Estimate segment field usages

server/src/main/java/org/elasticsearch/index/codec/DeduplicatingFieldInfosFormat.java

@@ -65,7 +65,7 @@ public FieldInfos read(Directory directory, SegmentInfo segmentInfo, String segm
                 fi.isParentField()
             );
         }
-        return new FieldInfos(deduplicated);
+        return new FieldInfosWithUsages(deduplicated);
     }
 
     private static Map<String, String> internStringStringMap(Map<String, String> m) {

server/src/main/java/org/elasticsearch/index/codec/FieldInfosWithUsages.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec;
+
+import org.apache.lucene.index.DocValuesType;
+import org.apache.lucene.index.FieldInfo;
+import org.apache.lucene.index.FieldInfos;
+import org.apache.lucene.index.IndexOptions;
+
+public class FieldInfosWithUsages extends FieldInfos {
+    private final int totalUsages;
+
+    public FieldInfosWithUsages(FieldInfo[] infos) {
+        super(infos);
+        this.totalUsages = computeUsages(infos);
+    }
+
+    public static int computeUsages(FieldInfo[] infos) {
+        int usages = 0;
+        for (FieldInfo fi : infos) {
+            if (fi.getIndexOptions() != IndexOptions.NONE) {
+                usages++;
+            }
+            if (fi.hasNorms()) {
+                usages++;
+            }
+            if (fi.hasVectors()) {
+                usages++;
+            }
+            if (fi.getDocValuesType() != DocValuesType.NONE) {
+                usages++;
+            }
+            if (fi.getPointDimensionCount() > 0) {
+                usages++;
+            }
+            if (fi.getVectorDimension() > 0) {
+                usages++;
+            }
+        }
+        return usages;
+    }
+
+    public int getTotalUsages() {
+        return totalUsages;
+    }
+}

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

@@ -83,6 +83,7 @@
 import org.elasticsearch.index.cache.query.TrivialQueryCachingPolicy;
 import org.elasticsearch.index.cache.request.ShardRequestCache;
 import org.elasticsearch.index.codec.CodecService;
+import org.elasticsearch.index.codec.FieldInfosWithUsages;
 import org.elasticsearch.index.engine.CommitStats;
 import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.index.engine.Engine.GetResult;
@@ -4093,11 +4094,21 @@ public void afterRefresh(boolean didRefresh) {
                 try (var searcher = getEngine().acquireSearcher("shard_field_stats", Engine.SearcherScope.INTERNAL)) {
                     int numSegments = 0;
                     int totalFields = 0;
+                    long usages = 0;
                     for (LeafReaderContext leaf : searcher.getLeafContexts()) {
                         numSegments++;
-                        totalFields += leaf.reader().getFieldInfos().size();
+                        var fieldInfos = leaf.reader().getFieldInfos();
+                        totalFields += fieldInfos.size();
+                        if (fieldInfos instanceof FieldInfosWithUsages ft) {
+                            if (usages != -1) {
+                                usages += ft.getTotalUsages();
+                            }
+                        } else {
+                            assert false : "Elasticsearch Codec should have wrapped FieldInfos with field usages: " + leaf;
+                            usages = -1;
+                        }
                     }
-                    shardFieldStats = new ShardFieldStats(numSegments, totalFields);
+                    shardFieldStats = new ShardFieldStats(numSegments, totalFields, usages);
                 } catch (AlreadyClosedException ignored) {
 
                 }

server/src/main/java/org/elasticsearch/index/shard/ShardFieldStats.java

@@ -14,7 +14,9 @@
  *
  * @param numSegments the number of segments
  * @param totalFields the total number of fields across the segments
+ * @param fieldUsages the number of usages for segment-level fields (e.g., doc_values, postings, norms, points)
+ *                    -1 if unavailable
  */
-public record ShardFieldStats(int numSegments, int totalFields) {
+public record ShardFieldStats(int numSegments, int totalFields, long fieldUsages) {
 
 }

server/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -1793,6 +1793,7 @@ public void testShardFieldStats() throws IOException {
         assertNotNull(stats);
         assertThat(stats.numSegments(), equalTo(0));
         assertThat(stats.totalFields(), equalTo(0));
+        assertThat(stats.fieldUsages(), equalTo(0L));
         // index some documents
         int numDocs = between(1, 10);
         for (int i = 0; i < numDocs; i++) {
@@ -1809,6 +1810,9 @@ public void testShardFieldStats() throws IOException {
         assertThat(stats.numSegments(), equalTo(1));
         // _id, _source, _version, _primary_term, _seq_no, f1, f1.keyword, f2, f2.keyword,
         assertThat(stats.totalFields(), equalTo(9));
+        // _id(term), _source(0), _version(dv), _primary_term(dv), _seq_no(point,dv), f1(postings,norms),
+        // f1.keyword(term,dv), f2(postings,norms), f2.keyword(term,dv),
+        assertThat(stats.fieldUsages(), equalTo(13L));
         // don't re-compute on refresh without change
         if (randomBoolean()) {
             shard.refresh("test");
@@ -1838,10 +1842,15 @@ public void testShardFieldStats() throws IOException {
         assertThat(stats.numSegments(), equalTo(2));
         // 9 + _id, _source, _version, _primary_term, _seq_no, f1, f1.keyword, f2, f2.keyword, f3, f3.keyword
         assertThat(stats.totalFields(), equalTo(21));
+        // first segment: 13, second segment: 13 + f3(postings,norms) + f3.keyword(term,dv), and __soft_deletes to previous segment
+        assertThat(stats.fieldUsages(), equalTo(31L));
         shard.forceMerge(new ForceMergeRequest().maxNumSegments(1).flush(true));
         stats = shard.getShardFieldStats();
         assertThat(stats.numSegments(), equalTo(1));
         assertThat(stats.totalFields(), equalTo(12));
+        // _id(term), _source(0), _version(dv), _primary_term(dv), _seq_no(point,dv), f1(postings,norms),
+        // f1.keyword(term,dv), f2(postings,norms), f2.keyword(term,dv), f3(postings,norms), f3.keyword(term,dv), __soft_deletes
+        assertThat(stats.fieldUsages(), equalTo(18L));
         closeShards(shard);
     }