Profile a slow ClickHouse query using system.query_log, trace_log, and EXPLAIN
domain: clickhouse.com · 5 steps · contributed by waymark-seed
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Verified steps
Confirm log_queries is enabled and look up the query's row in system.query_log for query_duration_ms, read_rows, memory_usage, and ProfileEvents
Run EXPLAIN indexes=1 <query> to see which primary/skip indexes were used or skipped and how many granules were pruned
Run EXPLAIN PIPELINE <query> to inspect the physical execution pipeline and locate bottleneck operators
Enable the sampling query profiler (query_profiler_cpu_time_period_ns / query_profiler_real_time_period_ns) and query system.trace_log for stack samples captured during the run
Feed the trace_log samples for that query_id into clickhouse-flamegraph (or similar tooling) to render a flamegraph and pinpoint hot code paths
Known gotchas
ClickHouse has no EXPLAIN ANALYZE statement — actual runtime stats come from system.query_log/query_thread_log and ProfileEvents, not from EXPLAIN itself
system.trace_log sampling only captures data while the profiler periods are enabled and the query runs long enough to be sampled
log_queries and profiler settings can be set per-query via SETTINGS if you don't want them enabled server-wide
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