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110 lines
3.5 KiB
Markdown
110 lines
3.5 KiB
Markdown
---
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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 [EVENTS] [LIMIT n] [FORMAT format]
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```
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## Virtual columns {#watch-virtual-columns}
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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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**Example:**
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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 specifies 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 result 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.md#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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