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AT

The SELECT statement can include an AT clause that allows you to query previous versions of your data by a specific snapshot ID or timestamp.

Databend automatically creates snapshots when data updates occur, so a snapshot can be considered as a view of your data at a time point in the past. You can access a snapshot by the snapshot ID or the timestamp at which the snapshot was created. For how to obtain the snapshot ID and timestamp, see Obtaining Snapshot ID and Timestamp.

This is part of the Databend's Time Travel feature that allows you to query, back up, and restore from a previous version of your data within the retention period (24 hours by default).

Syntax​

SELECT ...
FROM ...
AT ( { SNAPSHOT => <snapshot_id> | TIMESTAMP => <timestamp> } );

Obtaining Snapshot ID and Timestamp​

To return the snapshot IDs and timestamps of all the snapshots of a table, execute the following statement:

SELECT snapshot_id, 
       timestamp 
FROM   fuse_snapshot('<database_name>', '<table_name>');

For more information about the FUSE_SNAPSHOT function,see FUSE_SNAPSHOT.

Examples​

Query with a snapshot ID​

-- Return snapshot IDs
select snapshot_id,timestamp from fuse_snapshot('default', 'ontime2');
+----------------------------------+----------------------------+
| snapshot_id                      | timestamp                  |
+----------------------------------+----------------------------+
| 16729481923640f9864c1c8ddd0861e3 | 2022-06-28 09:09:40.190662 |
+----------------------------------+----------------------------+

-- Query with the snapshot ID
select * from ontime2 at (snapshot=>'16729481923640f9864c1c8ddd0861e3');

Query with a timestamp​

-- Create a table
create table demo(c varchar);

-- Insert two rows
insert into demo values('batch1.1'),('batch1.2');

-- Insert another row
insert into demo values('batch2.1');

-- Return timestamps
select timestamp from fuse_snapshot('default', 'demo'); 
+----------------------------+
| timestamp                  |
+----------------------------+
| 2022-06-22 08:58:54.509008 |
| 2022-06-22 08:58:36.254458 |
+----------------------------+

-- Travel to the time when the last row was inserted
select * from demo at (TIMESTAMP => '2022-06-22 08:58:54.509008'::TIMESTAMP); 
+----------+
| c        |
+----------+
| batch1.1 |
| batch1.2 |
| batch2.1 |
+----------+

-- Travel to the time when the first two rows were inserted
select * from demo at (TIMESTAMP => '2022-06-22 08:58:36.254458'::TIMESTAMP); 
+----------+
| c        |
+----------+
| batch1.1 |
| batch1.2 |
+----------+