Add a fast path that copies the first segment's index file as the merge base and only merges the tail segments into it. Limited to streaming indexes (HNSW, HNSW_RABITQ, FLAT) with matching index_type + quantize_type and no filter; IVF/VAMANA always rebuild (their Merge is dump-then-reopen and would drop the base docs).
Also, fix the incorrect concurrency of the compaction task.
When add_column is called on a multi-segment collection with a nullable
field and no expression, segment.cc previously sliced an Arrow ChunkedArray
with an offset that exceeded the array length, triggering SIGABRT in
Arrow's chunked_array.cc:170 assertion.
Fix the slicing logic in segment.cc to materialize null values per segment.
Add comprehensive tests in collection_test.cc and segment_test.cc covering
multi-segment add_column scenarios (nullable/non-nullable, with/without
expression, with/without unflushed data, drop+re-add).
Jalin Wang
Qinren Zhou
ZeFeng Yin
lichen2015
feihongxu0824
lichen2015
sanyi