Correctly calculate doc_count_error at the slice level for concurrent…

… segment search. Change slice_size heuristic to be equal to shard_size.

Signed-off-by: Jay Deng <jayd0104@gmail.com>

CHANGELOG.md

@@ -187,6 +187,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Restore support for Java 8 for RestClient ([#11562](https://github.com/opensearch-project/OpenSearch/pull/11562))
 - Add deleted doc count in _cat/shards ([#11678](https://github.com/opensearch-project/OpenSearch/pull/11678))
 - Capture information for additional query types and aggregation types ([#11582](https://github.com/opensearch-project/OpenSearch/pull/11582))
+- Use slice_size == shard_size heuristic in terms aggs for concurrent segment search and properly calculate the doc_count_error ([#11732](https://github.com/opensearch-project/OpenSearch/pull/11732))
 
 ### Deprecated
 

server/src/internalClusterTest/java/org/opensearch/search/aggregations/bucket/ShardSizeTermsIT.java

@@ -42,6 +42,7 @@
 import java.util.Map;
 
 import static org.opensearch.index.query.QueryBuilders.matchAllQuery;
+import static org.opensearch.search.SearchService.CLUSTER_CONCURRENT_SEGMENT_SEARCH_SETTING;
 import static org.opensearch.search.aggregations.AggregationBuilders.terms;
 import static org.hamcrest.Matchers.equalTo;
 
@@ -76,6 +77,10 @@ public void testNoShardSizeString() throws Exception {
     }
 
     public void testShardSizeEqualsSizeString() throws Exception {
+        assumeFalse(
+            "For concurrent segment search there will be additional error introduced during the slice level reduce and thus different buckets may be returned. See https://github.com/opensearch-project/OpenSearch/issues/11680",
+            internalCluster().clusterService().getClusterSettings().get(CLUSTER_CONCURRENT_SEGMENT_SEARCH_SETTING)
+        );
         createIdx("type=keyword");
 
         indexData();
@@ -211,6 +216,10 @@ public void testNoShardSizeLong() throws Exception {
     }
 
     public void testShardSizeEqualsSizeLong() throws Exception {
+        assumeFalse(
+            "For concurrent segment search there will be additional error introduced during the slice level reduce and thus different buckets may be returned. See https://github.com/opensearch-project/OpenSearch/issues/11680",
+            internalCluster().clusterService().getClusterSettings().get(CLUSTER_CONCURRENT_SEGMENT_SEARCH_SETTING)
+        );
         createIdx("type=long");
 
         indexData();
@@ -345,6 +354,10 @@ public void testNoShardSizeDouble() throws Exception {
     }
 
     public void testShardSizeEqualsSizeDouble() throws Exception {
+        assumeFalse(
+            "For concurrent segment search there will be additional error introduced during the slice level reduce and thus different buckets may be returned. See https://github.com/opensearch-project/OpenSearch/issues/11680",
+            internalCluster().clusterService().getClusterSettings().get(CLUSTER_CONCURRENT_SEGMENT_SEARCH_SETTING)
+        );
         createIdx("type=double");
 
         indexData();

server/src/internalClusterTest/java/org/opensearch/search/aggregations/bucket/TermsDocCountErrorIT.java

@@ -1118,6 +1118,11 @@ public void testFixedDocs() throws Exception {
             .get();
         assertSearchResponse(response);
 
+        // Force merge each shard down to 1 segment to verify results are the same between concurrent and non-concurrent search paths
+        // Else for concurrent segment search there will be additional error introduced during the slice level reduce and thus different
+        // buckets may be returned. See https://github.com/opensearch-project/OpenSearch/issues/11680",
+        client().admin().indices().prepareForceMerge("idx_fixed_docs_0", "idx_fixed_docs_1", "idx_fixed_docs_2").get();
+
         Terms terms = response.getAggregations().get("terms");
         assertThat(terms, notNullValue());
         assertThat(terms.getDocCountError(), equalTo(46L));