Number of active parts in a partition
Q: Why do I have several active parts in a partition? Why Clickhouse does not merge them immediately?
A: CH does not merge parts by time
Merge scheduler selects parts by own algorithm based on the current node workload / number of parts / size of parts.
CH merge scheduler balances between a big number of parts and a wasting resources on merges.
Merges are CPU/DISK IO expensive. If CH will merge every new part then all resources will be spend on merges and will no resources remain on queries (selects ).