Apply sequential read advice during vector merges

docs/changelog/153423.yaml

@@ -0,0 +1,5 @@
+area: Vector Search
+issues: []
+pr: 153423
+summary: Apply sequential read advice during merge for `bbq_disk` vectors
+type: enhancement

server/src/main/java/org/elasticsearch/index/codec/vectors/diskbbq/ES920DiskBBQVectorsReader.java

@@ -59,6 +59,15 @@ public class ES920DiskBBQVectorsReader extends IVFVectorsReader<IVFVectorsReader
         );
     }
 
+    private ES920DiskBBQVectorsReader(ES920DiskBBQVectorsReader other, GenericFlatVectorReaders genericReaders) {
+        super(other, genericReaders);
+    }
+
+    @Override
+    protected ES920DiskBBQVectorsReader mergeInstance(GenericFlatVectorReaders genericReaders) {
+        return new ES920DiskBBQVectorsReader(this, genericReaders);
+    }
+
     public CentroidIterator getPostingListPrefetchIterator(CentroidIterator centroidIterator, IndexInput postingListSlice)
         throws IOException {
         return new PrefetchingCentroidIterator(centroidIterator, postingListSlice);

server/src/main/java/org/elasticsearch/index/codec/vectors/diskbbq/IVFVectorsReader.java

@@ -161,6 +161,24 @@ protected IVFVectorsReader(
         }
     }
 
+    /**
+     * Copy constructor used to build a merge instance: shares everything with {@code other} but uses
+     * the provided flat vector readers (their merge instances).
+     */
+    protected IVFVectorsReader(IVFVectorsReader<E> other, GenericFlatVectorReaders genericReaders) {
+        this.state = other.state;
+        this.fieldInfos = other.fieldInfos;
+        this.fields = other.fields;
+        this.genericReaders = genericReaders;
+        this.centroidExtension = other.centroidExtension;
+        this.clusterExtension = other.clusterExtension;
+        this.versionDirectIo = other.versionDirectIo;
+        this.dynamicVisitRatio = other.dynamicVisitRatio;
+        this.versionMeta = other.versionMeta;
+        this.ivfCentroids = other.ivfCentroids;
+        this.ivfClusters = other.ivfClusters;
+    }
+
     public abstract CentroidIterator getCentroidIterator(
         FieldInfo fieldInfo,
         int numCentroids,
@@ -317,6 +335,24 @@ public final void checkIntegrity() throws IOException {
         CodecUtil.checksumEntireFile(ivfClusters);
     }
 
+    @Override
+    public final KnnVectorsReader getMergeInstance() throws IOException {
+        // Flat vectors are opened with RANDOM advice for search but read sequentially during a
+        // merge, so back the merge instance with the flat readers' merge instances (which switch
+        // their input to SEQUENTIAL). finishMerge() reverts them.
+        return mergeInstance(genericReaders.getMergeInstance());
+    }
+
+    /** Builds a merge instance of this reader backed by the given flat vector merge readers. */
+    protected abstract IVFVectorsReader<E> mergeInstance(GenericFlatVectorReaders genericReaders);
+
+    @Override
+    public final void finishMerge() throws IOException {
+        for (var reader : genericReaders.allReaders()) {
+            reader.finishMerge();
+        }
+    }
+
     protected FlatVectorsReader getReaderForField(String field) {
         FieldInfo info = fieldInfos.fieldInfo(field);
         if (info == null) throw new IllegalArgumentException("Could not find field [" + field + "]");

server/src/main/java/org/elasticsearch/index/codec/vectors/diskbbq/es94/ES940DiskBBQVectorsReader.java

@@ -73,6 +73,15 @@ public ES940DiskBBQVectorsReader(SegmentReadState state, GenericFlatVectorReader
         );
     }
 
+    private ES940DiskBBQVectorsReader(ES940DiskBBQVectorsReader other, GenericFlatVectorReaders genericReaders) {
+        super(other, genericReaders);
+    }
+
+    @Override
+    protected ES940DiskBBQVectorsReader mergeInstance(GenericFlatVectorReaders genericReaders) {
+        return new ES940DiskBBQVectorsReader(this, genericReaders);
+    }
+
     CentroidIterator getPostingListPrefetchIterator(CentroidIterator centroidIterator, IndexInput postingListSlice) throws IOException {
         // TODO we may want to prefetch more than one postings list, however, we will likely want to place a limit
         // so we don't bother prefetching many lists we won't end up scoring

server/src/main/java/org/elasticsearch/index/codec/vectors/diskbbq/es95/ES950DiskBBQVectorsReader.java

@@ -77,6 +77,15 @@ public ES950DiskBBQVectorsReader(SegmentReadState state, GenericFlatVectorReader
         );
     }
 
+    private ES950DiskBBQVectorsReader(ES950DiskBBQVectorsReader other, GenericFlatVectorReaders genericReaders) {
+        super(other, genericReaders);
+    }
+
+    @Override
+    protected ES950DiskBBQVectorsReader mergeInstance(GenericFlatVectorReaders genericReaders) {
+        return new ES950DiskBBQVectorsReader(this, genericReaders);
+    }
+
     CentroidIterator getPostingListPrefetchIterator(CentroidIterator centroidIterator, IndexInput postingListSlice) throws IOException {
         // TODO we may want to prefetch more than one postings list, however, we will likely want to place a limit
         // so we don't bother prefetching many lists we won't end up scoring

server/src/main/java/org/elasticsearch/index/codec/vectors/diskbbq/next/ESNextDiskBBQVectorsReader.java

@@ -79,6 +79,15 @@ public ESNextDiskBBQVectorsReader(SegmentReadState state, GenericFlatVectorReade
         );
     }
 
+    private ESNextDiskBBQVectorsReader(ESNextDiskBBQVectorsReader other, GenericFlatVectorReaders genericReaders) {
+        super(other, genericReaders);
+    }
+
+    @Override
+    protected ESNextDiskBBQVectorsReader mergeInstance(GenericFlatVectorReaders genericReaders) {
+        return new ESNextDiskBBQVectorsReader(this, genericReaders);
+    }
+
     CentroidIterator getPostingListPrefetchIterator(CentroidIterator centroidIterator, IndexInput postingListSlice) throws IOException {
         // TODO we may want to prefetch more than one postings list, however, we will likely want to place a limit
         // so we don't bother prefetching many lists we won't end up scoring