mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-09-21 17:20:50 +00:00
111 lines
3.5 KiB
Markdown
111 lines
3.5 KiB
Markdown
---
|
|
slug: /en/sql-reference/statements/watch
|
|
sidebar_position: 53
|
|
sidebar_label: WATCH
|
|
---
|
|
|
|
# WATCH Statement (Experimental)
|
|
|
|
:::note
|
|
This is an experimental feature that may change in backwards-incompatible ways in the future releases. Enable live views and `WATCH` query using `set allow_experimental_live_view = 1`.
|
|
:::
|
|
|
|
``` sql
|
|
WATCH [db.]live_view
|
|
[EVENTS]
|
|
[LIMIT n]
|
|
[FORMAT format]
|
|
```
|
|
|
|
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).
|
|
|
|
```sql
|
|
WATCH [db.]live_view [EVENTS] [LIMIT n] [FORMAT format]
|
|
```
|
|
|
|
## Virtual columns
|
|
|
|
The virtual `_version` column in the query result indicates the current result version.
|
|
|
|
**Example:**
|
|
|
|
```sql
|
|
CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
|
|
WATCH lv;
|
|
```
|
|
|
|
```bash
|
|
┌───────────────now()─┬─_version─┐
|
|
│ 2021-02-21 09:17:21 │ 1 │
|
|
└─────────────────────┴──────────┘
|
|
┌───────────────now()─┬─_version─┐
|
|
│ 2021-02-21 09:17:26 │ 2 │
|
|
└─────────────────────┴──────────┘
|
|
┌───────────────now()─┬─_version─┐
|
|
│ 2021-02-21 09:17:31 │ 3 │
|
|
└─────────────────────┴──────────┘
|
|
...
|
|
```
|
|
|
|
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.
|
|
|
|
**Example:**
|
|
|
|
```sql
|
|
INSERT INTO [db.]table WATCH [db.]live_view ...
|
|
```
|
|
|
|
## EVENTS Clause
|
|
|
|
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.
|
|
|
|
```sql
|
|
WATCH [db.]live_view EVENTS;
|
|
```
|
|
|
|
**Example:**
|
|
|
|
```sql
|
|
CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
|
|
WATCH lv EVENTS;
|
|
```
|
|
|
|
```bash
|
|
┌─version─┐
|
|
│ 1 │
|
|
└─────────┘
|
|
┌─version─┐
|
|
│ 2 │
|
|
└─────────┘
|
|
...
|
|
```
|
|
|
|
## LIMIT Clause
|
|
|
|
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.
|
|
|
|
```sql
|
|
WATCH [db.]live_view LIMIT 1;
|
|
```
|
|
|
|
**Example:**
|
|
|
|
```sql
|
|
CREATE LIVE VIEW lv WITH REFRESH 5 AS SELECT now();
|
|
WATCH lv EVENTS LIMIT 1;
|
|
```
|
|
|
|
```bash
|
|
┌─version─┐
|
|
│ 1 │
|
|
└─────────┘
|
|
```
|
|
|
|
## FORMAT Clause
|
|
|
|
The `FORMAT` clause works the same way as for the [SELECT](../../sql-reference/statements/select/format.md#format-clause).
|
|
|
|
:::note
|
|
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.
|
|
:::
|