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Deprecate LIVE VIEW
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Alexey Milovidov
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41f126eba0
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0d1bdff347
@ -90,152 +90,11 @@ Views look the same as normal tables. For example, they are listed in the result
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To delete a view, use [DROP VIEW](../../../sql-reference/statements/drop.md#drop-view). Although `DROP TABLE` works for VIEWs as well.
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## Live View [Experimental]
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## Live View [Deprecated]
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:::note
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This is an experimental feature that may change in backwards-incompatible ways in the future releases. Enable usage of live views and `WATCH` query using [allow_experimental_live_view](../../../operations/settings/settings.md#allow-experimental-live-view) setting. Input the command `set allow_experimental_live_view = 1`.
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:::
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This feature is deprecated and will be removed in the future.
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```sql
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CREATE LIVE VIEW [IF NOT EXISTS] [db.]table_name [WITH 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
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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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:::
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### Monitoring Live View Changes
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You can monitor changes in the `LIVE VIEW` query result using [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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│ 3 │ 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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```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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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 Live View 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 REFRESH Clause
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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](../../../operations/settings/settings.md#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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```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 does not exist..
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```
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### Live View 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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**See Also**
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- [ALTER LIVE VIEW](../alter/view.md#alter-live-view)
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For your convenience, the old documentation is located [here](https://pastila.nl/?00f32652/fdf07272a7b54bda7e13b919264e449f.md)
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## Window View [Experimental]
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