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Optimize JOINs - When and Algorithms
JOINs in ClickHouse can be expensive and should be minimized. When JOINs are necessary, choose the right algorithm: hash join for general use, merge join for sorted data, and direct join for dictionaries. Optimize JOINs by reducing right table size, using appropriate algorithms, filtering early, and ensuring tables are sorted. Consider alternatives like dictionaries or denormalization when possible.
JOINs are one of the most expensive operations in ClickHouse. Understanding when to use them, how to choose the right algorithm, and how to optimize them is crucial for writing efficient queries.
When to Use JOINs
Use JOINs when no better alternative exists (dictionaries aren't suitable, denormalization isn't practical, data relationships are complex), when the right table is small (can fit in memory, won't cause memory issues, performance is acceptable), or for infrequent queries (JOINs used occasionally, performance impact acceptable, not in hot query path).
Avoid JOINs when alternatives exist (can use dictionaries, denormalize data, use materialized views), when the right table is large (won't fit in memory, causes memory pressure, performance unacceptable), or for frequent queries (JOINs in hot query path, performance critical, need consistent low latency).