distributed_* session settings

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distributed_aggregation_memory_efficient

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distributed_connections_pool_size

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distributed_foreground_insert

• 0 — Data is inserted in background mode.
• 1 — Data is inserted in synchronous mode.

• Distributed Table Engine
• Managing Distributed Tables

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distributed_group_by_no_merge

• 0 — Disabled (final query processing is done on the initiator node).
• 1 - Do not merge aggregation states from different servers for distributed query processing (query completely processed on the shard, initiator only proxy the data), can be used in case it is for certain that there are different keys on different shards.
• 2 - Same as 1 but applies ORDER BY and LIMIT (it is not possible when the query processed completely on the remote node, like for distributed_group_by_no_merge=1) on the initiator (can be used for queries with ORDER BY and/or LIMIT).

SELECT *
FROM remote('127.0.0.{2,3}', system.one)
GROUP BY dummy
LIMIT 1
SETTINGS distributed_group_by_no_merge = 1
FORMAT PrettyCompactMonoBlock

┌─dummy─┐
│     0 │
│     0 │
└───────┘

SELECT *
FROM remote('127.0.0.{2,3}', system.one)
GROUP BY dummy
LIMIT 1
SETTINGS distributed_group_by_no_merge = 2
FORMAT PrettyCompactMonoBlock

┌─dummy─┐
│     0 │
└───────┘

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distributed_insert_skip_read_only_replicas

• 0 — INSERT was as usual, if it will go to read-only replica it will fail
• 1 — Initiator will skip read-only replicas before sending data to shards.

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distributed_product_mode

• Only applied for IN and JOIN subqueries.
• Only if the FROM section uses a distributed table containing more than one shard.
• If the subquery concerns a distributed table containing more than one shard.
• Not used for a table-valued remote function.

• deny — Default value. Prohibits using these types of subqueries (returns the “Double-distributed in/JOIN subqueries is denied” exception).
• local — Replaces the database and table in the subquery with local ones for the destination server (shard), leaving the normal IN/JOIN.
• global — Replaces the IN/JOIN query with GLOBAL IN/GLOBAL JOIN.
• allow — Allows the use of these types of subqueries.

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distributed_push_down_limit

• Sending extra rows over network;
• Processing rows behind the limit on the initiator.

• distributed_group_by_no_merge > 0.
• Query does not have GROUP BY/DISTINCT/LIMIT BY, but it has ORDER BY/LIMIT.
• Query has GROUP BY/DISTINCT/LIMIT BY with ORDER BY/LIMIT and:
  • optimize_skip_unused_shards is enabled.
  • optimize_distributed_group_by_sharding_key is enabled.

• 0 — Disabled.
• 1 — Enabled.

• distributed_group_by_no_merge
• optimize_skip_unused_shards
• optimize_distributed_group_by_sharding_key