ClickHouse/docs/en/operations/system_tables.md

# System tables

System tables are used for implementing part of the system's functionality, and for providing access to information about how the system is working.
You can't delete a system table (but you can perform DETACH).
System tables don't have files with data on the disk or files with metadata. The server creates all the system tables when it starts.
System tables are read-only.
They are located in the 'system' database.

## system.asynchronous_metrics {#system_tables-asynchronous_metrics}

Contains metrics which are calculated periodically in background. For example, the amount of RAM in use.

Columns:

- `metric` ([String](../data_types/string.md)) — Metric's name.
- `value` ([Float64](../data_types/float.md)) — Metric's value.

**Example**

```sql
SELECT * FROM system.asynchronous_metrics LIMIT 10
```

```text
┌─metric──────────────────────────────────┬──────value─┐
│ jemalloc.background_thread.run_interval │          0 │
│ jemalloc.background_thread.num_runs     │          0 │
│ jemalloc.background_thread.num_threads  │          0 │
│ jemalloc.retained                       │  422551552 │
│ jemalloc.mapped                         │ 1682989056 │
│ jemalloc.resident                       │ 1656446976 │
│ jemalloc.metadata_thp                   │          0 │
│ jemalloc.metadata                       │   10226856 │
│ UncompressedCacheCells                  │          0 │
│ MarkCacheFiles                          │          0 │
└─────────────────────────────────────────┴────────────┘
```

**See Also**

- [Monitoring](monitoring.md) — Base concepts of ClickHouse monitoring.
- [system.metrics](#system_tables-metrics) — Contains instantly calculated metrics.
- [system.events](#system_tables-events) — Contains a number of happened events.

## system.clusters

Contains information about clusters available in the config file and the servers in them.
Columns:

```
cluster String      — The cluster name.
shard_num UInt32 — The shard number in the cluster, starting from 1.
shard_weight UInt32 — The relative weight of the shard when writing data.
replica_num UInt32 — The replica number in the shard, starting from 1.
host_name String — The host name, as specified in the config.
String host_address — The host IP address obtained from DNS.
port UInt16 — The port to use for connecting to the server.
user String — The name of the user for connecting to the server.
```

## system.columns

Contains information about the columns in all the tables.

You can use this table to get information similar to the [DESCRIBE TABLE](../query_language/misc.md#misc-describe-table) query, but for multiple tables at once.

The `system.columns` table contains the following columns (the type of the corresponding column is shown in brackets):

- `database` (String) — Database name.
- `table` (String) — Table name.
- `name` (String) — Column name.
- `type` (String) — Column type.
- `default_kind` (String) — Expression type (`DEFAULT`, `MATERIALIZED`, `ALIAS`) for the default value, or an empty string if it is not defined.
- `default_expression` (String) — Expression for the default value, or an empty string if it is not defined.
- `data_compressed_bytes` (UInt64) — The size of compressed data, in bytes.
- `data_uncompressed_bytes` (UInt64) — The size of decompressed data, in bytes.
- `marks_bytes` (UInt64) — The size of marks, in bytes.
- `comment` (String) — The comment about column, or an empty string if it is not defined.
- `is_in_partition_key` (UInt8) — Flag that indicates whether the column is in partition expression.
- `is_in_sorting_key` (UInt8) — Flag that indicates whether the column is in sorting key expression.
- `is_in_primary_key` (UInt8) — Flag that indicates whether the column is in primary key expression.
- `is_in_sampling_key` (UInt8) — Flag that indicates whether the column is in sampling key expression.

## system.databases

This table contains a single String column called 'name' – the name of a database.
Each database that the server knows about has a corresponding entry in the table.
This system table is used for implementing the `SHOW DATABASES` query.

## system.detached_parts

Contains information about detached parts of
[MergeTree](table_engines/mergetree.md) tables. The `reason` column specifies
why the part was detached. For user-detached parts, the reason is empty. Such
parts can be attached with [ALTER TABLE ATTACH PARTITION|PART](../query_language/query_language/alter/#alter_attach-partition)
command. For the description of other columns, see [system.parts](#system_tables-parts).

## system.dictionaries

Contains information about external dictionaries.

Columns:

- `name String` — Dictionary name.
- `type String` — Dictionary type: Flat, Hashed, Cache.
- `origin String` — Path to the configuration file that describes the dictionary.
- `attribute.names Array(String)` — Array of attribute names provided by the dictionary.
- `attribute.types Array(String)` — Corresponding array of attribute types that are provided by the dictionary.
- `has_hierarchy UInt8` — Whether the dictionary is hierarchical.
- `bytes_allocated UInt64` — The amount of RAM the dictionary uses.
- `hit_rate Float64` — For cache dictionaries, the percentage of uses for which the value was in the cache.
- `element_count UInt64` — The number of items stored in the dictionary.
- `load_factor Float64` — The percentage full of the dictionary (for a hashed dictionary, the percentage filled in the hash table).
- `creation_time DateTime` — The time when the dictionary was created or last successfully reloaded.
- `last_exception String` — Text of the error that occurs when creating or reloading the dictionary if the dictionary couldn't be created.
- `source String` — Text describing the data source for the dictionary.

Note that the amount of memory used by the dictionary is not proportional to the number of items stored in it. So for flat and cached dictionaries, all the memory cells are pre-assigned, regardless of how full the dictionary actually is.

## system.events {#system_tables-events}

Contains information about the number of events that have occurred in the system. For example, in the table, you can find how many `SELECT` queries are processed from the moment of ClickHouse server start.

Columns:

- `event` ([String](../data_types/string.md)) — Event name.
- `value` ([UInt64](../data_types/int_uint.md)) — Count of events occurred.
- `description` ([String](../data_types/string.md)) — Description of an event.

**Example**

```sql
SELECT * FROM system.events LIMIT 5
```

```text
┌─event─────────────────────────────────┬─value─┬─description────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Query                                 │    12 │ Number of queries started to be interpreted and maybe executed. Does not include queries that are failed to parse, that are rejected due to AST size limits; rejected due to quota limits or limits on number of simultaneously running queries. May include internal queries initiated by ClickHouse itself. Does not count subqueries. │
│ SelectQuery                           │     8 │ Same as Query, but only for SELECT queries.                                                                                                                                                                                                                │
│ FileOpen                              │    73 │ Number of files opened.                                                                                                                                                                                                                                    │
│ ReadBufferFromFileDescriptorRead      │   155 │ Number of reads (read/pread) from a file descriptor. Does not include sockets.                                                                                                                                                                             │
│ ReadBufferFromFileDescriptorReadBytes │  9931 │ Number of bytes read from file descriptors. If the file is compressed, this will show compressed data size.                                                                                                                                                │
└───────────────────────────────────────┴───────┴────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
```

**See Also**

- [system.asynchronous_metrics](#system_tables-asynchronous_metrics) — Contains periodically calculated metrics.
- [system.metrics](#system_tables-metrics) — Contains instantly calculated metrics.
- [Monitoring](monitoring.md) — Base concepts of ClickHouse monitoring.

## system.functions

Contains information about normal and aggregate functions.

Columns:

- `name`(`String`) – The name of the function.
- `is_aggregate`(`UInt8`) — Whether the function is aggregate.

## system.graphite_retentions

Contains information about parameters [graphite_rollup](server_settings/settings.md#server_settings-graphite_rollup) which use in tables with [\*GraphiteMergeTree](table_engines/graphitemergetree.md) engines.

Columns:

- `config_name`     (String) - `graphite_rollup` parameter name.
- `regexp`          (String) - A pattern for the metric name.
- `function`        (String) - The name of the aggregating function.
- `age`             (UInt64) - The minimum age of the data in seconds.
- `precision`       (UInt64) - How precisely to define the age of the data in seconds.
- `priority`        (UInt16) - Pattern priority.
- `is_default`      (UInt8) - Is pattern default or not.
- `Tables.database` (Array(String)) - Array of databases names of tables, which use `config_name` parameter.
- `Tables.table`    (Array(String)) - Array of tables names, which use `config_name` parameter.


## system.merges

Contains information about merges and part mutations currently in process for tables in the MergeTree family.

Columns:

- `database String`  — The name of the database the table is in.
- `table String` — Table name.
- `elapsed Float64` — The time elapsed (in seconds) since the merge started.
- `progress Float64` — The percentage of completed work from 0 to 1.
- `num_parts UInt64` — The number of pieces to be merged.
- `result_part_name String` — The name of the part that will be formed as the result of merging.
- `is_mutation UInt8` - 1 if this process is a part mutation.
- `total_size_bytes_compressed UInt64` — The total size of the compressed data in the merged chunks.
- `total_size_marks UInt64` — The total number of marks in the merged parts.
- `bytes_read_uncompressed UInt64` — Number of bytes read, uncompressed.
- `rows_read UInt64` — Number of rows read.
- `bytes_written_uncompressed UInt64` — Number of bytes written, uncompressed.
- `rows_written UInt64` — Number of lines rows written.

## system.metrics {#system_tables-metrics}

Contains metrics which can be calculated instantly, or have an current value. For example, a number of simultaneously processed queries, the current value for replica delay. This table is always up to date.

Columns:

- `metric` ([String](../data_types/string.md)) — Metric's name.
- `value` ([Int64](../data_types/int_uint.md)) — Metric's value.
- `description` ([String](../data_types/string.md)) — Description of the metric.

**Example**

```sql
SELECT * FROM system.metrics LIMIT 10
```

```text
┌─metric─────────────────────┬─value─┬─description──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Query                      │     1 │ Number of executing queries                                                                                                                                                                      │
│ Merge                      │     0 │ Number of executing background merges                                                                                                                                                            │
│ PartMutation               │     0 │ Number of mutations (ALTER DELETE/UPDATE)                                                                                                                                                        │
│ ReplicatedFetch            │     0 │ Number of data parts fetching from replica                                                                                                                                                       │
│ ReplicatedSend             │     0 │ Number of data parts sending to replicas                                                                                                                                                         │
│ ReplicatedChecks           │     0 │ Number of data parts checking for consistency                                                                                                                                                    │
│ BackgroundPoolTask         │     0 │ Number of active tasks in BackgroundProcessingPool (merges, mutations, fetches or replication queue bookkeeping)                                                                                 │
│ BackgroundSchedulePoolTask │     0 │ Number of active tasks in BackgroundSchedulePool. This pool is used for periodic tasks of ReplicatedMergeTree like cleaning old data parts, altering data parts, replica re-initialization, etc. │
│ DiskSpaceReservedForMerge  │     0 │ Disk space reserved for currently running background merges. It is slightly more than total size of currently merging parts.                                                                     │
│ DistributedSend            │     0 │ Number of connections sending data, that was INSERTed to Distributed tables, to remote servers. Both synchronous and asynchronous mode.                                                          │
└────────────────────────────┴───────┴──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
```
**See Also**

- [system.asynchronous_metrics](#system_tables-asynchronous_metrics) — Contains periodically calculated metrics.
- [system.events](#system_tables-events) — Contains a umber of happened events.
- [Monitoring](monitoring.md) — Base concepts of ClickHouse monitoring.

## system.numbers

This table contains a single UInt64 column named 'number' that contains almost all the natural numbers starting from zero.
You can use this table for tests, or if you need to do a brute force search.
Reads from this table are not parallelized.

## system.numbers_mt

The same as 'system.numbers' but reads are parallelized. The numbers can be returned in any order.
Used for tests.

## system.one

This table contains a single row with a single 'dummy' UInt8 column containing the value 0.
This table is used if a SELECT query doesn't specify the FROM clause.
This is similar to the DUAL table found in other DBMSs.

## system.parts {#system_tables-parts}

Contains information about parts of [MergeTree](table_engines/mergetree.md) tables.

Each row describes one part of the data.

Columns:

- partition (String) – The partition name. To learn what a partition is, see the description of the [ALTER](../query_language/alter.md#query_language_queries_alter) query.

Formats:
- `YYYYMM` for automatic partitioning by month.
- `any_string` when partitioning manually.

- name (String) – Name of the data part.

- active (UInt8) – Indicates whether the part is active. If a part is active, it is used in a table; otherwise, it will be deleted. Inactive data parts remain after merging.

- marks (UInt64) – The number of marks. To get the approximate number of rows in a data part, multiply ``marks`` by the index granularity (usually 8192).

- marks_size (UInt64) – The size of the file with marks.

- rows (UInt64) – The number of rows.

- bytes (UInt64) – The number of bytes when compressed.

- modification_time (DateTime) – The modification time of the directory with the data part. This usually corresponds to the time of data part creation.|

- remove_time (DateTime) – The time when the data part became inactive.

- refcount (UInt32) – The number of places where the data part is used. A value greater than 2 indicates that the data part is used in queries or merges.

- min_date (Date) – The minimum value of the date key in the data part.

- max_date (Date) – The maximum value of the date key in the data part.

- min_block_number (UInt64) – The minimum number of data parts that make up the current part after merging.

- max_block_number (UInt64) – The maximum number of data parts that make up the current part after merging.

- level (UInt32) – Depth of the merge tree. If a merge was not performed, ``level=0``.

- primary_key_bytes_in_memory (UInt64) – The amount of memory (in bytes) used by primary key values.

- primary_key_bytes_in_memory_allocated (UInt64) – The amount of memory (in bytes) reserved for primary key values.

- database (String) – Name of the database.

- table (String) – Name of the table.

- engine (String) – Name of the table engine without parameters.

- is_frozen (UInt8) – Flag that shows partition data backup existence. 1, the backup exists. 0, the backup doesn't exist. For more details, see [FREEZE PARTITION](../query_language/alter.md#alter_freeze-partition)

## system.part_log {#system_tables-part-log}

The `system.part_log` table is created only if the [part_log](server_settings/settings.md#server_settings-part-log) server setting is specified.

This table contains information about the events that occurred with the [data parts](table_engines/custom_partitioning_key.md) in the [MergeTree](table_engines/mergetree.md) family tables. For instance, adding or merging data.

The `system.part_log` table contains the following columns:

- `event_type` (Enum) — Type of the event that occurred with the data part. Can have one of the following values: `NEW_PART` — inserting, `MERGE_PARTS` — merging, `DOWNLOAD_PART` — downloading, `REMOVE_PART` — removing or detaching using [DETACH PARTITION](../query_language/alter.md#alter_detach-partition), `MUTATE_PART` — updating.
- `event_date` (Date) — Event date.
- `event_time` (DateTime) — Event time.
- `duration_ms` (UInt64) — Duration.
- `database` (String) — Name of the database the data part is in.
- `table` (String) — Name of the table the data part is in.
- `part_name` (String) — Name of the data part.
- `partition_id` (String) — ID of the partition that the data part was inserted to. The column takes the 'all' value if the partitioning is by `tuple()`.
- `rows` (UInt64) — The number of rows in the data part.
- `size_in_bytes` (UInt64) — Size of the data part in bytes.
- `merged_from` (Array(String)) — An array of names of the parts which the current part was made up from (after the merge).
- `bytes_uncompressed` (UInt64) — Size of uncompressed bytes.
- `read_rows` (UInt64) — The number of rows was read during the merge.
- `read_bytes` (UInt64) — The number of bytes was read during the merge.
- `error` (UInt16) — The code number of the occurred error.
- `exception` (String) — Text message of the occurred error.

The `system.part_log` table is created after the first inserting data to the `MergeTree` table.

## system.processes

This system table is used for implementing the `SHOW PROCESSLIST` query.
Columns:

```
user String              – Name of the user who made the request. For distributed query processing, this is the user who helped the requestor server send the query to this server, not the user who made the distributed request on the requestor server.

address String           - The IP address the request was made from. The same for distributed processing.

elapsed Float64          - The time in seconds since request execution started.

rows_read UInt64         - The number of rows read from the table. For distributed processing, on the requestor server, this is the total for all remote servers.

bytes_read UInt64        - The number of uncompressed bytes read from the table. For distributed processing, on the requestor server, this is the total for all remote servers.

total_rows_approx UInt64 - The approximation of the total number of rows that should be read. For distributed processing, on the requestor server, this is the total for all remote servers. It can be updated during request processing, when new sources to process become known.

memory_usage UInt64      - How much memory the request uses. It might not include some types of dedicated memory.

query String             - The query text. For INSERT, it doesn't include the data to insert.

query_id String          - Query ID, if defined.
```

## system.query_log {#system_tables-query-log}

Contains information about queries execution. For each query, you can see processing start time, duration of processing, error message and other information.

!!! note
    The table doesn't contain input data for `INSERT` queries.

ClickHouse creates this table only if the [query_log](server_settings/settings.md#server_settings-query-log) server parameter is specified. This parameter sets the logging rules. For example, a logging interval or name of a table the queries will be logged in.

To enable query logging, set the parameter [log_queries](settings/settings.md#settings-log-queries) to 1. For details, see the [Settings](settings/settings.md) section.

The `system.query_log` table registers two kinds of queries:

1. Initial queries, that were run directly by the client.
2. Child queries that were initiated by other queries (for distributed query execution). For such a kind of queries, information about the parent queries is shown in the `initial_*` columns.

Columns:

- `type` (UInt8) — Type of event that occurred when executing the query. Possible values:
    - 1 — Successful start of query execution.
    - 2 — Successful end of query execution.
    - 3 — Exception before the start of query execution.
    - 4 — Exception during the query execution.
- `event_date` (Date) — Event date.
- `event_time` (DateTime) — Event time.
- `query_start_time` (DateTime) — Time of the query processing start.
- `query_duration_ms` (UInt64) — Duration of the query processing.
- `read_rows` (UInt64) — Number of read rows.
- `read_bytes` (UInt64) — Number of read bytes.
- `written_rows` (UInt64) — For `INSERT` queries, number of written rows. For other queries, the column value is 0.
- `written_bytes` (UInt64) — For `INSERT` queries, number of written bytes. For other queries, the column value is 0.
- `result_rows` (UInt64) — Number of rows in a result.
- `result_bytes` (UInt64) — Number of bytes in a result.
- `memory_usage` (UInt64) — Memory consumption by the query.
- `query` (String) — Query string.
- `exception` (String) — Exception message.
- `stack_trace` (String) — Stack trace (a list of methods called before the error occurred). An empty string, if the query is completed successfully.
- `is_initial_query` (UInt8) — Kind of query. Possible values:
    - 1 — Query was initiated by the client.
    - 0 — Query was initiated by another query for distributed query execution.
- `user` (String) — Name of the user initiated the current query.
- `query_id` (String) — ID of the query.
- `address` (FixedString(16)) — IP address the query was initiated from.
- `port` (UInt16) — A server port that was used to receive the query.
- `initial_user` (String) —  Name of the user who run the parent query (for distributed query execution).
- `initial_query_id` (String) — ID of the parent query.
- `initial_address` (FixedString(16)) — IP address that the parent query was launched from.
- `initial_port` (UInt16) — A server port that was used to receive the parent query from the client.
- `interface` (UInt8) — Interface that the query was initiated from. Possible values:
    - 1 — TCP.
    - 2 — HTTP.
- `os_user` (String) — User's OS.
- `client_hostname` (String) — Server name that the [clickhouse-client](../interfaces/cli.md) is connected to.
- `client_name` (String) — The [clickhouse-client](../interfaces/cli.md) name.
- `client_revision` (UInt32) — Revision of the [clickhouse-client](../interfaces/cli.md).
- `client_version_major` (UInt32) — Major version of the [clickhouse-client](../interfaces/cli.md).
- `client_version_minor` (UInt32) — Minor version of the [clickhouse-client](../interfaces/cli.md).
- `client_version_patch` (UInt32) — Patch component of the [clickhouse-client](../interfaces/cli.md) version.
- `http_method` (UInt8) — HTTP method initiated the query. Possible values:
    - 0 — The query was launched from the TCP interface.
    - 1 — `GET` method is used.
    - 2 — `POST` method is used.
- `http_user_agent` (String) — The `UserAgent` header passed in the HTTP request.
- `quota_key` (String) — The quota key specified in [quotas](quotas.md) setting.
- `revision` (UInt32) — ClickHouse revision.
- `thread_numbers` (Array(UInt32)) — Number of threads that are participating in query execution.
- `ProfileEvents.Names` (Array(String)) — Counters that measure the following metrics:
    - Time spent on reading and writing over the network.
    - Time spent on reading and writing to a disk.
    - Number of network errors.
    - Time spent on waiting when the network bandwidth is limited.
- `ProfileEvents.Values` (Array(UInt64)) — Values of metrics that are listed in the `ProfileEvents.Names` column.
- `Settings.Names` (Array(String)) — Names of settings that were changed when the client run a query. To enable logging of settings changing, set the `log_query_settings` parameter to 1.
- `Settings.Values` (Array(String)) — Values of settings that are listed in the `Settings.Names` column.

Each query creates one or two rows in the `query_log` table, depending on the status of the query:

1. If the query execution is successful, two events with types 1 and 2 are created (see the `type` column).
2. If the error occurred during the query processing, two events with types 1 and 4 are created.
3. If the error occurred before the query launching, a single event with type 3 is created.

By default, logs are added into the table at intervals of 7,5 seconds. You can set this interval in the [query_log](server_settings/settings.md#server_settings-query-log) server setting (see the `flush_interval_milliseconds` parameter). To flush the logs forcibly from the memory buffer into the table, use the `SYSTEM FLUSH LOGS` query.

When the table is deleted manually, it will be automatically created on the fly. Note that all the previous logs will be deleted.

!!! note
    The storage period for logs is unlimited; the logs aren't automatically deleted from the table. You need to organize the removing of non-actual logs yourself.

You can specify an arbitrary partitioning key for the `system.query_log` table in the [query_log](server_settings/settings.md#server_settings-query-log) server setting (see the `partition_by` parameter).

## system.replicas {#system_tables-replicas}

Contains information and status for replicated tables residing on the local server.
This table can be used for monitoring. The table contains a row for every Replicated\* table.

Example:

``` sql
SELECT *
FROM system.replicas
WHERE table = 'visits'
FORMAT Vertical
```

```
Row 1:
──────
database:           merge
table:              visits
engine:             ReplicatedCollapsingMergeTree
is_leader:          1
is_readonly:        0
is_session_expired: 0
future_parts:       1
parts_to_check:     0
zookeeper_path:     /clickhouse/tables/01-06/visits
replica_name:       example01-06-1.yandex.ru
replica_path:       /clickhouse/tables/01-06/visits/replicas/example01-06-1.yandex.ru
columns_version:    9
queue_size:         1
inserts_in_queue:   0
merges_in_queue:    1
log_max_index:      596273
log_pointer:        596274
total_replicas:     2
active_replicas:    2
```

Columns:

```
database:          Database name
table:              Table name
engine:            Table engine name

is_leader:          Whether the replica is the leader.

Only one replica at a time can be the leader. The leader is responsible for selecting background merges to perform.
Note that writes can be performed to any replica that is available and has a session in ZK, regardless of whether it is a leader.

is_readonly:        Whether the replica is in read-only mode.
This mode is turned on if the config doesn't have sections with ZooKeeper, if an unknown error occurred when reinitializing sessions in ZooKeeper, and during session reinitialization in ZooKeeper.

is_session_expired: Whether the session with ZooKeeper has expired.
Basically the same as 'is_readonly'.

future_parts:       The number of data parts that will appear as the result of INSERTs or merges that haven't been done yet.

parts_to_check:    The number of data parts in the queue for verification.
A part is put in the verification queue if there is suspicion that it might be damaged.

zookeeper_path:     Path to table data in ZooKeeper.
replica_name:       Replica name in ZooKeeper. Different replicas of the same table have different names.
replica_path:      Path to replica data in ZooKeeper. The same as concatenating 'zookeeper_path/replicas/replica_path'.

columns_version:    Version number of the table structure.
Indicates how many times ALTER was performed. If replicas have different versions, it means some replicas haven't made all of the ALTERs yet.

queue_size:         Size of the queue for operations waiting to be performed.
Operations include inserting blocks of data, merges, and certain other actions.
It usually coincides with 'future_parts'.

inserts_in_queue:   Number of inserts of blocks of data that need to be made.
Insertions are usually replicated fairly quickly. If this number is large, it means something is wrong.

merges_in_queue:    The number of merges waiting to be made.
Sometimes merges are lengthy, so this value may be greater than zero for a long time.

The next 4 columns have a non-zero value only where there is an active session with ZK.

log_max_index:      Maximum entry number in the log of general activity.
log_pointer:        Maximum entry number in the log of general activity that the replica copied to its execution queue, plus one.
If log_pointer is much smaller than log_max_index, something is wrong.

total_replicas:     The total number of known replicas of this table.
active_replicas:    The number of replicas of this table that have a session in ZooKeeper (i.e., the number of functioning replicas).
```

If you request all the columns, the table may work a bit slowly, since several reads from ZooKeeper are made for each row.
If you don't request the last 4 columns (log_max_index, log_pointer, total_replicas, active_replicas), the table works quickly.

For example, you can check that everything is working correctly like this:

``` sql
SELECT
    database,
    table,
    is_leader,
    is_readonly,
    is_session_expired,
    future_parts,
    parts_to_check,
    columns_version,
    queue_size,
    inserts_in_queue,
    merges_in_queue,
    log_max_index,
    log_pointer,
    total_replicas,
    active_replicas
FROM system.replicas
WHERE
       is_readonly
    OR is_session_expired
    OR future_parts > 20
    OR parts_to_check > 10
    OR queue_size > 20
    OR inserts_in_queue > 10
    OR log_max_index - log_pointer > 10
    OR total_replicas < 2
    OR active_replicas < total_replicas
```

If this query doesn't return anything, it means that everything is fine.

## system.settings

Contains information about settings that are currently in use.
I.e. used for executing the query you are using to read from the system.settings table.

Columns:

```
name String  — Setting name.
value String  — Setting value.
changed UInt8 — Whether the setting was explicitly defined in the config or explicitly changed.
```

Example:

``` sql
SELECT *
FROM system.settings
WHERE changed
```

```
┌─name───────────────────┬─value───────┬─changed─┐
│ max_threads            │ 8           │       1 │
│ use_uncompressed_cache │ 0           │       1 │
│ load_balancing         │ random      │       1 │
│ max_memory_usage       │ 10000000000 │       1 │
└────────────────────────┴─────────────┴─────────┘
```

## system.tables

Contains metadata of each table that the server knows about. Detached tables are not shown in `system.tables`.

This table contains the following columns (the type of the corresponding column is shown in brackets):

- `database` (String) — The name of database the table is in.
- `name` (String) — Table name.
- `engine` (String) — Table engine name (without parameters).
- `is_temporary` (UInt8) - Flag that indicates whether the table is temporary.
- `data_path` (String) - Path to the table data in the file system.
- `metadata_path` (String) - Path to the table metadata in the file system.
- `metadata_modification_time` (DateTime) - Time of latest modification of the table metadata.
- `dependencies_database` (Array(String)) - Database dependencies.
- `dependencies_table` (Array(String)) - Table dependencies ([MaterializedView](table_engines/materializedview.md) tables based on the current table).
- `create_table_query` (String) - The query that was used to create the table.
- `engine_full` (String) - Parameters of the table engine.
- `partition_key` (String) - The partition key expression specified in the table.
- `sorting_key` (String) - The sorting key expression specified in the table.
- `primary_key` (String) - The primary key expression specified in the table.
- `sampling_key` (String) - The sampling key expression specified in the table.

The `system.tables` is used in `SHOW TABLES` query implementation.

## system.zookeeper

The table does not exist if ZooKeeper is not configured. Allows reading data from the ZooKeeper cluster defined in the config.
The query must have a 'path' equality condition in the WHERE clause. This is the path in ZooKeeper for the children that you want to get data for.

The query `SELECT * FROM system.zookeeper WHERE path = '/clickhouse'` outputs data for all children on the `/clickhouse` node.
To output data for all root nodes, write path = '/'.
If the path specified in 'path' doesn't exist, an exception will be thrown.

Columns:

- `name String` — The name of the node.
- `path String` — The path to the node.
- `value String` — Node value.
- `dataLength Int32` — Size of the value.
- `numChildren Int32` — Number of descendants.
- `czxid Int64` — ID of the transaction that created the node.
- `mzxid Int64` — ID of the transaction that last changed the node.
- `pzxid Int64` — ID of the transaction that last deleted or added descendants.
- `ctime DateTime` — Time of node creation.
- `mtime DateTime` — Time of the last modification of the node.
- `version Int32` — Node version: the number of times the node was changed.
- `cversion Int32` — Number of added or removed descendants.
- `aversion Int32` — Number of changes to the ACL.
- `ephemeralOwner Int64` — For ephemeral nodes, the ID of hte session that owns this node.

Example:

``` sql
SELECT *
FROM system.zookeeper
WHERE path = '/clickhouse/tables/01-08/visits/replicas'
FORMAT Vertical
```

```
Row 1:
──────
name:           example01-08-1.yandex.ru
value:
czxid:          932998691229
mzxid:          932998691229
ctime:          2015-03-27 16:49:51
mtime:          2015-03-27 16:49:51
version:        0
cversion:       47
aversion:       0
ephemeralOwner: 0
dataLength:     0
numChildren:    7
pzxid:          987021031383
path:           /clickhouse/tables/01-08/visits/replicas

Row 2:
──────
name:           example01-08-2.yandex.ru
value:
czxid:          933002738135
mzxid:          933002738135
ctime:          2015-03-27 16:57:01
mtime:          2015-03-27 16:57:01
version:        0
cversion:       37
aversion:       0
ephemeralOwner: 0
dataLength:     0
numChildren:    7
pzxid:          987021252247
path:           /clickhouse/tables/01-08/visits/replicas
```

## system.mutations {#system_tables-mutations}

The table contains information about [mutations](../query_language/alter.md#alter-mutations) of MergeTree tables and their progress. Each mutation command is represented by a single row. The table has the following columns:

**database**, **table** - The name of the database and table to which the mutation was applied.

**mutation_id** - The ID of the mutation. For replicated tables these IDs correspond to znode names in the `<table_path_in_zookeeper>/mutations/` directory in ZooKeeper. For unreplicated tables the IDs correspond to file names in the data directory of the table.

**command** - The mutation command string (the part of the query after `ALTER TABLE [db.]table`).

**create_time** - When this mutation command was submitted for execution.

**block_numbers.partition_id**, **block_numbers.number** - A Nested column. For mutations of replicated tables contains one record for each partition: the partition ID and the block number that was acquired by the mutation (in each partition only parts that contain blocks with numbers less than the block number acquired by the mutation in that partition will be mutated). Because in non-replicated tables blocks numbers in all partitions form a single sequence, for mutatations of non-replicated tables the column will contain one record with a single block number acquired by the mutation.

**parts_to_do** - The number of data parts that need to be mutated for the mutation to finish.

**is_done** - Is the mutation done? Note that even if `parts_to_do = 0` it is possible that a mutation of a replicated table is not done yet because of a long-running INSERT that will create a new data part that will need to be mutated.

If there were problems with mutating some parts the following columns contain additional information:

**latest_failed_part** - The name of the most recent part that could not be mutated.

**latest_fail_time** - The time of the most recent part mutation failure.

**latest_fail_reason** - The exception message that caused the most recent part mutation failure.

[Original article](https://clickhouse.yandex/docs/en/operations/system_tables/) <!--hide-->