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83 lines
4.2 KiB
Markdown
---
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slug: /en/operations/system-tables/
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sidebar_position: 52
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sidebar_label: Overview
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pagination_next: 'en/operations/system-tables/asynchronous_metric_log'
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---
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# System Tables
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## Introduction {#system-tables-introduction}
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System tables provide information about:
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- Server states, processes, and environment.
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- Server’s internal processes.
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- Options used when the ClickHouse binary was built.
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System tables:
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- Located in the `system` database.
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- Available only for reading data.
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- Can’t be dropped or altered, but can be detached.
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Most of system tables store their data in RAM. A ClickHouse server creates such system tables at the start.
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Unlike other system tables, the system log tables [metric_log](../../operations/system-tables/metric_log.md), [query_log](../../operations/system-tables/query_log.md), [query_thread_log](../../operations/system-tables/query_thread_log.md), [trace_log](../../operations/system-tables/trace_log.md), [part_log](../../operations/system-tables/part_log.md), [crash_log](../../operations/system-tables/crash-log.md) and [text_log](../../operations/system-tables/text_log.md) are served by [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table engine and store their data in a filesystem by default. If you remove a table from a filesystem, the ClickHouse server creates the empty one again at the time of the next data writing. If system table schema changed in a new release, then ClickHouse renames the current table and creates a new one.
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System log tables can be customized by creating a config file with the same name as the table under `/etc/clickhouse-server/config.d/`, or setting corresponding elements in `/etc/clickhouse-server/config.xml`. Elements can be customized are:
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- `database`: database the system log table belongs to. This option is deprecated now. All system log tables are under database `system`.
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- `table`: table to insert data.
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- `partition_by`: specify [PARTITION BY](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) expression.
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- `ttl`: specify table [TTL](../../sql-reference/statements/alter/ttl.md) expression.
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- `flush_interval_milliseconds`: interval of flushing data to disk.
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- `engine`: provide full engine expression (starting with `ENGINE =` ) with parameters. This option is contradict with `partition_by` and `ttl`. If set together, the server would raise an exception and exit.
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An example:
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```xml
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<clickhouse>
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<query_log>
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<database>system</database>
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<table>query_log</table>
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<partition_by>toYYYYMM(event_date)</partition_by>
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<ttl>event_date + INTERVAL 30 DAY DELETE</ttl>
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<!--
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<engine>ENGINE = MergeTree PARTITION BY toYYYYMM(event_date) ORDER BY (event_date, event_time) SETTINGS index_granularity = 1024</engine>
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-->
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<flush_interval_milliseconds>7500</flush_interval_milliseconds>
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</query_log>
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</clickhouse>
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```
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By default, table growth is unlimited. To control a size of a table, you can use [TTL](../../sql-reference/statements/alter/ttl.md#manipulations-with-table-ttl) settings for removing outdated log records. Also you can use the partitioning feature of `MergeTree`-engine tables.
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## Sources of System Metrics {#system-tables-sources-of-system-metrics}
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For collecting system metrics ClickHouse server uses:
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- `CAP_NET_ADMIN` capability.
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- [procfs](https://en.wikipedia.org/wiki/Procfs) (only in Linux).
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**procfs**
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If ClickHouse server does not have `CAP_NET_ADMIN` capability, it tries to fall back to `ProcfsMetricsProvider`. `ProcfsMetricsProvider` allows collecting per-query system metrics (for CPU and I/O).
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If procfs is supported and enabled on the system, ClickHouse server collects these metrics:
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- `OSCPUVirtualTimeMicroseconds`
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- `OSCPUWaitMicroseconds`
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- `OSIOWaitMicroseconds`
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- `OSReadChars`
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- `OSWriteChars`
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- `OSReadBytes`
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- `OSWriteBytes`
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## Related content
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- Blog: [System Tables and a window into the internals of ClickHouse](https://clickhouse.com/blog/clickhouse-debugging-issues-with-system-tables)
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- Blog: [Essential monitoring queries - part 1 - INSERT queries](https://clickhouse.com/blog/monitoring-troubleshooting-insert-queries-clickhouse)
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- Blog: [Essential monitoring queries - part 2 - SELECT queries](https://clickhouse.com/blog/monitoring-troubleshooting-select-queries-clickhouse)
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