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ClickHouse is adding a cost-based join optimiser that can reorder outer, semi and anti joins, not just inner joins. The change could cut unnecessary database work while preserving query results, and is due to arrive in version 26.10.

What happened ClickHouse’s new optimiser uses dynamic programming to search for a cheaper join order. Until now, non-inner joins acted as barriers to reordering, limiting the plans the database could consider. The new approach uses a conflict detector to rule out transformations that would change a query’s result while allowing valid alternatives. The official blog says the feature will handle queries with up to 10 tables by default, with support for up to 12 if the relevant limit is raised. It covers LEFT, RIGHT and FULL outer joins, along with SEMI and ANTI joins. ClickHouse says tests across more than 90,000 join trees and 2.8 million candidate plans found no invalid or missing plans. Why it matters

Discuss: Should the default search limit stay conservative, or should database engines spend more effort finding the cheapest plan?

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