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Merge pull request #20217 from vzakaznikov/live-view-docs
Adding documentation for LIVE VIEWs
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@ -41,7 +41,6 @@ SELECT a, b, c FROM (SELECT ...)
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CREATE MATERIALIZED VIEW [IF NOT EXISTS] [db.]table_name [ON CLUSTER] [TO[db.]name] [ENGINE = engine] [POPULATE] AS SELECT ...
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```
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Materialized views store data transformed by the corresponding [SELECT](../../../sql-reference/statements/select/index.md) query.
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When creating a materialized view without `TO [db].[table]`, you must specify `ENGINE` – the table engine for storing data.
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@ -65,4 +64,191 @@ Views look the same as normal tables. For example, they are listed in the result
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There isn’t a separate query for deleting views. To delete a view, use [DROP TABLE](../../../sql-reference/statements/drop.md).
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## Live View (Experimental) {#live-view}
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!!! important "Important"
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This is an experimental feature that may change in backwards-incompatible ways in the future releases.
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Enable usage of live views and `WATCH` query using `set allow_experimental_live_view = 1`.
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```sql
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CREATE LIVE VIEW [IF NOT EXISTS] [db.]table_name [WITH [TIMEOUT [value_in_sec] [AND]] [REFRESH [value_in_sec]]] AS SELECT ...
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```
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Live views store result of the corresponding [SELECT](../../../sql-reference/statements/select/index.md) query and are updated any time the result of the query changes. Query result as well as partial result needed to combine with new data are stored in memory providing increased performance for repeated queries. Live views can provide push notifications when query result changes using the [WATCH](../../../sql-reference/statements/watch.md) query.
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Live views are triggered by insert into the innermost table specified in the query.
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Live views work similarly to how a query in a distributed table works. But instead of combining partial results from different servers they combine partial result from current data with partial result from the new data. When a live view query includes a subquery then the cached partial result is only stored for the innermost subquery.
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!!! info "Limitations"
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- [Table function](../../../sql-reference/table-functions/index.md) is not supported as the innermost table.
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- Tables that do not have inserts such as a [dictionary](../../../sql-reference/dictionaries/index.md), [system table](../../../operations/system-tables/index.md), a [normal view](#normal), or a [materialized view](#materialized) will not trigger a live view.
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- Only queries where one can combine partial result from the old data plus partial result from the new data will work. Live view will not work for queries that require the complete data set to compute the final result or aggregations where the state of the aggregation must be preserved.
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- Does not work with replicated or distributed tables where inserts are performed on different nodes.
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- Can't be triggered by multiple tables.
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See [WITH REFRESH](#live-view-with-refresh) to force periodic updates of a live view that in some cases can be used as a workaround.
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You can watch for changes in the live view query result using the [WATCH](../../../sql-reference/statements/watch.md) query
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```sql
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WATCH [db.]live_view
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```
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**Example:**
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```sql
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CREATE TABLE mt (x Int8) Engine = MergeTree ORDER BY x;
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CREATE LIVE VIEW lv AS SELECT sum(x) FROM mt;
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```
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Watch a live view while doing a parallel insert into the source table.
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```sql
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WATCH lv
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```
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```bash
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┌─sum(x)─┬─_version─┐
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│ 1 │ 1 │
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└────────┴──────────┘
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┌─sum(x)─┬─_version─┐
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│ 2 │ 2 │
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└────────┴──────────┘
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┌─sum(x)─┬─_version─┐
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│ 6 │ 3 │
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└────────┴──────────┘
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...
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```
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```sql
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INSERT INTO mt VALUES (1);
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INSERT INTO mt VALUES (2);
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INSERT INTO mt VALUES (3);
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```
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or add [EVENTS](../../../sql-reference/statements/watch.md#events-clause) clause to just get change events.
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```sql
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WATCH [db.]live_view EVENTS
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```
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**Example:**
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```sql
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WATCH lv EVENTS
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```
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```bash
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┌─version─┐
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│ 1 │
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└─────────┘
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┌─version─┐
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│ 2 │
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└─────────┘
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┌─version─┐
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│ 3 │
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└─────────┘
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...
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```
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You can execute [SELECT](../../../sql-reference/statements/select/index.md) query on a live view in the same way as for any regular view or a table. If the query result is cached it will return the result immediately without running the stored query on the underlying tables.
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```sql
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SELECT * FROM [db.]live_view WHERE ...
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```
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### Force Refresh {#live-view-alter-refresh}
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You can force live view refresh using the `ALTER LIVE VIEW [db.]table_name REFRESH` statement.
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### With Timeout {#live-view-with-timeout}
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When a live view is create with a `WITH TIMEOUT` clause then the live view will be dropped automatically after the specified number of seconds elapse since the end of the last [WATCH](../../../sql-reference/statements/watch.md) query that was watching the live view.
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```sql
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CREATE LIVE VIEW [db.]table_name WITH TIMEOUT [value_in_sec] AS SELECT ...
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```
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If the timeout value is not specified then the value specified by the `temporary_live_view_timeout` setting is used.
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**Example:**
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```sql
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CREATE TABLE mt (x Int8) Engine = MergeTree ORDER BY x;
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CREATE LIVE VIEW lv WITH TIMEOUT 15 AS SELECT sum(x) FROM mt;
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```
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### With Refresh {#live-view-with-refresh}
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When a live view is created with a `WITH REFRESH` clause then it will be automatically refreshed after the specified number of seconds elapse since the last refresh or trigger.
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```sql
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CREATE LIVE VIEW [db.]table_name WITH REFRESH [value_in_sec] AS SELECT ...
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```
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If the refresh value is not specified then the value specified by the `periodic_live_view_refresh` setting is used.
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**Example:**
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```sql
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CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
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WATCH lv
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```
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```bash
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┌───────────────now()─┬─_version─┐
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│ 2021-02-21 08:47:05 │ 1 │
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└─────────────────────┴──────────┘
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┌───────────────now()─┬─_version─┐
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│ 2021-02-21 08:47:10 │ 2 │
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└─────────────────────┴──────────┘
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┌───────────────now()─┬─_version─┐
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│ 2021-02-21 08:47:15 │ 3 │
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└─────────────────────┴──────────┘
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```
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You can combine `WITH TIMEOUT` and `WITH REFRESH` clauses using an `AND` clause.
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```sql
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CREATE LIVE VIEW [db.]table_name WITH TIMEOUT [value_in_sec] AND REFRESH [value_in_sec] AS SELECT ...
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```
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**Example:**
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```sql
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CREATE LIVE VIEW lv WITH TIMEOUT 15 AND REFRESH 5 AS SELECT now();
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```
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After 15 sec the live view will be automatically dropped if there are no active `WATCH` queries.
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```sql
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WATCH lv
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```
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```
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Code: 60. DB::Exception: Received from localhost:9000. DB::Exception: Table default.lv doesn't exist..
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```
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### Usage
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Most common uses of live view tables include:
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- Providing push notifications for query result changes to avoid polling.
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- Caching results of most frequent queries to provide immediate query results.
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- Watching for table changes and triggering a follow-up select queries.
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- Watching metrics from system tables using periodic refresh.
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### Settings {#live-view-settings}
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You can use the following settings to control the behaviour of live views.
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- `allow_experimental_live_view` - enable live views. Default is `0`.
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- `live_view_heartbeat_interval` - the heartbeat interval in seconds to indicate live query is alive. Default is `15` seconds.
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- `max_live_view_insert_blocks_before_refresh` - maximum number of inserted blocks after which
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mergeable blocks are dropped and query is re-executed. Default is `64` inserts.
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- `temporary_live_view_timeout` - interval after which live view with timeout is deleted. Default is `5` seconds.
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- `periodic_live_view_refresh` - interval after which periodically refreshed live view is forced to refresh. Default is `60` seconds.
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[Original article](https://clickhouse.tech/docs/en/sql-reference/statements/create/view/) <!--hide-->
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106
docs/en/sql-reference/statements/watch.md
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106
docs/en/sql-reference/statements/watch.md
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@ -0,0 +1,106 @@
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---
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toc_priority: 53
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toc_title: WATCH
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---
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# WATCH Statement (Experimental) {#watch}
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!!! important "Important"
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This is an experimental feature that may change in backwards-incompatible ways in the future releases.
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Enable live views and `WATCH` query using `set allow_experimental_live_view = 1`.
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``` sql
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WATCH [db.]live_view
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[EVENTS]
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[LIMIT n]
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[FORMAT format]
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```
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The `WATCH` query performs continuous data retrieval from a [live view](./create/view.md#live-view) table. Unless the `LIMIT` clause is specified it provides an infinite stream of query results from a [live view](./create/view.md#live-view).
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```sql
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WATCH [db.]live_view
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```
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The virtual `_version` column in the query result indicates the current result version.
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**Example:**
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```sql
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CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
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WATCH lv
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```
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```bash
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┌───────────────now()─┬─_version─┐
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│ 2021-02-21 09:17:21 │ 1 │
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└─────────────────────┴──────────┘
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┌───────────────now()─┬─_version─┐
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│ 2021-02-21 09:17:26 │ 2 │
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└─────────────────────┴──────────┘
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┌───────────────now()─┬─_version─┐
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│ 2021-02-21 09:17:31 │ 3 │
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└─────────────────────┴──────────┘
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...
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```
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By default, the requested data is returned to the client, while in conjunction with [INSERT INTO](../../sql-reference/statements/insert-into.md) it can be forwarded to a different table.
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```sql
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INSERT INTO [db.]table WATCH [db.]live_view ...
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```
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## EVENTS Clause {#events-clause}
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The `EVENTS` clause can be used to obtain a short form of the `WATCH` query where instead of the query result you will just get the latest query result version.
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```sql
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WATCH [db.]live_view EVENTS
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```
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**Example:**
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```sql
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CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
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WATCH lv EVENTS
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```
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```bash
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┌─version─┐
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│ 1 │
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└─────────┘
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┌─version─┐
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│ 2 │
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└─────────┘
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...
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```
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## LIMIT Clause {#limit-clause}
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The `LIMIT n` clause species the number of updates the `WATCH` query should wait for before terminating. By default there is no limit on the number of updates and therefore the query will not terminate. The value of `0` indicates that the `WATCH` query should not wait for any new query results and therefore will return immediately once query is evaluated.
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```sql
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WATCH [db.]live_view LIMIT 1
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```
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**Example:**
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```sql
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CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
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WATCH lv EVENTS LIMIT 1
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```
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```bash
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┌─version─┐
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│ 1 │
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└─────────┘
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```
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## FORMAT Clause {#format-clause}
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The `FORMAT` clause works the same way as for the [SELECT](../../sql-reference/statements/select/format.md#format-clause).
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!!! info "Note"
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The [JSONEachRowWithProgress](../../../interfaces/formats/#jsoneachrowwithprogress) format should be used when watching [live view](./create/view.md#live-view) tables over the HTTP interface. The progress messages will be added to the output to keep the long-lived HTTP connection alive until the query result changes. The interval between progress messages is controlled using the [live_view_heartbeat_interval](./create/view.md#live-view-settings) setting.
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