LUCENE-8619, after indexing 60 million shapes(~1.65 billion triangles) using LatLonShape, the index directory grew to a size of 265 GB when performing merging of different segments. After the processes were over the index size was 57 GB.
As an example imagine we are merging several segments to a new segment of size 10GB (4 dimensions). The BKD tree merging logic will create the following files:
1) Level 0: 4 copies of the data, each one sorted by one dimensions : 40GB
2) Level 1: 6 copies of half of the data, left and right : 30GB
3) Level 2: 6 copies of one quarter of the data, left and right : 15 GB
4) Level 3: 6 more copies halving the previous level, left and right : 7.5 GB
5) Level 4: 6 more copies halving the previous level, left and right : 3.75 GB
and so on... So it requires around 100GB to merge that segment.
In this issue is proposed to delay the creation of sorted copies to when they are needed. It reduces the total size required to half of what it is needed now.