clickhouse-query-optimization

SKILL.md

Search terms: query optimization, PREWHERE, JOIN, dictionary, EXPLAIN, performance, slow query, memory limit, timeout, full table scan, dictGet, query cache, DELETE, UPDATE, mutation, aggregation, GROUP BY, COUNT DISTINCT, uniq, querylog

The ORDER BY clause in table definition determines query performance. ClickHouse stores data sorted by these columns and uses a sparse primary index to skip irrelevant data.

Key insight: Always filter on the leftmost ORDER BY columns first for best performance.

20+ optimization rules for ClickHouse queries. Load when writing queries, debugging slow queries, fixing memory errors, or optimizing JOINs. Achieves 10-100x faster queries, 90% less memory usage. Source: obsessiondb/clickhouse-skills.

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