Don't load global ordinals with the map execution_hint (#38158)

The terms aggregator loads the global ordinals to retrieve the cardinality of the field to aggregate on. This information is then used to select the strategy to use for the aggregation (breadth_first or depth_first). However this should be avoided if the execution_hint is explicitly set to map since this mode doesn't really need the global ordinals. Since we still need the cardinality of the field this change picks the maximum cardinality in the segments as an estimation of the total cardinality to select the strategy to use (breadth_first or depth_first). This estimation is only used if the execution hint is set to map, otherwise the global ordinals are still used to retrieve the accurate cardinality.

Closes #37705

Author: jimczi

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/20_terms.yml

@@ -748,3 +748,70 @@ setup:
   - is_false: aggregations.str_terms.buckets.1.key_as_string
 
   - match: { aggregations.str_terms.buckets.1.doc_count: 2 }
+
+---
+"Global ordinals are not loaded with the map execution hint":
+
+  - skip:
+      version: " - 6.6.99"
+      reason:  bug fixed in 6.7
+
+  - do:
+      index:
+        refresh: true
+        index: test_1
+        type: test
+        id: 1
+        routing: 1
+        body: { "str": "abc" }
+
+  - do:
+      index:
+        refresh: true
+        index: test_1
+        type: test
+        id: 2
+        routing: 1
+        body: { "str": "abc" }
+
+  - do:
+      index:
+        refresh: true
+        index: test_1
+        type: test
+        id: 3
+        routing: 1
+        body: { "str": "bcd" }
+
+  - do:
+      indices.refresh: {}
+
+  - do:
+      search:
+        index: test_1
+        body: { "size" : 0, "aggs" : { "str_terms" : { "terms" : { "field" : "str", "execution_hint" : "map" } } } }
+
+  - length: { aggregations.str_terms.buckets: 2 }
+
+  - do:
+      indices.stats:
+        index: test_1
+        metric: fielddata
+        fielddata_fields: str
+
+  - match: { indices.test_1.total.fielddata.memory_size_in_bytes: 0}
+
+  - do:
+      search:
+        index: test_1
+        body: { "size" : 0, "aggs" : { "str_terms" : { "terms" : { "field" : "str", "execution_hint" : "global_ordinals" } } } }
+
+  - length: { aggregations.str_terms.buckets: 2 }
+
+  - do:
+      indices.stats:
+        index: test_1
+        metric: fielddata
+        fielddata_fields: str
+
+  - gt: { indices.test_1.total.fielddata.memory_size_in_bytes: 0}

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/TermsAggregatorFactory.java

@@ -133,7 +133,7 @@ protected Aggregator doCreateInternal(ValuesSource valuesSource, Aggregator pare
             if (valuesSource instanceof ValuesSource.Bytes.WithOrdinals == false) {
                 execution = ExecutionMode.MAP;
             }
-            final long maxOrd = getMaxOrd(valuesSource, context.searcher());
+            final long maxOrd = execution == ExecutionMode.GLOBAL_ORDINALS ? getMaxOrd(valuesSource, context.searcher()) : -1;
             if (execution == null) {
                 execution = ExecutionMode.GLOBAL_ORDINALS;
             }