Merge IndexHNSW

[codeworse]

Is there an efficient way to merge IndexHNSW? It seems odd to me that there is no implementation of merge_from in IndexHNSWFlat/PQ and etc. It could be easily added by doing storage->merge_from(other.storage) inside(and in case of nullptr storage throw an exception)

[mdouze]

Stitching graph indices is not trivial without incurring an accuracy loss or memory overhead.
See an overview here
https://dl.acm.org/doi/pdf/10.1145/3627535.3638475