Estimate segment field usages (#112760)

We have introduced a new memory estimation method in serverless, based 
on the number of segments and the fields within them. This new approach
works well overall, but it still falls short in cases where most fields
are used more than once - for example, in both doc_values and postings,
or doc_values and points. This change exposes the total usage of fields
in segments, allowing us to adjust the memory estimate for these cases.

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 +1,49 @@
+/*
+ * 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.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 +4094,20 @@ 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 {
+                            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);
     }