Introduce a constant_keyword field.

docs/reference/how-to/search-speed.asciidoc

@@ -418,3 +418,115 @@ The <<text,`text`>> field has an <<index-prefixes,`index_prefixes`>> option that
 indexes prefixes of all terms and is automatically leveraged by query parsers to
 run prefix queries. If your use-case involves running lots of prefix queries,
 this can speed up queries significantly.
+
+[[faster-filtering-with-constant-keyword]]
+=== Use <<constant-keyword,`constant_keyword`>> to speed up filtering
+
+There is a general rule that the cost of a filter is mostly a function of the
+number of matched documents. Imagine that you have an index containing cycles.
+There are a large number of bicycles and many searches perform a filter on
+`cycle_type: bicycle`. This very common filter is unfortunately also very costly
+since it matches most documents. There is a simple way to avoid running this
+filter: move bicycles to their own index and filter bicycles by searching this
+index instead of adding a filter to the query.
+
+Unfortunately this can make client-side logic tricky, which is where
+`constant_keyword` helps. By mapping `cycle_type` as a `constant_keyword` with
+value `bicycle` on the index that contains bicycles, clients can keep running
+the exact same queries as they used to run on the monolithic index and
+Elasticsearch will do the right thing on the bicycles index by ignoring filters
+on `cycle_type` if the value is `bicycle` and returning no hits otherwise.
+
+Here is what mappings could look like:
+
+[source,console]
+--------------------------------------------------
+PUT bicycles
+{
+  "mappings": {
+    "properties": {
+      "cycle_type": {
+        "type": "constant_keyword",
+        "value": "bicycle"
+      },
+      "name": {
+        "type": "text"
+      }
+    }
+  }
+}
+
+PUT other_cycles
+{
+  "mappings": {
+    "properties": {
+      "cycle_type": {
+        "type": "keyword"
+      },
+      "name": {
+        "type": "text"
+      }
+    }
+  }
+}
+--------------------------------------------------
+
+We are splitting our index in two: one that will contain only bicycles, and
+another one that contains other cycles: unicycles, tricycles, etc. Then at
+search time, we need to search both indices, but we don't need to modify
+queries.
+
+
+[source,console]
+--------------------------------------------------
+GET bicycles,other_cycles/_search
+{
+  "query": {
+    "bool": {
+      "must": {
+        "match": {
+          "description": "dutch"
+        }
+      },
+      "filter": {
+        "term": {
+          "cycle_type": "bicycle"
+        }
+      }
+    }
+  }
+}
+--------------------------------------------------
+// TEST[continued]
+
+On the `bicycles` index, Elasticsearch will simply ignore the `cycle_type`
+filter and rewrite the search request to the one below:
+
+[source,console]
+--------------------------------------------------
+GET bicycles,other_cycles/_search
+{
+  "query": {
+    "match": {
+      "description": "dutch"
+    }
+  }
+}
+--------------------------------------------------
+// TEST[continued]
+
+On the `other_cycles` index, Elasticsearch will quickly figure out that
+`bicycle` doesn't exist in the terms dictionary of the `cycle_type` field and
+return a search response with no hits.
+
+This is a powerful way of making queries cheaper by putting common values in a
+dedicated index. This idea can also be combined across multiple fields: for
+instance if you track the color of each cycle and your `bicycles` index ends up
+having a majority of black bikes, you could split it into a `bicycles-black`
+and a `bicycles-other-colors` indices.
+
+The `constant_keyword` is not strictly required for this optimization: it is
+also possible to update the client-side logic in order to route queries to the
+relevant indices based on filters. However `constant_keyword` makes it
+transparently and allows to decouple search requests from the index topology in
+exchange of very little overhead.

docs/reference/mapping/types.asciidoc

@@ -57,6 +57,8 @@ string::         <<text,`text`>> and <<keyword,`keyword`>>
 
 <<histogram>>:: `histogram` for pre-aggregated numerical values for percentiles aggregations.
 
+<<constant-keyword>>:: Specialization of `keyword` for the case when all documents have the same value.
+
 [float]
 [[types-array-handling]]
 === Arrays
@@ -126,4 +128,6 @@ include::types/text.asciidoc[]
 
 include::types/token-count.asciidoc[]
 
-include::types/shape.asciidoc[]
+include::types/shape.asciidoc[]
+
+include::types/constant-keyword.asciidoc[]

docs/reference/mapping/types/constant-keyword.asciidoc

@@ -0,0 +1,85 @@
+[role="xpack"]
+[testenv="basic"]
+
+[[constant-keyword]]
+=== Constant keyword datatype
+++++
+<titleabbrev>Constant keyword</titleabbrev>
+++++
+
+Constant keyword is a specialization of the <<keyword,`keyword`>> field for
+the case that all documents in the index have the same value.
+
+[source,console]
+--------------------------------
+PUT logs-debug
+{
+  "mappings": {
+    "properties": {
+      "@timestamp": {
+        "type": "date"
+      },
+      "message": {
+        "type": "text"
+      },
+      "level": {
+        "type": "constant_keyword",
+        "value": "debug"
+      }
+    }
+  }
+}
+--------------------------------
+
+`constant_keyword` supports the same queries and aggregations as `keyword`
+fields do, but takes advantage of the fact that all documents have the same
+value per index to execute queries more efficiently.
+
+It is both allowed to submit documents that don't have a value for the field or
+that have a value equal to the value configured in mappings. The two below
+indexing requests are equivalent:
+
+[source,console]
+--------------------------------
+POST logs-debug/_doc
+{
+  "date": "2019-12-12",
+  "message": "Starting up Elasticsearch",
+  "level": "debug"
+}
+
+POST logs-debug/_doc
+{
+  "date": "2019-12-12",
+  "message": "Starting up Elasticsearch"
+}
+--------------------------------
+//TEST[continued]
+
+However providing a value that is different from the one configured in the
+mapping is disallowed.
+
+In case no `value` is provided in the mappings, the field will automatically
+configure itself based on the value contained in the first indexed document.
+While this behavior can be convenient, note that it means that a single
+poisonous document can cause all other documents to be rejected if it had a
+wrong value.
+
+The `value` of the field cannot be changed after it has been set.
+
+[[constant-keyword-params]]
+==== Parameters for constant keyword fields
+
+The following mapping parameters are accepted:
+
+[horizontal]
+
+<<mapping-field-meta,`meta`>>::
+
+    Metadata about the field.
+
+`value`::
+
+    The value to associate with all documents in the index. If this parameter
+    is not provided, it is set based on the first document that gets indexed.
+

docs/reference/rest-api/info.asciidoc

@@ -71,6 +71,10 @@ Example response:
           "available" : true,
           "enabled" : true
       },
+      "constant_keyword" : {
+         "available" : true,
+         "enabled" : true
+      },
       "enrich" : {
           "available" : true,
           "enabled" : true

server/src/main/java/org/elasticsearch/index/fielddata/plain/ConstantIndexFieldData.java

@@ -91,6 +91,9 @@ public Collection<Accountable> getChildResources() {
 
         @Override
         public SortedSetDocValues getOrdinalsValues() {
+            if (value == null) {
+                return DocValues.emptySortedSet();
+            }
             final BytesRef term = new BytesRef(value);
             final SortedDocValues sortedValues = new AbstractSortedDocValues() {
 

x-pack/plugin/core/src/main/java/org/elasticsearch/license/XPackLicenseState.java

@@ -626,6 +626,10 @@ public boolean isDataScienceAllowed() {
         return allowForAllLicenses();
     }
 
+    public boolean isConstantKeywordAllowed() {
+        return allowForAllLicenses();
+    }
+
     /**
      * @return true if security is available to be used with the current license type
      */

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/XPackField.java

@@ -55,6 +55,8 @@ public final class XPackField {
     public static final String ANALYTICS = "analytics";
     /** Name constant for the enrich plugin. */
     public static final String ENRICH = "enrich";
+    /** Name constant for the constant-keyword plugin. */
+    public static final String CONSTANT_KEYWORD = "constant_keyword";
 
     private XPackField() {}
 

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/action/XPackInfoFeatureAction.java

@@ -42,10 +42,11 @@ public class XPackInfoFeatureAction extends ActionType<XPackInfoFeatureResponse>
     public static final XPackInfoFeatureAction SPATIAL = new XPackInfoFeatureAction(XPackField.SPATIAL);
     public static final XPackInfoFeatureAction ANALYTICS = new XPackInfoFeatureAction(XPackField.ANALYTICS);
     public static final XPackInfoFeatureAction ENRICH = new XPackInfoFeatureAction(XPackField.ENRICH);
+    public static final XPackInfoFeatureAction CONSTANT_KEYWORD = new XPackInfoFeatureAction(XPackField.CONSTANT_KEYWORD);
 
     public static final List<XPackInfoFeatureAction> ALL = Arrays.asList(
         SECURITY, MONITORING, WATCHER, GRAPH, MACHINE_LEARNING, LOGSTASH, EQL, SQL, ROLLUP, INDEX_LIFECYCLE, SNAPSHOT_LIFECYCLE, CCR,
-        TRANSFORM, FLATTENED, VECTORS, VOTING_ONLY, FROZEN_INDICES, SPATIAL, ANALYTICS, ENRICH
+        TRANSFORM, FLATTENED, VECTORS, VOTING_ONLY, FROZEN_INDICES, SPATIAL, ANALYTICS, ENRICH, CONSTANT_KEYWORD
     );
 
     private XPackInfoFeatureAction(String name) {

x-pack/plugin/mapper-constant-keyword/build.gradle

@@ -0,0 +1,24 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+evaluationDependsOn(xpackModule('core'))
+
+apply plugin: 'elasticsearch.esplugin'
+
+esplugin {
+  name 'constant-keyword'
+  description 'Module for the constant-keyword field type, which is a specialization of keyword for the case when all documents have the same value.'
+  classname 'org.elasticsearch.xpack.constantkeyword.ConstantKeywordMapperPlugin'
+  extendedPlugins = ['x-pack-core']
+}
+archivesBaseName = 'x-pack-constant-keyword'
+
+dependencies {
+  compileOnly project(path: xpackModule('core'), configuration: 'default')
+  testCompile project(path: xpackModule('core'), configuration: 'testArtifacts')
+}
+
+integTest.enabled = false

x-pack/plugin/mapper-constant-keyword/src/main/java/org/elasticsearch/xpack/constantkeyword/ConstantKeywordInfoTransportAction.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.constantkeyword;
+
+import org.elasticsearch.action.support.ActionFilters;
+import org.elasticsearch.common.inject.Inject;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.license.XPackLicenseState;
+import org.elasticsearch.transport.TransportService;
+import org.elasticsearch.xpack.core.XPackField;
+import org.elasticsearch.xpack.core.action.XPackInfoFeatureAction;
+import org.elasticsearch.xpack.core.action.XPackInfoFeatureTransportAction;
+
+public class ConstantKeywordInfoTransportAction extends XPackInfoFeatureTransportAction {
+
+    private final XPackLicenseState licenseState;
+
+    @Inject
+    public ConstantKeywordInfoTransportAction(TransportService transportService, ActionFilters actionFilters,
+                                        Settings settings, XPackLicenseState licenseState) {
+        super(XPackInfoFeatureAction.CONSTANT_KEYWORD.name(), transportService, actionFilters);
+        this.licenseState = licenseState;
+    }
+
+    @Override
+    public String name() {
+        return XPackField.CONSTANT_KEYWORD;
+    }
+
+    @Override
+    public boolean available() {
+        return licenseState.isConstantKeywordAllowed();
+    }
+
+    @Override
+    public boolean enabled() {
+        return true;
+    }
+
+}

x-pack/plugin/mapper-constant-keyword/src/main/java/org/elasticsearch/xpack/constantkeyword/ConstantKeywordMapperPlugin.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.constantkeyword;
+
+import org.elasticsearch.action.ActionRequest;
+import org.elasticsearch.action.ActionResponse;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.mapper.Mapper;
+import org.elasticsearch.plugins.ActionPlugin;
+import org.elasticsearch.plugins.MapperPlugin;
+import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.xpack.constantkeyword.mapper.ConstantKeywordFieldMapper;
+import org.elasticsearch.xpack.core.action.XPackInfoFeatureAction;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+
+import static java.util.Collections.singletonMap;
+
+public class ConstantKeywordMapperPlugin extends Plugin implements MapperPlugin, ActionPlugin {
+
+    public ConstantKeywordMapperPlugin(Settings settings) {}
+
+    @Override
+    public Map<String, Mapper.TypeParser> getMappers() {
+        return singletonMap(ConstantKeywordFieldMapper.CONTENT_TYPE, new ConstantKeywordFieldMapper.TypeParser());
+    }
+
+    @Override
+    public List<ActionHandler<? extends ActionRequest, ? extends ActionResponse>> getActions() {
+        return Arrays.asList(
+            new ActionHandler<>(XPackInfoFeatureAction.CONSTANT_KEYWORD, ConstantKeywordInfoTransportAction.class));
+    }
+}