Fallback to tryRead(...) if look ahead finds multiple ordinals

Author: martijnvg

server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/ES819TSDBDocValuesProducer.java

@@ -398,7 +398,7 @@ public BlockLoader.Block tryRead(BlockLoader.BlockFactory factory, BlockLoader.D
                         }
                         // TODO: Since ordinals are sorted, start at 0 (offset by startValue), scan until lastValue,
                         // then fill remaining positions with lastValue.
-                        return null;
+                        // Falling back to tryRead(...) is safe here, given that current block index wasn't altered by looking ahead.
                     }
                     try (var builder = factory.singletonOrdinalsBuilder(this, docs.count() - offset, true)) {
                         BlockLoader.SingletonLongBuilder delegate = new SingletonLongToSingletonOrdinalDelegate(builder);

server/src/test/java/org/elasticsearch/index/codec/tsdb/es819/ES819TSDBDocValuesFormatTests.java

@@ -1139,20 +1139,11 @@ public int get(int docId) {
                                         return docId;
                                     }
                                 }, start);
-                                Set<String> seenValues = new HashSet<>();
-                                for (int p = start; p <= end; p++) {
-                                    String hostName = hostnames.get(((Number) idBlock.get(p)).intValue());
-                                    seenValues.add(hostName);
-                                }
-                                if (seenValues.size() == 1) {
-                                    assertNotNull(hostBlock);
-                                    assertThat(hostBlock.size(), equalTo(end - start + 1));
-                                    for (int i = 0; i < hostBlock.size(); i++) {
-                                        String actualHostName = BytesRefs.toString(hostBlock.get(i));
-                                        assertThat(actualHostName, equalTo(hostnames.get(((Number) idBlock.get(i + start)).intValue())));
-                                    }
-                                } else {
-                                    assertNull(hostBlock);
+                                assertNotNull(hostBlock);
+                                assertThat(hostBlock.size(), equalTo(end - start + 1));
+                                for (int i = 0; i < hostBlock.size(); i++) {
+                                    String actualHostName = BytesRefs.toString(hostBlock.get(i));
+                                    assertThat(actualHostName, equalTo(hostnames.get(((Number) idBlock.get(i + start)).intValue())));
                                 }
                                 if (start == idBlock.size() - 1) {
                                     break;