KILL Statements

There are two kinds of kill statements: to kill a query and to kill a mutation

KILL QUERY

KILL QUERY [ON CLUSTER cluster]
  WHERE <where expression to SELECT FROM system.processes query>
  [SYNC|ASYNC|TEST]
  [FORMAT format]

Attempts to forcibly terminate the currently running queries. The queries to terminate are selected from the system.processes table using the criteria defined in the WHERE clause of the KILL query.

Examples:

First, you'll need to get the list of incomplete queries. This SQL query provides them according to those running the longest:

List from a single ClickHouse node:

SELECT
  initial_query_id,
  query_id,
  formatReadableTimeDelta(elapsed) AS time_delta,
  query,
  *
  FROM system.processes
  WHERE query ILIKE 'SELECT%'
  ORDER BY time_delta DESC;

List from a ClickHouse cluster:

SELECT
  initial_query_id,
  query_id,
  formatReadableTimeDelta(elapsed) AS time_delta,
  query,
  *
  FROM clusterAllReplicas(default, system.processes)
  WHERE query ILIKE 'SELECT%'
  ORDER BY time_delta DESC;

Kill the query:

-- Forcibly terminates all queries with the specified query_id:
KILL QUERY WHERE query_id='2-857d-4a57-9ee0-327da5d60a90'

-- Synchronously terminates all queries run by 'username':
KILL QUERY WHERE user='username' SYNC

If you are killing a query in ClickHouse Cloud or in a self-managed cluster, then be sure to use the ON CLUSTER [cluster-name] option, in order to ensure the query is killed on all replicas:::

Read-only users can only stop their own queries.

By default, the asynchronous version of queries is used (ASYNC), which does not wait for confirmation that queries have stopped.

The synchronous version (SYNC) waits for all queries to stop and displays information about each process as it stops. The response contains the kill_status column, which can take the following values:

1. finished – The query was terminated successfully.
2. waiting – Waiting for the query to end after sending it a signal to terminate.
3. The other values ​​explain why the query can’t be stopped.

A test query (TEST) only checks the user’s rights and displays a list of queries to stop.

KILL MUTATION

The presence of long-running or incomplete mutations often indicates that a ClickHouse service is running poorly. The asynchronous nature of mutations can cause them to consume all available resources on a system. You may need to either:

• Pause all new mutations, INSERTs , and SELECTs and allow the queue of mutations to complete.
• Or manually kill some of these mutations by sending a KILL command.

KILL MUTATION [ON CLUSTER cluster]
  WHERE <where expression to SELECT FROM system.mutations query>
  [TEST]
  [FORMAT format]

Tries to cancel and remove mutations that are currently executing. Mutations to cancel are selected from the system.mutations table using the filter specified by the WHERE clause of the KILL query.

A test query (TEST) only checks the user’s rights and displays a list of mutations to stop.

Examples:

Get a count() of the number of incomplete mutations:

Count of mutations from a single ClickHouse node:

SELECT count(*)
FROM system.mutations
WHERE is_done = 0;

Count of mutations from a ClickHouse cluster of replicas:

SELECT count(*)
FROM clusterAllReplicas('default', system.mutations)
WHERE is_done = 0;

Query the list of incomplete mutations:

List of mutations from a single ClickHouse node:

SELECT mutation_id, *
FROM system.mutations
WHERE is_done = 0;

List of mutations from a ClickHouse cluster:

SELECT mutation_id, *
FROM clusterAllReplicas('default', system.mutations)
WHERE is_done = 0;

Kill the mutations as needed:

-- Cancel and remove all mutations of the single table:
KILL MUTATION WHERE database = 'default' AND table = 'table'

-- Cancel the specific mutation:
KILL MUTATION WHERE database = 'default' AND table = 'table' AND mutation_id = 'mutation_3.txt'

The query is useful when a mutation is stuck and cannot finish (e.g. if some function in the mutation query throws an exception when applied to the data contained in the table).

Changes already made by the mutation are not rolled back.