parallel_* session settings

ClickHouse session settings in the parallel_* generated group.

These settings are available in system.settings and are autogenerated from source.

parallel_distributed_insert_select

Enables parallel distributed INSERT ... SELECT query.

If we execute INSERT INTO distributed_table_a SELECT ... FROM distributed_table_b queries and both tables use the same cluster, and both tables are either replicated or non-replicated, then this query is processed locally on every shard.

Possible values:

• 0 — Disabled.
• 1 — SELECT will be executed on each shard from the underlying table of the distributed engine.
• 2 — SELECT and INSERT will be executed on each shard from/to the underlying table of the distributed engine.

Since v25.4, INSERT ... SELECT from a ReplicatedMergeTree or SharedMergeTree source can also be parallelized across replicas. To enable it:

• parallel_distributed_insert_select = 2
• enable_parallel_replicas = 1

parallel_hash_join_threshold

When hash-based join algorithm is applied, this threshold helps to decide between using hash and parallel_hash (only if estimation of the right table size is available). The former is used when we know that the right table size is below the threshold.

parallel_non_joined_rows_processing

Allow multiple threads to process non-joined rows from the right table in parallel during RIGHT and FULL JOINs. This can speed up the non-joined phase when using the parallel_hash join algorithm with large tables. When disabled, non-joined rows are processed by a single thread.

parallel_replica_offset

This is internal setting that should not be used directly and represents an implementation detail of the ‘parallel replicas’ mode. This setting will be automatically set up by the initiator server for distributed queries to the index of the replica participating in query processing among parallel replicas.

parallel_view_processing

Enables pushing to attached views concurrently instead of sequentially.