> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-vortex-format.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

> Documentation for ALTER

# ALTER

Most `ALTER TABLE` queries modify table settings or data:

| Modifier                                                             |
| -------------------------------------------------------------------- |
| [COLUMN](/reference/statements/alter/column)                         |
| [PARTITION](/reference/statements/alter/partition)                   |
| [DELETE](/reference/statements/alter/delete)                         |
| [UPDATE](/reference/statements/alter/update)                         |
| [ORDER BY](/reference/statements/alter/order-by)                     |
| [SAMPLE BY](/reference/statements/alter/sample-by)                   |
| [INDEX](/reference/statements/alter/skipping-index)                  |
| [PROJECTION](/reference/statements/alter/projection)                 |
| [CONSTRAINT](/reference/statements/alter/constraint)                 |
| [TTL](/reference/statements/alter/ttl)                               |
| [STATISTICS](/reference/statements/alter/statistics)                 |
| [SETTING](/reference/statements/alter/setting)                       |
| [APPLY DELETED MASK](/reference/statements/alter/apply-deleted-mask) |
| [APPLY PATCHES](/reference/statements/alter/apply-patches)           |

<Note>
  Most `ALTER TABLE` queries are supported only for [\*MergeTree](/reference/engines/table-engines/mergetree-family/index), [Merge](/reference/engines/table-engines/special/merge) and [Distributed](/reference/engines/table-engines/special/distributed) tables.
</Note>

These `ALTER` statements manipulate views:

| Statement                                                        | Description                                                                  |
| ---------------------------------------------------------------- | ---------------------------------------------------------------------------- |
| [ALTER TABLE ... MODIFY QUERY](/reference/statements/alter/view) | Modifies a [Materialized view](/reference/statements/create/view) structure. |

These `ALTER` statements modify entities related to role-based access control:

| Statement                                                        |
| ---------------------------------------------------------------- |
| [USER](/reference/statements/alter/user)                         |
| [ROLE](/reference/statements/alter/role)                         |
| [QUOTA](/reference/statements/alter/quota)                       |
| [ROW POLICY](/reference/statements/alter/row-policy)             |
| [MASKING POLICY](/reference/statements/alter/masking-policy)     |
| [SETTINGS PROFILE](/reference/statements/alter/settings-profile) |

| Statement                                                                         | Description                                                                                     |
| --------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------- |
| [ALTER TABLE ... MODIFY COMMENT](/reference/statements/alter/comment)             | Adds, modifies, or removes comments to the table, regardless if it was set before or not.       |
| [ALTER DATABASE ... MODIFY COMMENT](/reference/statements/alter/database-comment) | Adds, modifies, or removes comments to the database, regardless if it was set before or not.    |
| [ALTER NAMED COLLECTION](/reference/statements/alter/named-collection)            | Modifies [Named Collections](/concepts/features/configuration/server-config/named-collections). |

<h2 id="mutations">
  Mutations
</h2>

`ALTER` queries that are intended to manipulate table data are implemented with a mechanism called "mutations", most notably [ALTER TABLE ... DELETE](/reference/statements/alter/delete) and [ALTER TABLE ... UPDATE](/reference/statements/alter/update). They are asynchronous background processes similar to merges in [MergeTree](/reference/engines/table-engines/mergetree-family/index) tables that to produce new "mutated" versions of parts.

For `*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`](/reference/system-tables/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`](/reference/statements/kill#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.

<h2 id="synchronicity-of-alter-queries">
  Synchronicity of ALTER Queries
</h2>

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.

For `ALTER` queries that creates mutations (e.g.: including, but not limited to `UPDATE`, `DELETE`, `MATERIALIZE INDEX`, `MATERIALIZE PROJECTION`, `MATERIALIZE COLUMN`, `APPLY DELETED MASK`, `APPLY PATCHES`, `CLEAR STATISTIC`, `MATERIALIZE STATISTIC`) the synchronicity is defined by the [mutations\_sync](/reference/settings/session-settings/mutations#mutations_sync) setting.

For other `ALTER` queries which only modify the metadata, you can use the [alter\_sync](/reference/settings/session-settings/alter#alter_sync) setting to set up waiting.

You can specify how long (in seconds) to wait for inactive replicas to execute all `ALTER` queries with the [replication\_wait\_for\_inactive\_replica\_timeout](/reference/settings/session-settings/other#replication_wait_for_inactive_replica_timeout) setting.

<Note>
  For all `ALTER` queries, if `alter_sync = 2` and some replicas are not active for more than the time, specified in the `replication_wait_for_inactive_replica_timeout` setting, then an exception `UNFINISHED` is thrown.
</Note>

<h3 id="concurrent-alter-assignment-on-one-table">
  Concurrent `ALTER` assignment on one table
</h3>

On replicated tables, submitting several separate `ALTER` statements against the same table in quick succession can fail with `CANNOT_ASSIGN_ALTER` (code 517). The replicated path raises this when the replica has not yet applied some previous `ALTER`s (metadata version still behind the common metadata — the server may say the replica "still not applied some of previous alters" or "Probably too many alters executing concurrently"). That condition can remain true even after an earlier `ALTER` has already been assigned. This is a **general concurrent metadata-`ALTER` / mutation** condition — it is not limited to mutation-only statements. Ordinary concurrent metadata alters (`ADD` / `DROP` / `MODIFY`, and similar) can raise the same retryable code (see for example the retry path covered by `tests/queries/0_stateless/03518_alter_logical_race.sh`).

Approaches that avoid the race:

* Combine independent metadata operations into a **single** multi-clause `ALTER` when the grammar allows it (for example multiple `ADD INDEX` clauses).
* Serialize `ALTER` statements and retry on code 517 until previous `ALTER`s have been applied on the replica.
* For mutation-producing `ALTER`s, wait for the previous mutation to finish using a documented observable such as [`mutations_sync`](/reference/settings/session-settings/mutations#mutations_sync) or `is_done` in [`system.mutations`](/reference/system-tables/mutations) before submitting the next one.

<h3 id="combining-materialize-index-clauses">
  Combining `MATERIALIZE INDEX` clauses
</h3>

Multiple `MATERIALIZE INDEX` clauses can appear in one `ALTER`. The covered case in-tree is packing several `ADD INDEX` clauses together with `MATERIALIZE INDEX` for those same new indexes in a single statement (see `tests/queries/0_stateless/02911_add_index_and_materialize_index.sql`). That packed `ADD INDEX` + `MATERIALIZE INDEX` form mixes an `AlterCommand` segment with a `MutationCommand` segment, so **`DatabaseReplicated` rejects it** with `QUERY_IS_PROHIBITED` (`InterpreterAlterQuery::validateReplicatedDatabaseSegments`). Treat the `02911` example as valid for ordinary (non-`DatabaseReplicated`) databases; on `DatabaseReplicated`, keep metadata changes and materialize mutations in separate statements.

In the current implementation, each `MATERIALIZE INDEX` clause is resolved against the table metadata snapshot when the mutation is prepared, so materialize-only multi-clause forms on already-existing indexes follow the same preparation path (mutation-only, so they stay within one segment). That exact shape is not yet covered by a focused stateless test; treat it as current implementation behavior rather than a separately guaranteed contract until such coverage exists.

If you need ordered mutation apply, you can still issue one `MATERIALIZE INDEX` per statement and wait with [`mutations_sync`](/reference/settings/session-settings/mutations#mutations_sync).

<h2 id="related-content">
  Related content
</h2>

* Blog: [Handling Updates and Deletes in ClickHouse](https://clickhouse.com/blog/handling-updates-and-deletes-in-clickhouse)