Switch from using docvalue_fields to extracting values from _source

docs/reference/sql/endpoints/translate.asciidoc

@@ -24,9 +24,6 @@ Which returns:
 {
     "size" : 10,
     "docvalue_fields" : [
-        {
-            "field": "page_count"
-        },
         {
             "field": "release_date",
             "format": "epoch_millis"
@@ -35,7 +32,8 @@ Which returns:
     "_source": {
         "includes": [
             "author",
-            "name"
+            "name",
+            "page_count"
         ],
         "excludes": []
     },

docs/reference/sql/limitations.asciidoc

@@ -161,3 +161,21 @@ By default,`geo_points` fields are indexed and have doc values. However only lat
 indexed with some loss of precision from the original values (4.190951585769653E-8 for the latitude and
 8.381903171539307E-8 for longitude). The altitude component is accepted but not stored in doc values nor indexed.
 Therefore calling `ST_Z` function in the filtering, grouping or sorting will return `null`.
+
+[float]
+[[fields-from-source]]
+=== Retrieving from `_source`
+
+Most of {es-sql}'s columns are retrieved from the document's `_source` and there is no attempt to get the columns content from
+`docvalue_fields` not even in the case <<mapping-source-field,`_source`>> field is disabled in the mapping explicitly.
+If a column, for which there is no source stored, is asked for in a query, {es-sql} will not return it. Field types that don't follow
+this restriction are: `keyword`, `date`, `scaled_float`, `geo_point`, `geo_shape` since they are NOT returned from `_source` but
+from `docvalue_fields`.
+
+[float]
+[[fields-from-docvalues]]
+=== Retrieving from `docvalue_fields`
+
+When the number of columns retrieveable from `docvalue_fields` is greater than the configured <<dynamic-index-settings,`index.max_docvalue_fields_search` setting>>
+the query will fail with `IllegalArgumentException: Trying to retrieve too many docvalue_fields` error. Either the mentioned {es}
+setting needs to be adjusted or fewer columns retrieveable from `docvalue_fields` need to be selected.

test/framework/src/main/java/org/elasticsearch/test/NotEqualMessageBuilder.java

@@ -150,7 +150,8 @@ public void compare(String field, @Nullable Object actual, Object expected) {
             field(field, "same [" + expected + "]");
             return;
         }
-        field(field, "expected [" + expected + "] but was [" + actual + "]");
+        field(field, "expected " + expected.getClass().getSimpleName() + " [" + expected + "] but was "
+                + actual.getClass().getSimpleName() + " [" + actual + "]");
     }
 
     private void indent() {

test/framework/src/test/java/org/elasticsearch/test/hamcrest/ElasticsearchAssertionsTests.java

@@ -122,7 +122,7 @@ public void testAssertXContentEquivalentErrors() throws IOException {
             AssertionError error = expectThrows(AssertionError.class,
                     () -> assertToXContentEquivalent(BytesReference.bytes(builder), BytesReference.bytes(otherBuilder),
                             builder.contentType()));
-            assertThat(error.getMessage(), containsString("f2: expected [value2] but was [differentValue2]"));
+            assertThat(error.getMessage(), containsString("f2: expected String [value2] but was String [differentValue2]"));
         }
         {
             XContentBuilder builder = JsonXContent.contentBuilder();
@@ -155,7 +155,7 @@ public void testAssertXContentEquivalentErrors() throws IOException {
             AssertionError error = expectThrows(AssertionError.class,
                     () -> assertToXContentEquivalent(BytesReference.bytes(builder), BytesReference.bytes(otherBuilder),
                             builder.contentType()));
-            assertThat(error.getMessage(), containsString("2: expected [three] but was [four]"));
+            assertThat(error.getMessage(), containsString("2: expected String [three] but was String [four]"));
         }
         {
             XContentBuilder builder = JsonXContent.contentBuilder();

x-pack/plugin/ml/qa/ml-with-security/src/test/java/org/elasticsearch/smoketest/MlWithSecurityInsufficientRoleIT.java

@@ -30,7 +30,7 @@ public void test() throws IOException {
         } catch (AssertionError ae) {
             // Some tests assert on searches of wildcarded ML indices rather than on ML endpoints.  For these we expect no hits.
             if (ae.getMessage().contains("hits.total didn't match expected value")) {
-                assertThat(ae.getMessage(), containsString("but was [0]"));
+                assertThat(ae.getMessage(), containsString("but was Integer [0]"));
             } else {
                 assertThat(ae.getMessage(),
                         either(containsString("action [cluster:monitor/xpack/ml")).or(containsString("action [cluster:admin/xpack/ml")));

x-pack/plugin/sql/qa/single-node/src/test/java/org/elasticsearch/xpack/sql/qa/single_node/CliExplainIT.java

@@ -96,15 +96,11 @@ public void testExplainWithWhere() throws IOException {
         assertThat(readLine(), startsWith("  },"));
         assertThat(readLine(), startsWith("  \"_source\" : {"));
         assertThat(readLine(), startsWith("    \"includes\" : ["));
+        assertThat(readLine(), startsWith("      \"i\""));
         assertThat(readLine(), startsWith("      \"test_field\""));
         assertThat(readLine(), startsWith("    ],"));
         assertThat(readLine(), startsWith("    \"excludes\" : [ ]"));
         assertThat(readLine(), startsWith("  },"));
-        assertThat(readLine(), startsWith("  \"docvalue_fields\" : ["));
-        assertThat(readLine(), startsWith("    {"));
-        assertThat(readLine(), startsWith("      \"field\" : \"i\""));
-        assertThat(readLine(), startsWith("    }"));
-        assertThat(readLine(), startsWith("  ],"));
         assertThat(readLine(), startsWith("  \"sort\" : ["));
         assertThat(readLine(), startsWith("    {"));
         assertThat(readLine(), startsWith("      \"_doc\" :"));

x-pack/plugin/sql/qa/single-node/src/test/java/org/elasticsearch/xpack/sql/qa/single_node/FieldExtractorIT.java

@@ -0,0 +1,13 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.sql.qa.single_node;
+
+import org.elasticsearch.xpack.sql.qa.FieldExtractorTestCase;
+
+public class FieldExtractorIT extends FieldExtractorTestCase {
+
+}