Improve the error message when loading text fielddata. (#52753)

Emphasize keyword over fielddata as the preferred way to use String fields for aggregations or sorting.

client/rest-high-level/src/test/java/org/elasticsearch/client/GraphIT.java

@@ -131,7 +131,7 @@ public class GraphIT extends ESRestHighLevelClientTestCase {
         assertThat(exploreResponse.isTimedOut(), Matchers.is(false));
         ShardOperationFailedException[] failures = exploreResponse.getShardFailures();
         assertThat(failures.length, Matchers.equalTo(1));
-        assertTrue(failures[0].reason().contains("Fielddata is disabled"));
+        assertTrue(failures[0].reason().contains("Text fields are not optimised for operations that require per-document field data"));
 
     }
 

server/src/main/java/org/elasticsearch/index/mapper/TextFieldMapper.java

@@ -757,9 +757,10 @@ public class TextFieldMapper extends FieldMapper {
         @Override
         public IndexFieldData.Builder fielddataBuilder(String fullyQualifiedIndexName) {
             if (fielddata == false) {
-                throw new IllegalArgumentException("Fielddata is disabled on text fields by default. Set fielddata=true on [" + name()
-                        + "] in order to load fielddata in memory by uninverting the inverted index. Note that this can however "
-                                + "use significant memory. Alternatively use a keyword field instead.");
+                throw new IllegalArgumentException("Text fields are not optimised for operations that require per-document "
+                    + "field data like aggregations and sorting, so these operations are disabled by default. Please use a "
+                    + "keyword field instead. Alternatively, set fielddata=true on [" + name() + "] in order to load "
+                    + "field data by uninverting the inverted index. Note that this can use significant memory.");
             }
             return new PagedBytesIndexFieldData.Builder(fielddataMinFrequency, fielddataMaxFrequency, fielddataMinSegmentSize);
         }

server/src/test/java/org/elasticsearch/index/mapper/TextFieldMapperTests.java

@@ -516,7 +516,7 @@ public class TextFieldMapperTests extends ESSingleNodeTestCase {
             FieldMapper fieldMapper = (FieldMapper) disabledMapper.mappers().getMapper("field");
             fieldMapper.fieldType().fielddataBuilder("test");
         });
-        assertThat(e.getMessage(), containsString("Fielddata is disabled"));
+        assertThat(e.getMessage(), containsString("Text fields are not optimised for operations that require per-document field data"));
 
         mapping = Strings.toString(XContentFactory.jsonBuilder().startObject().startObject("type")
                 .startObject("properties").startObject("field")