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System Tables and a Window into the Internals of ClickHouse

author avatar
Derek Chia
Dec 21, 2022


Ever wondered how to debug an issue in ClickHouse? Need a specific statistic, or are you curious about the queries being executed by your users and those that are failing? Or maybe you need to identify the currently applied settings. Look no further than system tables! In this post, we explore the system tables in ClickHouse and show how we in ClickHouse support use them to debug issues and understand your cluster usage with practical examples.

Introduction to System Tables

System tables in ClickHouse are virtual tables that provide information about server states, processes, and the operating environment. These system tables are located in the system database and are only available for reading by the users. They cannot be dropped or altered, but their partition can be detached and old records can be removed using TTL. System tables offer great insight into the internal operations of ClickHouse and can be a valuable source of information when optimizing queries, monitoring system performance, or troubleshooting a system crash.

In general, there are a few types of system tables in ClickHouse, and some useful ones contain system information related to your database, tables, columns, and parts. There are also tables showing real-time information such as metrics and events, providing a snapshot view of the current system events. Users may also find historical records in system log tables such as metric_log, query_log, part_log, etc. In a cluster, distribution_queue and replication_queue can be used to troubleshoot a distributed setup. Tables related to settings, users, and roles also provide information on the current configuration and user privileges.

Most system tables store their data in memory, but system log tables such as metric_log, query_log and part_log use the MergeTree table engine and store their data in the filesystem by default. This persistent storage ensures that logs are still available for analysis after a server restart.

Where are the System Tables?

The complete list of system tables is accessible via the SHOW TABLES FROM system statement. You can also find an expanded description for most system tables in our documentation.

SHOW TABLES FROM system ┌─name───────────────────────────┐ │ aggregate_function_combinators │ │ asynchronous_inserts │ │ asynchronous_metric_log │ │ asynchronous_metric_log_0 │ │ asynchronous_metrics │ │ backups │ │ build_options │ │ certificates │ │ clusters │ │ collations │ │ columns │ │ contributors │ │ current_roles │ │ data_skipping_indices │ │ data_type_families │ │ databases │

Like any other table, we can run typical select queries e.g. SELECT * FROM system.databases, to retrieve rows from a specified table.

SELECT * FROM system.databases LIMIT 2 FORMAT Vertical Row 1: ────── name: INFORMATION_SCHEMA engine: Memory data_path: /var/lib/clickhouse/ metadata_path: uuid: 00000000-0000-0000-0000-000000000000 comment: Row 2: ────── name: blogs engine: Replicated data_path: /var/lib/clickhouse/store/ metadata_path: /var/lib/clickhouse/store/912/9125f586-0e3f-48f6-85b0-ccc76380e1a2/ uuid: 9125f586-0e3f-48f6-85b0-ccc76380e1a2 comment: 2 rows in set. Elapsed: 0.001 sec.

Aggregating on these tables enables us to write more complex queries and gain a deeper understanding of the state of ClickHouse.

SELECT engine, count() AS count FROM system.databases GROUP BY engine ┌─engine─────┬─count─┐ │ Memory │ 2 │ │ Atomic │ 1 │ │ Replicated │ 8 │ └────────────┴───────┘ 3 rows in set. Elapsed: 0.015 sec.

So, what are some of the valuable insights that we can gather from the system tables?

Hot tips for querying system tables

In this section, we will highlight some useful system tables that can help answer the common questions we may have when using ClickHouse.

What settings were changed from the default value?

First, we begin by reviewing the list of settings (using system.settings) that were changed from the default value. During troubleshooting, this is an excellent first step to analyze if the changed settings could affect system behavior.

SELECT * FROM system.settings WHERE changed LIMIT 2 FORMAT Vertical Row 1: ────── name: max_insert_threads value: 4 changed: 1 description: The maximum number of threads to execute the INSERT SELECT query. Values 0 or 1 means that INSERT SELECT is not run in parallel. Higher values will lead to higher memory usage. Parallel INSERT SELECT has effect only if the SELECT part is run on parallel, see 'max_threads' setting. min: ᴺᵁᴸᴸ max: ᴺᵁᴸᴸ readonly: 0 type: UInt64 Row 2: ────── name: max_threads value: 60 changed: 1 description: The maximum number of threads to execute the request. By default, it is determined automatically. min: ᴺᵁᴸᴸ max: ᴺᵁᴸᴸ readonly: 0 type: MaxThreads 2 rows in set. Elapsed: 0.003 sec.

What are the long-running queries? Which queries took up the most memory?

Next, we dive into the query log table (system.query_log) that holds a wealth of information about executed queries. It is often the go-to table for identifying long-running, memory-intensive, or failed queries.

Using the query below, we can generate an overview of the queries that took the longest to execute. We also select other columns such as memory_usage, userCPU, and systemCPU to give us a glimpse of the resources utilized. On top of this, the function normalizedQueryHash hashes similar queries into identical 64-bit hash values, allowing us to further aggregate the value and monitor performance for similar queries.

The same query below can also be used to find queries that took up the most memory. Simply replace the sorting key with memory_usage. Note that every successful query will result in two entries recorded in the query_log. The first query will have the type QueryStart and the last will be QueryFinish. We are particularly interested in the QueryFinish rows as these will record the timing and resources used to execute the queries.

SELECT type, event_time, query_duration_ms, initial_query_id, formatReadableSize(memory_usage) AS memory, `ProfileEvents.Values`[indexOf(`ProfileEvents.Names`, 'UserTimeMicroseconds')] AS userCPU, `ProfileEvents.Values`[indexOf(`ProfileEvents.Names`, 'SystemTimeMicroseconds')] AS systemCPU, normalizedQueryHash(query) AS normalized_query_hash, substring(normalizeQuery(query) AS query, 1, 100) FROM system.query_log ORDER BY query_duration_ms DESC LIMIT 2 FORMAT Vertical Row 1: ────── type: QueryFinish event_time: 2022-11-26 11:50:14 query_duration_ms: 600802 initial_query_id: feb4c490-b420-47d3-a7ee-8c87fc68bf45 memory: 631.64 MiB userCPU: 27404274713 systemCPU: 234596117 normalized_query_hash: 17959601262672325984 substring(normalizeQuery(query), 1, 100): SELECT count() AS c FROM wikistat GROUP BY time Row 2: ────── type: QueryFinish event_time: 2022-11-26 15:05:39 query_duration_ms: 545026 initial_query_id: 8196b460-7a6a-434e-9324-14fc765a9a76 memory: 690.21 MiB userCPU: 28103266351 systemCPU: 324925435 normalized_query_hash: 8457232685578498203 substring(normalizeQuery(query), 1, 100): SELECT `time`, count() AS `c` FROM `default`.`wikistat` GROUP BY `time` ORDER BY `time` ASC WITH FIL 2 rows in set. Elapsed: 0.244 sec. Processed 8.49 million rows, 4.70 GB (34.75 million rows/s., 19.22 GB/s.)

Which queries have failed?

Not all queries are crafted perfectly with some failing to execute. ExceptionBeforeStart and ExceptionWhileProcessing are two types of exception events that could happen when executing a query. Below is a query that filters for these exceptions and displays the exception message and stack trace, along with columns such as used_aggregate_functions, etc. This information can be helpful for troubleshooting.

SELECT type, query_start_time, query_duration_ms, query_id, query_kind, is_initial_query, normalizeQuery(query) AS normalized_query, concat(toString(read_rows), ' rows / ', formatReadableSize(read_bytes)) AS read, concat(toString(written_rows), ' rows / ', formatReadableSize(written_bytes)) AS written, concat(toString(result_rows), ' rows / ', formatReadableSize(result_bytes)) AS result, formatReadableSize(memory_usage) AS `memory usage`, exception, concat('\n', stack_trace) AS stack_trace, user, initial_user, multiIf(empty(client_name), http_user_agent, concat(client_name, ' ', toString(client_version_major), '.', toString(client_version_minor), '.', toString(client_version_patch))) AS client, client_hostname, databases, tables, columns, used_aggregate_functions, used_aggregate_function_combinators, used_database_engines, used_data_type_families, used_dictionaries, used_formats, used_functions, used_storages, used_table_functions, thread_ids, ProfileEvents, Settings FROM system.query_log WHERE type IN ['3', '4'] ORDER BY query_start_time DESC LIMIT 1 FORMAT Vertical Row 1: ────── type: ExceptionBeforeStart query_start_time: 2022-12-12 09:50:52 query_duration_ms: 0 query_id: eec8ab27-51a6-4cde-ae3d-c306c13de5eb query_kind: Select is_initial_query: 1 normalized_query: select x from taxi_zone_dictionary read: 0 rows / 0.00 B written: 0 rows / 0.00 B result: 0 rows / 0.00 B memory usage: 0.00 B exception: Code: 47. DB::Exception: Missing columns: 'x' while processing query: 'SELECT x FROM taxi_zone_dictionary', required columns: 'x'. (UNKNOWN_IDENTIFIER) (version (official build)) stack_trace: 0. DB::Exception::Exception(std::__1::basic_string, std::__1::allocator> const&, int, bool) @ 0xbd145e8 in /usr/bin/clickhouse 1. DB::TreeRewriterResult::collectUsedColumns(std::__1::shared_ptr const&, bool, bool) @ 0x10ad376c in /usr/bin/clickhouse 2. DB::TreeRewriter::analyzeSelect(std::__1::shared_ptr&, DB::TreeRewriterResult&&, DB::SelectQueryOptions const&, std::__1::vector> const&, std::__1::vector, std::__1::allocator>, std::__1::allocator, std::__1::allocator>>> const&, std::__1::shared_ptr) const @ 0x10ad7fac in /usr/bin/clickhouse 3. ? @ 0x1083b550 in /usr/bin/clickhouse 4. DB::InterpreterSelectQuery::InterpreterSelectQuery(std::__1::shared_ptr const&, std::__1::shared_ptr const&, std::__1::optional, std::__1::shared_ptr const&, DB::SelectQueryOptions const&, std::__1::vector, std::__1::allocator>, std::__1::allocator, std::__1::allocator>>> const&, std::__1::shared_ptr const&, std::__1::shared_ptr) @ 0x10838454 in /usr/bin/clickhouse 5. DB::InterpreterSelectWithUnionQuery:: buildCurrentChildInterpreter(std::__1::shared_ptr const&, std::__1::vector, std::__1::allocator>, std::__1::allocator, std::__1::allocator>>> const&) @ 0x108d1dcc in /usr/bin/clickhouse 6. DB::InterpreterSelectWithUnionQuery:: InterpreterSelectWithUnionQuery(std::__1::shared_ptr const&, std::__1::shared_ptr, DB::SelectQueryOptions const&, std::__1::vector, std::__1::allocator>, std::__1::allocator, std::__1::allocator>>> const&) @ 0x108cfb68 in /usr/bin/clickhouse 7. DB::InterpreterFactory::get(std::__1::shared_ptr&, std::__1::shared_ptr, DB::SelectQueryOptions const&) @ 0x107fe174 in /usr/bin/clickhouse 8. ? @ 0x10b70ab8 in /usr/bin/clickhouse 9. DB::executeQuery(std::__1::basic_string, std::__1::allocator> const&, std::__1::shared_ptr, bool, DB::QueryProcessingStage::Enum) @ 0x10b6e684 in /usr/bin/clickhouse 10. DB::TCPHandler::runImpl() @ 0x11637db0 in /usr/bin/clickhouse 11. DB::TCPHandler::run() @ 0x11648ec4 in /usr/bin/clickhouse 12. Poco::Net::TCPServerConnection::start() @ 0x1225a98c in /usr/bin/clickhouse 13. Poco::Net::TCPServerDispatcher::run() @ 0x1225c520 in /usr/bin/clickhouse 14. Poco::PooledThread::run() @ 0x12416c5c in /usr/bin/clickhouse 15. Poco::ThreadImpl::runnableEntry(void*) @ 0x12414524 in /usr/bin/clickhouse 16. start_thread @ 0x7624 in /usr/lib/aarch64-linux-gnu/ 17. ? @ 0xd149c in /usr/lib/aarch64-linux-gnu/ user: default initial_user: default client: ClickHouse 22.10.2 client_hostname: derek-clickhouse databases: [] tables: [] columns: [] used_aggregate_functions: [] used_aggregate_function_combinators: [] used_database_engines: [] used_data_type_families: [] used_dictionaries: [] used_formats: [] used_functions: [] used_storages: [] used_table_functions: [] thread_ids: [] ProfileEvents: {} Settings: {} 1 row in set. Elapsed: 0.019 sec.

What are the common errors?

Next, we explore the system.errors table. This table contains error codes and the number of times each error has been triggered. Furthermore, we can see when the error last occurred coupled with the exact error message. The last_error_trace column also contains a stack trace for debugging and is helpful for introspecting the server state.

SELECT name, code, value, last_error_time, last_error_message, last_error_trace AS remote FROM system.errors LIMIT 1 FORMAT Vertical Row 1: ────── name: CANNOT_READ_FROM_ISTREAM code: 23 value: 1016 last_error_time: 2022-12-21 11:43:06 last_error_message: Cannot read from istream at offset 0 remote: [228387450,270427334,306047695,310642709,310640492, 310861745,310860816,310860718,315390197,129746296,129744797,229143926, 229154103,229129110,229149953,140698656110089,140698655211827] 1 row in set. Elapsed: 0.002 sec.

Are parts being created when the rows are inserted?

Engines in the MergeTree family are designed to write data quickly to a table in small parts, before merging these into larger parts in the background. To confirm that the rows inserted are successfully written into the disk as parts, we can review the system.part_log and check that new parts are created in a timely manner.

SELECT event_time, event_time_microseconds, rows FROM system.part_log WHERE (database = 'default') AND (table = 'github_events') AND (event_type IN ['NewPart']) ORDER BY event_time ASC LIMIT 10 ┌──────────event_time─┬────event_time_microseconds─┬───rows─┐ │ 2022-12-12 10:54:42 │ 2022-12-12 10:54:42.373583 │ 573440 │ │ 2022-12-12 10:54:45 │ 2022-12-12 10:54:45.116786 │ 507904 │ │ 2022-12-12 10:54:47 │ 2022-12-12 10:54:47.374676 │ 312032 │ │ 2022-12-12 10:54:49 │ 2022-12-12 10:54:49.598769 │ 434176 │ │ 2022-12-12 10:54:51 │ 2022-12-12 10:54:51.824833 │ 368638 │ │ 2022-12-12 10:54:53 │ 2022-12-12 10:54:53.964555 │ 548864 │ │ 2022-12-12 10:54:56 │ 2022-12-12 10:54:56.286868 │ 524288 │ │ 2022-12-12 10:54:58 │ 2022-12-12 10:54:58.892573 │ 253948 │ │ 2022-12-12 10:55:01 │ 2022-12-12 10:55:01.404872 │ 450560 │ │ 2022-12-12 10:55:03 │ 2022-12-12 10:55:03.630993 │ 328850 │ └─────────────────────┴────────────────────────────┴────────┘ 10 rows in set. Elapsed: 0.012 sec. Processed 4.96 thousand rows, 292.42 KB (404.05 thousand rows/s., 23.80 MB/s.)

What is the status of the in-progress merges?

As newly created parts are constantly merged in the background, we can watch for long-running merges using the system.merges table. Merges that take a long time to complete could mean that certain system resources (e.g. CPU, disk IO) have reached a saturation point.

SELECT hostName(), database, table, round(elapsed, 0) AS time, round(progress, 4) AS percent, formatReadableTimeDelta((elapsed / progress) - elapsed) AS ETA, num_parts, formatReadableSize(memory_usage) AS memory_usage, result_part_name FROM system.merges ORDER BY (elapsed / percent) - elapsed ASC FORMAT Vertical Row 1: ────── hostName(): c-mint-mb-85-server-0 database: default table: minicrawl time: 831 percent: 0.6428 ETA: 7 minutes and 41 seconds num_parts: 6 memory_usage: 1.50 GiB result_part_name: all_839_1124_4 2 rows in set. Elapsed: 0.360 sec.

Are there parts with errors?

To identify errors during part merges, we can again examine the system.part_log table to reveal the number of times a data part error occurred for a particular event type. The error codes are resolved to the respective error names and act as a feedback mechanism for us to adjust our queries or provide additional resources. A full list of error codes and names can be found here.

SELECT event_date, event_type, table, error AS error_code, errorCodeToName(error) AS error_code_name, count() as c FROM system.part_log WHERE (error_code != 0) AND (event_date > (now() - toIntervalMonth(1))) GROUP BY event_date, event_type, error, table ORDER BY event_date DESC, event_type ASC, table ASC, error ASC ┌─event_date─┬─event_type───┬─table──┬─error_code─┬─error_code_name─────────┬────c────┐ │ 2022-12-12 │ MergeParts │ events │ 241 │ MEMORY_LIMIT_EXCEEDED │ 77 │ │ 2022-12-06 │ MergeParts │ events │ 241 │ MEMORY_LIMIT_EXCEEDED │ 16 │ │ 2022-11-28 │ NewPart │ x │ 389 │ INSERT_WAS_DEDUPLICATED │ 38 │ │ 2022-11-28 │ NewPart │ x │ 394 │ QUERY_WAS_CANCELLED │ 1 │ │ 2022-11-28 │ MergeParts │ events │ 236 │ ABORTED │ 25 │ │ 2022-11-28 │ MutatePart │ events │ 236 │ ABORTED │ 68 │ │ 2022-11-27 │ MergeParts │ events │ 236 │ ABORTED │ 1 │ │ 2022-11-27 │ MutatePart │ events │ 236 │ ABORTED │ 9 │ │ 2022-11-26 │ MergeParts │ events │ 236 │ ABORTED │ 26 │ │ 2022-11-26 │ MutatePart │ events │ 236 │ ABORTED │ 282 │ │ 2022-11-25 │ NewPart │ x │ 394 │ QUERY_WAS_CANCELLED │ 1 │ │ 2022-11-25 │ MutatePart │ events │ 236 │ ABORTED │ 14 │ │ 2022-11-24 │ MergeParts │ events │ 236 │ ABORTED │ 55 │ │ 2022-11-24 │ MergeParts │ events │ 241 │ MEMORY_LIMIT_EXCEEDED │ 158 │ │ 2022-11-24 │ DownloadPart │ events │ 1000 │ POCO_EXCEPTION │ 4 │ │ 2022-11-24 │ MutatePart │ events │ 236 │ ABORTED │ 119 │ │ 2022-11-23 │ MergeParts │ events │ 241 │ MEMORY_LIMIT_EXCEEDED │ 174 │ │ 2022-11-23 │ DownloadPart │ events │ 1000 │ POCO_EXCEPTION │ 12 │ │ 2022-11-22 │ MergeParts │ events │ 241 │ MEMORY_LIMIT_EXCEEDED │ 70 │ └────────────┴──────────────┴────────┴────────────┴─────────────────────────┴─────────┘ 19 rows in set. Elapsed: 0.008 sec. Processed 73.25 thousand rows, 1.61 MB (8.99 million rows/s., 198.00 MB/s.)

Are there long-running mutations that are stuck?

ALTER queries, also known as mutations, manipulate table data by rewriting the whole data parts. Thus, this can be a resource-intensive operation and can take a long time to complete if a large number of parts need to be modified and potentially impact normal merge operations. The query below lists the in-progress mutations and displays the reason for failure, if any.

SELECT database, table, mutation_id, command, create_time, parts_to_do_names, parts_to_do, is_done, latest_failed_part, latest_fail_time, latest_fail_reason FROM system.mutations WHERE NOT is_done ORDER BY create_time DESC Row 1: ────── database: default table: events_wide_new mutation_id: 0000000001 command: DROP COLUMN col798 create_time: 2022-12-12 16:19:53 parts_to_do_names: ['20221212_6_41_2_86','20221212_42_71_2_86','20221212_72_99_2_86','20221212_106_139_2'] parts_to_do: 4 is_done: 0 latest_failed_part: latest_fail_time: 1970-01-01 00:00:00 latest_fail_reason: 1 row in set. Elapsed: 0.002 sec.

How much disk space are the tables using?

ClickHouse compresses data really well with the use of LZ4 compression codec by default (in ClickHouse Cloud we actually use ZSTD - see here for more details why). However, there are times when we might be curious to find out how much disk space each table is using. The query below makes use of table to show us the disk space each non-system table is taking up in total (total_bytes_on_disk), as well as the total size of compressed data parts (data_compressed_bytes) and also the size when these data parts are uncompressed (data_uncompressed_bytes). In the example below, we can observe from the compression_ratio column that the compressed data only take up less than 40% of disk space! For those interested in further minimizing your storage, check out our blog post on Optimizing ClickHouse with Schemas and Codecs.

SELECT hostName(), database, table, sum(rows) AS rows, formatReadableSize(sum(bytes_on_disk)) AS total_bytes_on_disk, formatReadableSize(sum(data_compressed_bytes)) AS total_data_compressed_bytes, formatReadableSize(sum(data_uncompressed_bytes)) AS total_data_uncompressed_bytes, round(sum(data_compressed_bytes) / sum(data_uncompressed_bytes), 3) AS compression_ratio FROM WHERE database != 'system' GROUP BY hostName(), database, table ORDER BY sum(bytes_on_disk) DESC FORMAT Vertical Row 1: ────── hostName(): c-mint-mb-85-server-0 database: default table: reddit rows: 9946243959 total_bytes_on_disk: 718.47 GiB total_data_compressed_bytes: 717.39 GiB total_data_uncompressed_bytes: 2.22 TiB compression_ratio: 0.315 Row 2: ────── hostName(): c-mint-mb-85-server-0 database: default table: wikistat rows: 417565645200 total_bytes_on_disk: 579.44 GiB total_data_compressed_bytes: 554.31 GiB total_data_uncompressed_bytes: 14.12 TiB compression_ratio: 0.038 2 rows in set. Elapsed: 0.004 sec.

What is the status of the parts that are moving?

Other than parts merging in the background, parts and partitions can also be moved between disks and volumes. For example, it is common to first store the recently written parts on a hot volume (SSD) and then move them automatically to a cold volume (HDD) when they have passed a certain age. This operation can be done using the TTL clause or be triggered with the ALTER statement. When the parts are moving, we can monitor the status using the recently introduced system.moves table to watch for the elapsed time and the destination disk. The query below shows that a part (all_1_22_2) is in the process of moving to an s3 disk.

ALTER TABLE ontime MOVE PART 'all_1_22_2' TO VOLUME 'external';

SELECT * FROM system.moves FORMAT Vertical Row 1: ────── database: default table: ontime elapsed: 8.900590354 target_disk_name: s3 target_disk_path: /var/lib/clickhouse/disks/s3_disk/ part_name: all_1_22_2 part_size: 1643771811 thread_id: 10071 1 row in set. Elapsed: 0.160 sec.

Querying system tables from all nodes in a cluster

When querying for system tables in a cluster, take note that the query is only executed on the local node where the query is issued. To retrieve rows from all nodes in a cluster with shards and replicas, we need to use the clusterAllReplicas table function. The query below is sent to a cluster with two shards, each with two replicas. Based on the hostName, we can see that the resulting rows were gathered from all four nodes.

SELECT hostName(), is_initial_query, query_id, initial_query_id, query FROM clusterAllReplicas('default', system.processes) FORMAT Vertical Row 1: ────── hostName(): c-mint-mb-85-server-0 is_initial_query: 1 query_id: c8f1cfb2-7eed-4ecd-a303-cc20ef5d9d0f initial_query_id: c8f1cfb2-7eed-4ecd-a303-cc20ef5d9d0f query: SELECT hostName(), is_initial_query, query_id, initial_query_id, query FROM clusterAllReplicas('default', system.processes) FORMAT Vertical Row 2: ────── hostName(): c-mint-mb-85-server-1 is_initial_query: 0 query_id: e487bae2-0886-46ef-8510-87c218f45332 initial_query_id: c8f1cfb2-7eed-4ecd-a303-cc20ef5d9d0f query: SELECT hostName(), `is_initial_query`, `query_id`, `initial_query_id`, `query` FROM `system`.`processes` Row 3: ────── hostName(): c-mint-mb-85-server-2 is_initial_query: 0 query_id: 271abbde-64b9-46ea-9391-9f402cc013ef initial_query_id: c8f1cfb2-7eed-4ecd-a303-cc20ef5d9d0f query: SELECT hostName(), `is_initial_query`, `query_id`, `initial_query_id`, `query` FROM `system`.`processes` 3 rows in set. Elapsed: 0.006 sec.


In this post, we’ve introduced how system tables can be used to query for the current and historical state of ClickHouse. We’ve provided examples using system tables to answer some common questions when using ClickHouse. In a future post, we’ll explore these tables in more detail and how they can be used to monitor ClickHouse for common challenges on INSERT and SELECT queries.

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