Remove ElasticsearchQueryCachingPolicy (#27190)

We have an hidden setting called `index.queries.cache.term_queries` that disables caching of term queries in the query cache.
Though term queries are not cached in the Lucene UsageTrackingQueryCachingPolicy since version 6.5.
This makes the es policy useless but also makes it impossible to re-enable caching for term queries.
This change appeared in Lucene 6.5 so this setting is no-op since version 5.4 of Elasticsearch
The change in this PR removes the setting and the custom policy.

Author: jimczi

core/src/main/java/org/elasticsearch/common/settings/IndexScopedSettings.java

@@ -145,7 +145,6 @@ public final class IndexScopedSettings extends AbstractScopedSettings {
         IndexModule.INDEX_STORE_PRE_LOAD_SETTING,
         IndexModule.INDEX_QUERY_CACHE_ENABLED_SETTING,
         IndexModule.INDEX_QUERY_CACHE_EVERYTHING_SETTING,
-        IndexModule.INDEX_QUERY_CACHE_TERM_QUERIES_SETTING,
         FsDirectoryService.INDEX_LOCK_FACTOR_SETTING,
         EngineConfig.INDEX_CODEC_SETTING,
         EngineConfig.INDEX_OPTIMIZE_AUTO_GENERATED_IDS,

core/src/main/java/org/elasticsearch/index/IndexModule.java

@@ -101,11 +101,6 @@ public final class IndexModule {
     public static final Setting<Boolean> INDEX_QUERY_CACHE_EVERYTHING_SETTING =
         Setting.boolSetting("index.queries.cache.everything", false, Property.IndexScope);
 
-    // This setting is an escape hatch in case not caching term queries would slow some users down
-    // Do not document.
-    public static final Setting<Boolean> INDEX_QUERY_CACHE_TERM_QUERIES_SETTING =
-        Setting.boolSetting("index.queries.cache.term_queries", false, Property.IndexScope);
-
     private final IndexSettings indexSettings;
     private final AnalysisRegistry analysisRegistry;
     // pkg private so tests can mock

core/src/main/java/org/elasticsearch/index/shard/ElasticsearchQueryCachingPolicy.java

@@ -291,11 +291,7 @@ public IndexShard(
         if (IndexModule.INDEX_QUERY_CACHE_EVERYTHING_SETTING.get(settings)) {
             cachingPolicy = QueryCachingPolicy.ALWAYS_CACHE;
         } else {
-            QueryCachingPolicy cachingPolicy = new UsageTrackingQueryCachingPolicy();
-            if (IndexModule.INDEX_QUERY_CACHE_TERM_QUERIES_SETTING.get(settings) == false) {
-                cachingPolicy = new ElasticsearchQueryCachingPolicy(cachingPolicy);
-            }
-            this.cachingPolicy = cachingPolicy;
+            cachingPolicy = new UsageTrackingQueryCachingPolicy();
         }
         indexShardOperationPermits = new IndexShardOperationPermits(shardId, logger, threadPool);
         searcherWrapper = indexSearcherWrapper;

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -429,9 +429,6 @@ public void randomIndexTemplate() throws IOException {
             if (randomBoolean()) {
                 randomSettingsBuilder.put(IndexModule.INDEX_QUERY_CACHE_EVERYTHING_SETTING.getKey(), randomBoolean());
             }
-            if (randomBoolean()) {
-                randomSettingsBuilder.put(IndexModule.INDEX_QUERY_CACHE_TERM_QUERIES_SETTING.getKey(), randomBoolean());
-            }
             PutIndexTemplateRequestBuilder putTemplate = client().admin().indices()
                 .preparePutTemplate("random_index_template")
                 .setPatterns(Collections.singletonList("*"))