merge_max_block_size
The number of rows that are read from the merged parts into memory. Possible values:- Any positive integer.
merge_max_block_size rows, then
merges and writes the result into a new part. The read block is placed in RAM,
so merge_max_block_size affects the size of the RAM required for the merge.
Thus, merges can consume a large amount of RAM for tables with very wide rows
(if the average row size is 100kb, then when merging 10 parts,
(100kb * 10 * 8192) = ~ 8GB of RAM). By decreasing merge_max_block_size,
you can reduce the amount of RAM required for a merge but slow down a merge.
merge_max_block_size_bytes
How many bytes in blocks should be formed for merge operations. By default has the same value asindex_granularity_bytes.