WIP: SAI skip vector graph refinement on memtable flush

Author: michaeljmarshall

src/java/org/apache/cassandra/index/sai/disk/v1/V1OnDiskFormat.java

@@ -319,6 +319,6 @@ private boolean isVectorDataComponent(IndexContext context, IndexComponentType i
     @Override
     public int jvectorFileFormatVersion()
     {
-        throw new UnsupportedOperationException("JVector is not supported in V2OnDiskFormat");
+        throw new UnsupportedOperationException("JVector is not supported");
     }
 }

src/java/org/apache/cassandra/index/sai/disk/vector/CassandraOnHeapGraph.java

@@ -169,7 +169,8 @@ public CassandraOnHeapGraph(IndexContext context, boolean forSearching, Memtable
                                         indexConfig.getConstructionBeamWidth(),
                                         indexConfig.getNeighborhoodOverflow(1.0f), // no overflow means add will be a bit slower but flush will be faster
                                         indexConfig.getAlpha(dimension > 3 ? 1.2f : 2.0f),
-                                        indexConfig.isHierarchyEnabled() && jvectorVersion >= 4);
+                                        indexConfig.isHierarchyEnabled() && jvectorVersion >= 4,
+                                        memtable != null); // Skip graph refinement on memtable to speed up flush
         searchers = ThreadLocal.withInitial(() -> new GraphSearcherAccessManager(new GraphSearcher(builder.getGraph())));
     }