Module promotion 

Sparse → dense promotion at fill ratio > DENSE_PROMOTION_THRESHOLD.

The threshold is fixed in code rather than in the schema because it’s a storage-level decision, not a workload one — at fill ratios above 70%, the per-cell overhead of coordinate columns exceeds the cost of storing nulls densely.

Constants

DENSE_PROMOTION_THRESHOLD

Fill ratio above which a sparse tile is rewritten into a dense tile (auto-promotion threshold).

Functions

should_promote_to_dense

True if nnz / cells_per_tile > threshold.

sparse_to_dense

Convert a sparse tile to a dense tile by materialising every cell.