thecoop commented on code in PR #16696:
URL: https://github.com/apache/lucene/pull/16696#discussion_r4061811279
##########
lucene/core/src/java/org/apache/lucene/codecs/hnsw/FlatVectorsWriter.java:
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@@ -63,4 +66,65 @@ public final IORunnable mergeOneField(FieldInfo fieldInfo,
MergeState mergeState
public abstract void mergeOneFlatVectorField(FieldInfo fieldInfo, MergeState
mergeState)
throws IOException;
+
+ /**
+ * Merges one field and, while writing the merged vectors, optionally
prepares what a merge scorer
+ * needs, so that the merged float vectors need not be read back.
+ *
+ * <p>A wrapping writer should forward this method along with {@link
#mergeOneFlatVectorField}.
+ * Forwarding only the latter stays correct, but the wrapped writer then
prepares nothing.
+ * Forwarding helps only when the wrapping format opens the wrapped writer's
own reader over the
+ * merged segment. Otherwise {@link MergeScorerData#scorerSupplier} rejects
the prepared data and
+ * the merge uses its fallback.
+ *
+ * @param fieldInfo field to merge
+ * @param mergeState merge state
+ * @param needsMergeScorer whether to prepare scorer data for a vector
count. The writer tests
+ * this predicate against the count before deletions, which can exceed
the number of vectors
+ * written.
+ * @return prepared data that the caller must close, or {@code null} if none
was prepared, which
+ * is what the default implementation returns and what any writer
returns for a field it
+ * cannot prepare for. The caller then builds the scorer itself.
+ * @throws IOException if an I/O error occurs
+ */
+ public MergeScorerData mergeOneFlatVectorFieldForMergeScorer(
Review Comment:
I'm not a _big_ fan of nullable return types, does using `Optional` here and
in the interface method make sense instead?
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