Ensure single segment in test (elastic#131077) (elastic#131275)

Author: idegtiarenko

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/lucene/read/ValuesSourceReaderOperatorTests.java

@@ -18,6 +18,7 @@
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
 import org.apache.lucene.index.NoMergePolicy;
+import org.apache.lucene.index.TieredMergePolicy;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.tests.index.RandomIndexWriter;
@@ -390,11 +391,11 @@ private IndexReader initIndex(Directory directory, int size, int commitEvery) th
         return DirectoryReader.open(directory);
     }
 
-    private IndexReader initIndexLongField(Directory directory, int size, int commitEvery) throws IOException {
+    private IndexReader initIndexLongField(Directory directory, int size, int commitEvery, boolean forceMerge) throws IOException {
         try (
             IndexWriter writer = new IndexWriter(
                 directory,
-                newIndexWriterConfig().setMergePolicy(NoMergePolicy.INSTANCE).setMaxBufferedDocs(IndexWriterConfig.DISABLE_AUTO_FLUSH)
+                newIndexWriterConfig().setMergePolicy(new TieredMergePolicy()).setMaxBufferedDocs(IndexWriterConfig.DISABLE_AUTO_FLUSH)
             )
         ) {
             for (int d = 0; d < size; d++) {
@@ -413,6 +414,10 @@ private IndexReader initIndexLongField(Directory directory, int size, int commit
                     writer.commit();
                 }
             }
+
+            if (forceMerge) {
+                writer.forceMerge(1);
+            }
         }
         return DirectoryReader.open(directory);
     }
@@ -924,7 +929,7 @@ private void testLoadLong(boolean shuffle, boolean manySegments) throws IOExcept
         int numDocs = between(10, 500);
         initMapping();
         keyToTags.clear();
-        reader = initIndexLongField(directory, numDocs, manySegments ? commitEvery(numDocs) : numDocs);
+        reader = initIndexLongField(directory, numDocs, manySegments ? commitEvery(numDocs) : numDocs, manySegments == false);
 
         DriverContext driverContext = driverContext();
         List<Page> input = CannedSourceOperator.collectPages(sourceOperator(driverContext, numDocs));