Add fallback support to Lucene104ScalarQuantizedVectorsFormat getFloatVectorValues when there are no full-precision vectors present (#15415)

* Add fallback support to Lucene104ScalarQuantizedVectorsFormat.getFloatVectorValues when there are no full-precision vectors present

* Removed some unused params and tidy changes

* Added entry in Changes.txt

* Added name in Changes.txt log

* Addressed comments

* Renamed function params

---------

Co-authored-by: Pulkit Gupta <[email protected]>

Author: Pulkitg64

lucene/CHANGES.txt

@@ -176,6 +176,9 @@ New Features
    `Lucene104HnswScalarQuantizedVectorsFormat(ScalarEncoding.SINGLE_BIT_QUERY_NIBBLE, int, int)`
     (Ben Trent)
 
+ * GITHUB#15415: Add fallback support to Lucene104ScalarQuantizedVectorsFormat getFloatVectorValues when there are
+   no full-precision vectors present (Pulkit Gupta)
+
 Improvements
 ---------------------
 * GITHUB#15148: Add support uint8 distance and allow 8 bit scalar quantization (Trevor McCulloch)

lucene/core/src/java/org/apache/lucene/codecs/lucene104/Lucene104ScalarQuantizedVectorsReader.java

@@ -210,6 +210,23 @@ public FloatVectorValues getFloatVectorValues(String field) throws IOException {
               + " expected: "
               + VectorEncoding.FLOAT32);
     }
+
+    FloatVectorValues rawFloatVectorValues = rawVectorsReader.getFloatVectorValues(field);
+
+    if (rawFloatVectorValues.size() == 0) {
+      return OffHeapScalarQuantizedFloatVectorValues.load(
+          fi.ordToDocDISIReaderConfiguration,
+          fi.dimension,
+          fi.size,
+          fi.scalarEncoding,
+          fi.similarityFunction,
+          vectorScorer,
+          fi.centroid,
+          fi.vectorDataOffset,
+          fi.vectorDataLength,
+          quantizedVectorData);
+    }
+
     OffHeapScalarQuantizedVectorValues sqvv =
         OffHeapScalarQuantizedVectorValues.load(
             fi.ordToDocDISIReaderConfiguration,
@@ -224,7 +241,7 @@ public FloatVectorValues getFloatVectorValues(String field) throws IOException {
             fi.vectorDataOffset,
             fi.vectorDataLength,
             quantizedVectorData);
-    return new ScalarQuantizedVectorValues(rawVectorsReader.getFloatVectorValues(field), sqvv);
+    return new ScalarQuantizedVectorValues(rawFloatVectorValues, sqvv);
   }
 
   @Override