ALTER queries modify table settings or data:
ALTER settings modify entities related to role-based access control:
ALTER queries that are intended to manipulate table data are implemented with a mechanism called “mutations”, most notably ALTER TABLE … DELETE and ALTER TABLE … UPDATE. They are asynchronous background processes similar to merges in MergeTree tables that to produce new “mutated” versions of parts.
*MergeTree tables mutations execute by rewriting whole data parts. There is no atomicity - parts are substituted for mutated parts as soon as they are ready and a
SELECT query that started executing during a mutation will see data from parts that have already been mutated along with data from parts that have not been mutated yet.
Mutations are totally ordered by their creation order and are applied to each part in that order. Mutations are also partially ordered with
INSERT INTO queries: data that was inserted into the table before the mutation was submitted will be mutated and data that was inserted after that will not be mutated. Note that mutations do not block inserts in any way.
A mutation query returns immediately after the mutation entry is added (in case of replicated tables to ZooKeeper, for non-replicated tables - to the filesystem). The mutation itself executes asynchronously using the system profile settings. To track the progress of mutations you can use the
system.mutations table. A mutation that was successfully submitted will continue to execute even if ClickHouse servers are restarted. There is no way to roll back the mutation once it is submitted, but if the mutation is stuck for some reason it can be cancelled with the
KILL MUTATION query.
Entries for finished mutations are not deleted right away (the number of preserved entries is determined by the
finished_mutations_to_keep storage engine parameter). Older mutation entries are deleted.
For non-replicated tables, all
ALTER queries are performed synchronously. For replicated tables, the query just adds instructions for the appropriate actions to
ZooKeeper, and the actions themselves are performed as soon as possible. However, the query can wait for these actions to be completed on all the replicas.
ALTER ... ATTACH|DETACH|DROP queries, you can use the
replication_alter_partitions_sync setting to set up waiting. Possible values:
0 – do not wait;
1 – only wait for own execution (default);
2 – wait for all.
ALTER TABLE ... UPDATE|DELETE queries the synchronicity is defined by the mutations_sync setting.