Query performance

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This document goes over:

  • What tools are available to understand and measure query performance
  • Importance of page cache
  • General tips and tricks for performant queries



clickhouse-client is a command-line application for running queries against ClickHouse.

When executing queries, it details progress, execution time, how many rows and gigabytes of data were processed, and how much CPU was used.

clickhouse-client progress reporting

You can get additional logging from ClickHouse by setting SET send_logs_level = 'trace' before running a query.


ClickHouse saves all queries it runs into system.query_log table.

It includes information on:

  • What query was run and when
  • How long did it take to execute
  • How many resources did it take up: memory, rows/bytes read
  • In case of errors, exception information

Some tips for querying the query_log:

  • For distributed queries, filter by is_initial_query to disambiguate distributed queries i.e. is_initial_query = 1 denotes the query sent to the coordinator node, whereas is_initial_query = 0 denotes internally-generated queries sent to gather data from other shards
  • Use clusterAllReplicas('<cluster_name>', system.query_log) to query all nodes
  • Filter on type = 'QueryFinish' if you only want data on queries that completed successfully. Forgetting to do so can skew averages since the results will include QueryStart events, which have columns such as query_duration_ms set to 0.
  • ProfileEvents column contains a lot of useful performance data on each query, not all of which is documented. Check the source for a full list all measurements.

At PostHog, we also add metadata to each query via the log_comment setting to make results easier to analyze. This includes information on the source of the query and how it was constructed. See this runbook for more details.

An example query to get recent slow queries:

formatReadableSize(read_bytes) as read_size,
formatReadableSize(result_bytes) as result_size,
formatReadableSize(memory_usage) as memory,
FROM system.query_log
query NOT LIKE '%query_log%'
AND type = 'QueryFinish'
AND event_time > now() - toIntervalDay(3)
AND query_duration_ms > 30000
/* If using log_comment, consider including something like:
AND JSONExtract(log_comment, 'kind') = 'request'
ORDER BY query_duration_ms desc

Note that this table is not distributed - on a cluster setting you might need to run query against each node separately or do ad-hoc distributed queries.


Previous pages in this manual showed various examples of using the ClickHouse EXPLAIN statement to your advantage.

Various forms of explain can detail:

  • If and how much data ClickHouse was able to avoid processing thanks to schema setup
  • If and how ClickHouse "optimizes" the query by moving columns to PREWHERE
  • Detailing how the query is planned to be executed

Read more about EXPLAIN in ClickHouse's EXPLAIN Statement docs.

Flame graphs

For CPU-bound calculations, flamegraphs can help visualize what ClickHouse worked on during query execution.

We've built flamegraph support into PostHog. You can find tools to generate flamegraphs for queries under PostHog instance settings.

Importance of the page cache

When running queries, you might encounter an odd artifact: the first time you run a query, it's really slow but it speeds up significantly when run again.

This behavior is due to the Linux page cache. In broad terms, the operating system caches recently read files into memory, speeding up subsequent reads of the same data.

As most queries in ClickHouse are dependent on fast I/O to execute fast, this can have a significant effect on query performance. It is a reason why at PostHog our ClickHouse nodes have a lot of memory available.

Effect on benchmarking

This behavior can be a problem for profiling: users constructing new queries might not hit the page cache and receive a worse experience than benchmarking may show.

This means it's often important to wipe page cache on ClickHouse when doing queries. This can be achieved with the following command on a ClickHouse node:

sudo sh -c "/usr/bin/echo 3 > /proc/sys/vm/drop_caches"

Note that the above will only drop the cache on the given node, but distributed queries might still be affected from the page cache on nodes in the other shards.

For completely clean benchmarking, you might also want to drop ClickHouse's internal mark cache.

Tips for achieving well-performing queries

Previous pages in the ClickHouse manual have highlighted the importance of setting up the correct schema and detailing how queries work in a distributed setting.

This section highlights some general rules of thumb that can help speed up queries:

Also always do your benchmarking in a realistic setting: on large datasets on powerful machines.

Next in the ClickHouse manual: Operations


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