read_in_order_two_level_merge_threshold
Minimal number of parts to read to run preliminary merge step during multithread reading in order of primary key.read_in_order_use_buffering
Use buffering before merging while reading in order of primary key. It increases the parallelism of query executioread_in_order_use_virtual_row
Use virtual row while reading in order of primary key or its monotonic function fashion. It is useful when searching over multiple parts as only relevant ones are touched.read_in_order_use_virtual_row_per_block
When enabled together withread_in_order_use_virtual_row, emit a virtual row after each block read (not only at the beginning of each part).
This allows MergingSortedTransform to reprioritize sources more frequently, which is useful when downstream filters discard many rows and data is distributed unevenly across parts.
Note that it disables read_in_order_use_buffering optimization and preliminary merge (read_in_order_two_level_merge_threshold) for reading.