ClickHouse Testing

Functional Tests

Functional tests are the most simple and convenient to use. Most of ClickHouse features can be tested with functional tests and they are mandatory to use for every change in ClickHouse code that can be tested that way.

Each functional test sends one or multiple queries to the running ClickHouse server and compares the result with reference.

Tests are located in dbms/tests/queries directory. There are two subdirectories: stateless and stateful. Stateless tests run queries without any preloaded test data - they often create small synthetic datasets on the fly, within the test itself. Stateful tests require preloaded test data from Yandex.Metrica and not available to general public. We tend to use only stateless tests and avoid adding new stateful tests.

Each test can be one of two types: .sql and .sh. .sql test is the simple SQL script that is piped to clickhouse-client --multiquery --testmode. .sh test is a script that is run by itself.

To run all tests, use dbms/tests/clickhouse-test tool. Look --help for the list of possible options. You can simply run all tests or run subset of tests filtered by substring in test name: ./clickhouse-test substring.

The most simple way to invoke functional tests is to copy clickhouse-client to /usr/bin/, run clickhouse-server and then run ./clickhouse-test from its own directory.

To add new test, create a .sql or .sh file in dbms/tests/queries/0_stateless directory, check it manually and then generate .reference file in the following way: clickhouse-client -n --testmode < 00000_test.sql > 00000_test.reference or ./ > ./00000_test.reference.

Tests should use (create, drop, etc) only tables in test database that is assumed to be created beforehand; also tests can use temporary tables.

If you want to use distributed queries in functional tests, you can leverage remote table function with 127.0.0.{1..2} addresses for the server to query itself; or you can use predefined test clusters in server configuration file like test_shard_localhost.

Some tests are marked with zookeeper, shard or long in their names. zookeeper is for tests that are using ZooKeeper. shard is for tests that requires server to listen 127.0.0.*; distributed or global have the same meaning. long is for tests that run slightly longer that one second. You can disable these groups of tests using --no-zookeeper, --no-shard and --no-long options, respectively.

Known bugs

If we know some bugs that can be easily reproduced by functional tests, we place prepared functional tests in dbms/tests/queries/bugs directory. These tests will be moved to dbms/tests/queries/0_stateless when bugs are fixed.

Integration Tests

Integration tests allow to test ClickHouse in clustered configuration and ClickHouse interaction with other servers like MySQL, Postgres, MongoDB. They are useful to emulate network splits, packet drops, etc. These tests are run under Docker and create multiple containers with various software.

See dbms/tests/integration/ on how to run these tests.

Note that integration of ClickHouse with third-party drivers is not tested. Also we currently don't have integration tests with our JDBC and ODBC drivers.

Unit Tests

Unit tests are useful when you want to test not the ClickHouse as a whole, but a single isolated library or class. You can enable or disable build of tests with ENABLE_TESTS CMake option. Unit tests (and other test programs) are located in tests subdirectories across the code. To run unit tests, type ninja test. Some tests use gtest, but some are just programs that return non-zero exit code on test failure.

It's not necessarily to have unit tests if the code is already covered by functional tests (and functional tests are usually much more simple to use).

Performance Tests

Performance tests allow to measure and compare performance of some isolated part of ClickHouse on synthetic queries. Tests are located at dbms/tests/performance. Each test is represented by .xml file with description of test case. Tests are run with clickhouse performance-test tool (that is embedded in clickhouse binary). See --help for invocation.

Each test run one or miltiple queries (possibly with combinations of parameters) in a loop with some conditions for stop (like "maximum execution speed is not changing in three seconds") and measure some metrics about query performance (like "maximum execution speed"). Some tests can contain preconditions on preloaded test dataset.

If you want to improve performance of ClickHouse in some scenario, and if improvements can be observed on simple queries, it is highly recommended to write a performance test. It always makes sense to use perf top or other perf tools during your tests.

Test Tools And Scripts

Some programs in tests directory are not prepared tests, but are test tools. For example, for Lexer there is a tool dbms/src/Parsers/tests/lexer that just do tokenization of stdin and writes colorized result to stdout. You can use these kind of tools as a code examples and for exploration and manual testing.

You can also place pair of files .sh and .reference along with the tool to run it on some predefined input - then script result can be compared to .reference file. These kind of tests are not automated.

Miscellanous Tests

There are tests for external dictionaries located at dbms/tests/external_dictionaries and for machine learned models in dbms/tests/external_models. These tests are not updated and must be transferred to integration tests.

There is separate test for quorum inserts. This test run ClickHouse cluster on separate servers and emulate various failure cases: network split, packet drop (between ClickHouse nodes, between ClickHouse and ZooKeeper, between ClickHouse server and client, etc.), kill -9, kill -STOP and kill -CONT , like Jepsen. Then the test checks that all acknowledged inserts was written and all rejected inserts was not.

Quorum test was written by separate team before ClickHouse was open-sourced. This team no longer work with ClickHouse. Test was accidentially written in Java. For these reasons, quorum test must be rewritten and moved to integration tests.

Manual Testing

When you develop a new feature, it is reasonable to also test it manually. You can do it with the following steps:

Build ClickHouse. Run ClickHouse from the terminal: change directory to dbms/src/programs/clickhouse-server and run it with ./clickhouse-server. It will use configuration (config.xml, users.xml and files within config.d and users.d directories) from the current directory by default. To connect to ClickHouse server, run dbms/src/programs/clickhouse-client/clickhouse-client.

Note that all clickhouse tools (server, client, etc) are just symlinks to a single binary named clickhouse. You can find this binary at dbms/src/programs/clickhouse. All tools can also be invoked as clickhouse tool instead of clickhouse-tool.

Alternatively you can install ClickHouse package: either stable release from Yandex repository or you can build package for yourself with ./release in ClickHouse sources root. Then start the server with sudo service clickhouse-server start (or stop to stop the server). Look for logs at /etc/clickhouse-server/clickhouse-server.log.

When ClickHouse is already installed on your system, you can build a new clickhouse binary and replace the existing binary:

sudo service clickhouse-server stop
sudo cp ./clickhouse /usr/bin/
sudo service clickhouse-server start

Also you can stop system clickhouse-server and run your own with the same configuration but with logging to terminal:

sudo service clickhouse-server stop
sudo -u clickhouse /usr/bin/clickhouse server --config-file /etc/clickhouse-server/config.xml

Example with gdb:

sudo -u clickhouse gdb --args /usr/bin/clickhouse server --config-file /etc/clickhouse-server/config.xml

If the system clickhouse-server is already running and you don't want to stop it, you can change port numbers in your config.xml (or override them in a file in config.d directory), provide appropriate data path, and run it.

clickhouse binary has almost no dependencies and works across wide range of Linux distributions. To quick and dirty test your changes on a server, you can simply scp your fresh built clickhouse binary to your server and then run it as in examples above.

Testing Environment

Before publishing release as stable we deploy it on testing environment. Testing environment is a cluster that process 1/39 part of Yandex.Metrica data. We share our testing environment with Yandex.Metrica team. ClickHouse is upgraded without downtime on top of existing data. We look at first that data is processed successfully without lagging from realtime, the replication continue to work and there is no issues visible to Yandex.Metrica team. First check can be done in the following way:

SELECT hostName() AS h, any(version()), any(uptime()), max(UTCEventTime), count() FROM remote('example01-01-{1..3}t', merge, hits) WHERE EventDate >= today() - 2 GROUP BY h ORDER BY h;

In some cases we also deploy to testing environment of our friend teams in Yandex: Market, Cloud, etc. Also we have some hardware servers that are used for development purposes.

Load Testing

After deploying to testing environment we run load testing with queries from production cluster. This is done manually.

Make sure you have enabled query_log on your production cluster.

Collect query log for a day or more:

clickhouse-client --query="SELECT DISTINCT query FROM system.query_log WHERE event_date = today() AND query LIKE '%ym:%' AND query NOT LIKE '%system.query_log%' AND type = 2 AND is_initial_query" > queries.tsv

This is a way complicated example. type = 2 will filter queries that are executed successfully. query LIKE '%ym:%' is to select relevant queries from Yandex.Metrica. is_initial_query is to select only queries that are initiated by client, not by ClickHouse itself (as parts of distributed query processing).

scp this log to your testing cluster and run it as following:

clickhouse benchmark --concurrency 16 < queries.tsv

(probably you also want to specify a --user)

Then leave it for a night or weekend and go take a rest.

You should check that clickhouse-server doesn't crash, memory footprint is bounded and performance not degrading over time.

Precise query execution timings are not recorded and not compared due to high variability of queries and environment.

Build Tests

Build tests allow to check that build is not broken on various alternative configurations and on some foreign systems. Tests are located at ci directory. They run build from source inside Docker, Vagrant, and sometimes with qemu-user-static inside Docker. These tests are under development and test runs are not automated.


Normally we release and run all tests on a single variant of ClickHouse build. But there are alternative build variants that are not thoroughly tested. Examples:

  • build on FreeBSD;
  • build on Debian with libraries from system packages;
  • build with shared linking of libraries;
  • build on AArch64 platform;
  • build on PowerPc platform.

For example, build with system packages is bad practice, because we cannot guarantee what exact version of packages a system will have. But this is really needed by Debian maintainers. For this reason we at least have to support this variant of build. Another example: shared linking is a common source of trouble, but it is needed for some enthusiasts.

Though we cannot run all tests on all variant of builds, we want to check at least that various build variants are not broken. For this purpose we use build tests.

Testing For Protocol Compatibility

When we extend ClickHouse network protocol, we test manually that old clickhouse-client works with new clickhouse-server and new clickhouse-client works with old clickhouse-server (simply by running binaries from corresponding packages).

Help From The Compiler

Main ClickHouse code (that is located in dbms directory) is built with -Wall -Wextra -Werror and with some additional enabled warnings. Although these options are not enabled for third-party libraries.

Clang has even more useful warnings - you can look for them with -Weverything and pick something to default build.

For production builds, gcc is used (it still generates slightly more efficient code than clang). For development, clang is usually more convenient to use. You can build on your own machine with debug mode (to save battery of your laptop), but please note that compiler is able to generate more warnings with -O3 due to better control flow and inter-procedure analysis. When building with clang, libc++ is used instead of libstdc++ and when building with debug mode, debug version of libc++ is used that allows to catch more errors at runtime.


Address sanitizer. We run functional and integration tests under ASan on per-commit basis.

Valgrind (Memcheck). We run functional tests under Valgrind overnight. It takes multiple hours. Currently there is one known false positive in re2 library, see this article.

Undefined behaviour sanitizer. We run functional and integration tests under ASan on per-commit basis.

Thread sanitizer. We run functional tests under TSan on per-commit basis. We still don't run integration tests under TSan on per-commit basis.

Memory sanitizer. Currently we still don't use MSan.

Debug allocator. Debug version of jemalloc is used for debug build.


We use simple fuzz test to generate random SQL queries and to check that the server doesn't die. Fuzz testing is performed with Address sanitizer. You can find it in This test should be run continuously (overnight and longer).

As of December 2018, we still don't use isolated fuzz testing of library code.

Security Audit

People from Yandex Cloud department do some basic overview of ClickHouse capabilities from the security standpoint.

Static Analyzers

We run PVS-Studio on per-commit basis. We have evaluated clang-tidy, Coverity, cppcheck, PVS-Studio, tscancode. You will find instructions for usage in dbms/tests/instructions/ directory. Also you can read the article in russian.

If you use CLion as an IDE, you can leverage some clang-tidy checks out of the box.


FORTIFY_SOURCE is used by default. It is almost useless, but still makes sense in rare cases and we don't disable it.

Code Style

Code style rules are described here.

To check for some common style violations, you can use utils/check-style script.

To force proper style of your code, you can use clang-format. File .clang-format is located at the sources root. It mostly corresponding with our actual code style. But it's not recommended to apply clang-format to existing files because it makes formatting worse. You can use clang-format-diff tool that you can find in clang source repository.

Alternatively you can try uncrustify tool to reformat your code. Configuration is in uncrustify.cfg in the sources root. It is less tested than clang-format.

CLion has its own code formatter that has to be tuned for our code style.

Metrica B2B Tests

Each ClickHouse release is tested with Yandex Metrica and AppMetrica engines. Testing and stable versions of ClickHouse are deployed on VMs and run with a small copy of Metrica engine that is processing fixed sample of input data. Then results of two instances of Metrica engine are compared together.

These tests are automated by separate team. Due to high number of moving parts, tests are fail most of the time by completely unrelated reasons, that are very difficult to figure out. Most likely these tests have negative value for us. Nevertheless these tests was proved to be useful in about one or two times out of hundreds.

Test Coverage

As of July 2018 we don't track test coverage.

Test Automation

We run tests with Yandex internal CI and job automation system named "Sandbox".

Build jobs and tests are run in Sandbox on per commit basis. Resulting packages and test results are published in GitHub and can be downloaded by direct links. Artifacts are stored eternally. When you send a pull request on GitHub, we tag it as "can be tested" and our CI system will build ClickHouse packages (release, debug, with address sanitizer, etc) for you.

We don't use Travis CI due to the limit on time and computational power. We don't use Jenkins. It was used before and now we are happy we are not using Jenkins.