Iterators;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import lombok.RequiredArgsConstructor;
+import org.apache.commons.lang3.tuple.Pair;
+
+/**
+ * The Implementation of Content to represent {@link JsonNode}.
+ */
+@RequiredArgsConstructor
+public class ElasticsearchJsonContent implements Content {
+
+  private final JsonNode value;
+
+  @Override
+  public Integer intValue() {
+    return value().intValue();
+  }
+
+  @Override
+  public Long longValue() {
+    return value().longValue();
+  }
+
+  @Override
+  public Short shortValue() {
+    return value().shortValue();
+  }
+
+  @Override
+  public Byte byteValue() {
+    return (byte) value().shortValue();
+  }
+
+  @Override
+  public Float floatValue() {
+    return value().floatValue();
+  }
+
+  @Override
+  public Double doubleValue() {
+    return value().doubleValue();
+  }
+
+  @Override
+  public String stringValue() {
+    return value().asText();
+  }
+
+  @Override
+  public Boolean booleanValue() {
+    return value().booleanValue();
+  }
+
+  @Override
+  public Iterator<Map.Entry<String, Content>> map() {
+    LinkedHashMap<String, Content> map = new LinkedHashMap<>();
+    final JsonNode mapValue = value();
+    mapValue
+        .fieldNames()
+        .forEachRemaining(
+            field -> map.put(field, new ElasticsearchJsonContent(mapValue.get(field))));
+    return map.entrySet().iterator();
+  }
+
+  @Override
+  public Iterator<? extends Content> array() {
+    return Iterators.transform(value.elements(), ElasticsearchJsonContent::new);
+  }
+
+  @Override
+  public boolean isNull() {
+    return value == null || value.isNull();
+  }
+
+  @Override
+  public boolean isNumber() {
+    return value().isNumber();
+  }
+
+  @Override
+  public boolean isString() {
+    return value().isTextual();
+  }
+
+  @Override
+  public Object objectValue() {
+    return value();
+  }
+
+  @Override
+  public Pair<Double, Double> geoValue() {
+    return Pair.of(value().get("lat").doubleValue(), value().get("lon").doubleValue());
+  }
+
+  /**
+   * Return the first element if is Elasticsearch Array.
+   * https://www.elastic.co/guide/en/elasticsearch/reference/current/array.html.
+   */
+  private JsonNode value() {
+    return value.isArray() ? value.get(0) : value;
+  }
+}

elasticsearch/src/main/java/com/amazon/opendistroforelasticsearch/sql/elasticsearch/data/utils/ObjectContent.java

@@ -0,0 +1,125 @@
+/*
+ *     Copyright 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ *     Licensed under the Apache License, Version 2.0 (the "License").
+ *     You may not use this file except in compliance with the License.
+ *     A copy of the License is located at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *     or in the "license" file accompanying this file. This file 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 com.amazon.opendistroforelasticsearch.sql.elasticsearch.data.utils;
+
+import java.util.AbstractMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import lombok.RequiredArgsConstructor;
+import org.apache.commons.lang3.tuple.Pair;
+
+/**
+ * The Implementation of Content to represent {@link Object}.
+ */
+@RequiredArgsConstructor
+public class ObjectContent implements Content {
+
+  private final Object value;
+
+  @Override
+  public Integer intValue() {
+    return parseNumberValue(value, Integer::valueOf, Number::intValue);
+  }
+
+  @Override
+  public Long longValue() {
+    return parseNumberValue(value, Long::valueOf, Number::longValue);
+  }
+
+  @Override
+  public Short shortValue() {
+    return parseNumberValue(value, Short::valueOf, Number::shortValue);
+  }
+
+  @Override
+  public Byte byteValue() {
+    return parseNumberValue(value, Byte::valueOf, Number::byteValue);
+  }
+
+  @Override
+  public Float floatValue() {
+    return parseNumberValue(value, Float::valueOf, Number::floatValue);
+  }
+
+  @Override
+  public Double doubleValue() {
+    return parseNumberValue(value, Double::valueOf, Number::doubleValue);
+  }
+
+  @Override
+  public String stringValue() {
+    return (String) value;
+  }
+
+  @Override
+  public Boolean booleanValue() {
+    return (Boolean) value;
+  }
+
+  @Override
+  public Object objectValue() {
+    return value;
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public Iterator<Map.Entry<String, Content>> map() {
+    return ((Map<String, Object>) value).entrySet().stream()
+        .map(entry -> (Map.Entry<String, Content>) new AbstractMap.SimpleEntry<String, Content>(
+            entry.getKey(),
+            new ObjectContent(entry.getValue())))
+        .iterator();
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public Iterator<? extends Content> array() {
+    return ((List<Object>) value).stream().map(ObjectContent::new).iterator();
+  }
+
+  @Override
+  public boolean isNull() {
+    return value == null;
+  }
+
+  @Override
+  public boolean isNumber() {
+    return value instanceof Number;
+  }
+
+  @Override
+  public boolean isString() {
+    return value instanceof String;
+  }
+
+  @Override
+  public Pair<Double, Double> geoValue() {
+    final String[] split = ((String) value).split(",");
+    return Pair.of(Double.valueOf(split[0]), Double.valueOf(split[1]));
+  }
+
+  private <T> T parseNumberValue(Object value, Function<String, T> stringTFunction,
+                                 Function<Number, T> numberTFunction) {
+    if (value instanceof String) {
+      return stringTFunction.apply((String) value);
+    } else {
+      return numberTFunction.apply((Number) value);
+    }
+  }
+}