Enable SourceLookup to leverage sequential stored fields reader (#63035)

modules/percolator/src/test/java/org/elasticsearch/percolator/PercolatorMatchedSlotSubFetchPhaseTests.java

@@ -65,7 +65,13 @@ public void testHitsExecute() throws Exception {
                 LeafReaderContext context = reader.leaves().get(0);
                 // A match:
                 {
-                    HitContext hit = new HitContext(new SearchHit(0), context, 0, new SourceLookup(), new HashMap<>());
+                    HitContext hit = new HitContext(
+                        new SearchHit(0),
+                        context,
+                        0,
+                        new SourceLookup(),
+                        new HashMap<>()
+                    );
                     PercolateQuery.QueryStore queryStore = ctx -> docId -> new TermQuery(new Term("field", "value"));
                     MemoryIndex memoryIndex = new MemoryIndex();
                     memoryIndex.addField("field", "value", new WhitespaceAnalyzer());
@@ -86,7 +92,13 @@ public void testHitsExecute() throws Exception {
 
                 // No match:
                 {
-                    HitContext hit = new HitContext(new SearchHit(0), context, 0, new SourceLookup(), new HashMap<>());
+                    HitContext hit = new HitContext(
+                        new SearchHit(0),
+                        context,
+                        0,
+                        new SourceLookup(),
+                        new HashMap<>()
+                    );
                     PercolateQuery.QueryStore queryStore = ctx -> docId -> new TermQuery(new Term("field", "value"));
                     MemoryIndex memoryIndex = new MemoryIndex();
                     memoryIndex.addField("field", "value1", new WhitespaceAnalyzer());
@@ -106,7 +118,13 @@ public void testHitsExecute() throws Exception {
 
                 // No query:
                 {
-                    HitContext hit = new HitContext(new SearchHit(0), context, 0, new SourceLookup(), new HashMap<>());
+                    HitContext hit = new HitContext(
+                        new SearchHit(0),
+                        context,
+                        0,
+                        new SourceLookup(),
+                        new HashMap<>()
+                    );
                     PercolateQuery.QueryStore queryStore = ctx -> docId -> null;
                     MemoryIndex memoryIndex = new MemoryIndex();
                     memoryIndex.addField("field", "value", new WhitespaceAnalyzer());

server/src/main/java/org/elasticsearch/search/lookup/SourceLookup.java

@@ -21,15 +21,19 @@
 import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.LeafReaderContext;
 import org.elasticsearch.ElasticsearchParseException;
+import org.elasticsearch.common.CheckedBiConsumer;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.collect.Tuple;
+import org.elasticsearch.common.lucene.index.SequentialStoredFieldsLeafReader;
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.common.xcontent.support.XContentMapValues;
 import org.elasticsearch.index.fieldvisitor.FieldsVisitor;
 import org.elasticsearch.search.fetch.subphase.FetchSourceContext;
 
+import java.io.IOException;
+import java.io.UncheckedIOException;
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
@@ -40,6 +44,7 @@
 public class SourceLookup implements Map {
 
     private LeafReader reader;
+    CheckedBiConsumer<Integer, FieldsVisitor, IOException> fieldReader;
 
     private int docId = -1;
 
@@ -75,7 +80,7 @@ public Map<String, Object> loadSourceIfNeeded() {
         }
         try {
             FieldsVisitor sourceFieldVisitor = new FieldsVisitor(true);
-            reader.document(docId, sourceFieldVisitor);
+            fieldReader.accept(docId, sourceFieldVisitor);
             BytesReference source = sourceFieldVisitor.source();
             if (source == null) {
                 this.source = emptyMap();
@@ -91,20 +96,40 @@ public Map<String, Object> loadSourceIfNeeded() {
         return this.source;
     }
 
-    public static Tuple<XContentType, Map<String, Object>> sourceAsMapAndType(BytesReference source) throws ElasticsearchParseException {
+    private static Tuple<XContentType, Map<String, Object>> sourceAsMapAndType(BytesReference source) throws ElasticsearchParseException {
         return XContentHelper.convertToMap(source, false);
     }
 
     public static Map<String, Object> sourceAsMap(BytesReference source) throws ElasticsearchParseException {
         return sourceAsMapAndType(source).v2();
     }
 
-    public void setSegmentAndDocument(LeafReaderContext context, int docId) {
+    public void setSegmentAndDocument(
+        LeafReaderContext context,
+        int docId
+    ) {
         if (this.reader == context.reader() && this.docId == docId) {
             // if we are called with the same document, don't invalidate source
             return;
         }
-        this.reader = context.reader();
+        if (this.reader != context.reader()) {
+            this.reader = context.reader();
+            // only reset reader and fieldReader when reader changes
+            try {
+                if (context.reader() instanceof SequentialStoredFieldsLeafReader) {
+                    // All the docs to fetch are adjacent but Lucene stored fields are optimized
+                    // for random access and don't optimize for sequential access - except for merging.
+                    // So we do a little hack here and pretend we're going to do merges in order to
+                    // get better sequential access.
+                    SequentialStoredFieldsLeafReader lf = (SequentialStoredFieldsLeafReader) context.reader();
+                    fieldReader = lf.getSequentialStoredFieldsReader()::visitDocument;
+                } else {
+                    fieldReader = context.reader()::document;
+                }
+            } catch (IOException e) {
+                throw new UncheckedIOException(e);
+            }
+        }
         this.source = null;
         this.sourceAsBytes = null;
         this.docId = docId;

server/src/test/java/org/elasticsearch/search/fetch/subphase/FetchSourcePhaseTests.java

@@ -159,7 +159,13 @@ private HitContext hitExecuteMultiple(XContentBuilder source, boolean fetchSourc
         // We don't need a real index, just a LeafReaderContext which cannot be mocked.
         MemoryIndex index = new MemoryIndex();
         LeafReaderContext leafReaderContext = index.createSearcher().getIndexReader().leaves().get(0);
-        HitContext hitContext = new HitContext(searchHit, leafReaderContext, 1, new SourceLookup(), new HashMap<>());
+        HitContext hitContext = new HitContext(
+            searchHit,
+            leafReaderContext,
+            1,
+            new SourceLookup(),
+            new HashMap<>()
+        );
         hitContext.sourceLookup().setSource(source == null ? null : BytesReference.bytes(source));
 
         FetchSourcePhase phase = new FetchSourcePhase();