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

Contain metrics used for profiling and monitoring. They usually reflect the number of events currently in the system, or the total resources consumed by the system. Example: The number of SELECT queries currently running; the amount of memory in use.system.asynchronous_metricsandsystem.metrics differ in their sets of metrics and how they are calculated.

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 tables. You can use this table to get information similar to DESCRIBE TABLE, but for multiple tables at once.

database String — The name of the database the table is in.
table String – Table name.
name String — Column name.
type String — Column type.
default_type 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.

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.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

Contains information about the number of events that have occurred in the system. This is used for profiling and monitoring purposes. Example: The number of processed SELECT queries. Columns: 'event String' – the event name, and 'value UInt64' – the quantity.

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.merges

Contains information about merges 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.
• 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 partss.
• 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.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

Contains information about parts of MergeTree 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.

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.

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.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:

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:

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:

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

This table contains the String columns 'database', 'name', and 'engine'. The table also contains three virtual columns: metadata_modification_time (DateTime type), create_table_query, and engine_full (String type). Each table that the server knows about is entered in the 'system.tables' table. This system table is used for implementing SHOW TABLES queries.

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:

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

Original article