Optimize performance of unscored queries

src/main/java/it/cavallium/dbengine/lucene/DocumentStoredSingleFieldVisitor.java

@@ -0,0 +1,62 @@
+package it.cavallium.dbengine.lucene;
+
+import java.io.IOException;
+import java.util.HashSet;
+import java.util.Objects;
+import java.util.Set;
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.FieldType;
+import org.apache.lucene.document.StoredField;
+import org.apache.lucene.document.TextField;
+import org.apache.lucene.index.FieldInfo;
+import org.apache.lucene.index.StoredFieldVisitor;
+import org.apache.lucene.index.StoredFieldVisitor.Status;
+
+public class DocumentStoredSingleFieldVisitor extends StoredFieldVisitor {
+	private final Document doc = new Document();
+	private final String fieldToAdd;
+
+	public DocumentStoredSingleFieldVisitor(String fieldToAdd) {
+		this.fieldToAdd = fieldToAdd;
+	}
+
+	public DocumentStoredSingleFieldVisitor() {
+		this.fieldToAdd = null;
+	}
+
+	public void binaryField(FieldInfo fieldInfo, byte[] value) throws IOException {
+		this.doc.add(new StoredField(fieldInfo.name, value));
+	}
+
+	public void stringField(FieldInfo fieldInfo, String value) throws IOException {
+		FieldType ft = new FieldType(TextField.TYPE_STORED);
+		ft.setStoreTermVectors(fieldInfo.hasVectors());
+		ft.setOmitNorms(fieldInfo.omitsNorms());
+		ft.setIndexOptions(fieldInfo.getIndexOptions());
+		this.doc.add(new StoredField(fieldInfo.name, (String)Objects.requireNonNull(value, "String value should not be null"), ft));
+	}
+
+	public void intField(FieldInfo fieldInfo, int value) {
+		this.doc.add(new StoredField(fieldInfo.name, value));
+	}
+
+	public void longField(FieldInfo fieldInfo, long value) {
+		this.doc.add(new StoredField(fieldInfo.name, value));
+	}
+
+	public void floatField(FieldInfo fieldInfo, float value) {
+		this.doc.add(new StoredField(fieldInfo.name, value));
+	}
+
+	public void doubleField(FieldInfo fieldInfo, double value) {
+		this.doc.add(new StoredField(fieldInfo.name, value));
+	}
+
+	public Status needsField(FieldInfo fieldInfo) {
+		return Objects.equals(this.fieldToAdd, fieldInfo.name) ? Status.YES : Status.NO;
+	}
+
+	public Document getDocument() {
+		return this.doc;
+	}
+}

src/main/java/it/cavallium/dbengine/lucene/LuceneUtils.java

@@ -188,7 +188,9 @@ public class LuceneUtils {
 		if (docId > indexReader.maxDoc()) {
 			throw new IOException("Document " + docId + " > maxDoc (" +indexReader.maxDoc() + ")");
 		}
-		Document d = indexReader.document(docId, Set.of(keyFieldName));
+		DocumentStoredSingleFieldVisitor visitor = new DocumentStoredSingleFieldVisitor(keyFieldName);
+		indexReader.document(docId, visitor);
+		Document d = visitor.getDocument();
 		if (d.getFields().isEmpty()) {
 			throw new NoSuchElementException(
 					"Can't get key (field \"" + keyFieldName + "\") of document docId: " + docId + ". Available fields: []");

src/main/java/it/cavallium/dbengine/lucene/searcher/ScoredSimpleLuceneShardSearcher.java

@@ -135,7 +135,7 @@ class ScoredSimpleLuceneShardSearcher implements LuceneShardSearcher {
 										s -> {}
 								)
 								.subscribeOn(scheduler)
-								.concatMap(topFieldDoc -> LuceneUtils
+								.flatMapSequential(topFieldDoc -> LuceneUtils
 										.convertHits(topFieldDoc.scoreDocs, indexSearchers, keyFieldName, scheduler)
 								);
 					});

src/main/java/it/cavallium/dbengine/lucene/searcher/SimpleLuceneLocalSearcher.java

@@ -93,7 +93,7 @@ public class SimpleLuceneLocalSearcher implements LuceneLocalSearcher {
 											s -> {}
 									)
 									.subscribeOn(scheduler)
-									.concatMap(topFieldDoc -> LuceneUtils
+									.flatMapSequential(topFieldDoc -> LuceneUtils
 											.convertHits(topFieldDoc.scoreDocs, IndexSearchers.unsharded(indexSearcher), keyFieldName, scheduler)
 									);
 						});

src/main/java/it/cavallium/dbengine/lucene/searcher/UnscoredLuceneShardSearcher.java

@@ -126,7 +126,7 @@ class UnscoredLuceneShardSearcher implements LuceneShardSearcher {
 										s -> {}
 								)
 								.subscribeOn(scheduler)
-								.concatMap(topFieldDoc -> LuceneUtils
+								.flatMapSequential(topFieldDoc -> LuceneUtils
 										.convertHits(topFieldDoc.scoreDocs, indexSearchers, keyFieldName, scheduler)
 								);
 					});