Bug fix: correct the column when SELECT function as field (#462)

Author: chloe-zh

src/main/java/com/amazon/opendistroforelasticsearch/sql/executor/csv/CSVResultsExtractor.java

@@ -310,7 +310,11 @@ private List<String> createHeadersAndFillDocsMap(final boolean flat, final Searc
                 doc.put("_type", hit.getType());
             }
 
-            mergeHeaders(csvHeaders, doc, flat);
+            // select function as field is a special case where each hit has non-null field (function)
+            // and sourceAsMap is all columns in index (the same as 'SELECT *')
+            if (fields.isEmpty()) {
+                mergeHeaders(csvHeaders, doc, flat);
+            }
             docsAsMap.add(doc);
         }
 

src/test/java/com/amazon/opendistroforelasticsearch/sql/esintgtest/CsvFormatResponseIT.java

@@ -660,6 +660,14 @@ public void sensitiveCharacterSanitizeAndQuotedTest() throws IOException {
         Assert.assertTrue(lines.get(0).contains("\",,,@cmd|' /C notepad'!_xlbgnm.A1\""));
     }
 
+    @Test
+    public void selectFunctionAsFieldTest() throws IOException {
+        String query = "select log(age) from " + TEST_INDEX_ACCOUNT;
+        CSVResult result = executeCsvRequest(query, false, false, false, false);
+        List<String> headers = result.getHeaders();
+        Assert.assertEquals(1, headers.size());
+    }
+
     private void verifyFieldOrder(final String[] expectedFields) throws IOException {
 
         final String fields = String.join(", ", expectedFields);