The AS OF SYSTEM TIME timestamp clause causes statements to execute using the database contents "as of" a specified time in the past.
You can use this clause to read historical data (also known as "time travel queries") and to improve performance by decreasing transaction conflicts. See Use AS OF SYSTEM TIME to decrease conflicts with long-running queries.
Historical data is available only within the garbage collection window, which is determined by the ttlseconds field in the replication zone configuration. All CockroachDB Serverless clusters have a default garbage collection window of 4500 seconds (1 hour and 15 minutes) that cannot be altered.
Synopsis
The AS OF SYSTEM TIME clause is supported in multiple SQL contexts,
including but not limited to:
- In
SELECTclauses, at the very end of theFROMsub-clause. - In
BACKUP, after the parameters of theTOsub-clause. - In
RESTORE, after the parameters of theFROMsub-clause. - In
BEGIN, after theBEGINkeyword. - In
SET, after theSET TRANSACTIONkeyword.
Parameters
The timestamp argument supports the following formats:
| Format | Notes |
|---|---|
INT |
Nanoseconds since the Unix epoch. |
negative INTERVAL |
Added to statement_timestamp(), and thus must be negative. |
STRING |
A TIMESTAMP, INT of nanoseconds, or negative INTERVAL. |
follower_read_timestamp() |
A function that returns the TIMESTAMP statement_timestamp() - 4.2s. Using this function will set the time as close as possible to the present time while remaining safe for exact staleness follower reads. |
with_min_timestamp(TIMESTAMPTZ, [nearest_only]) |
The minimum timestamp at which to perform the bounded staleness read. The actual timestamp of the read may be equal to or later than the provided timestamp, but cannot be before the provided timestamp. This is useful to request a read from nearby followers, if possible, while enforcing causality between an operation at some point in time and any dependent reads. This function accepts an optional nearest_only argument that will error if the reads cannot be serviced from a nearby replica. |
with_max_staleness(INTERVAL, [nearest_only]) |
The maximum staleness interval with which to perform the bounded staleness read. The timestamp of the read can be at most this stale with respect to the current time. This is useful to request a read from nearby followers, if possible, while placing some limit on how stale results can be. Note that with_max_staleness(INTERVAL) is equivalent to with_min_timestamp(now() - INTERVAL). This function accepts an optional nearest_only argument that will error if the reads cannot be serviced from a nearby replica. |
To set AS OF SYSTEM TIME follower_read_timestamp() on all implicit and explicit read-only transactions by default, set the default_transaction_use_follower_reads session variable to on. When default_transaction_use_follower_reads=on and follower reads are enabled, all read-only transactions use follower reads.
Examples
Select historical data (time-travel)
Imagine this example represents the database's current data:
> SELECT name, balance
FROM accounts
WHERE name = 'Edna Barath';
+-------------+---------+
| name | balance |
+-------------+---------+
| Edna Barath | 750 |
| Edna Barath | 2200 |
+-------------+---------+
We could instead retrieve the values as they were on October 3, 2016 at 12:45 UTC:
> SELECT name, balance
FROM accounts
AS OF SYSTEM TIME '2016-10-03 12:45:00'
WHERE name = 'Edna Barath';
+-------------+---------+
| name | balance |
+-------------+---------+
| Edna Barath | 450 |
| Edna Barath | 2000 |
+-------------+---------+
Using different timestamp formats
Assuming the following statements are run at 2016-01-01 12:00:00, they would execute as of 2016-01-01 08:00:00:
> SELECT * FROM t AS OF SYSTEM TIME '2016-01-01 08:00:00'
> SELECT * FROM t AS OF SYSTEM TIME 1451635200000000000
> SELECT * FROM t AS OF SYSTEM TIME '1451635200000000000'
> SELECT * FROM t AS OF SYSTEM TIME '-4h'
> SELECT * FROM t AS OF SYSTEM TIME INTERVAL '-4h'
Selecting from multiple tables
It is not yet possible to select from multiple tables at different timestamps. The entire query runs at the specified time in the past.
When selecting over multiple tables in a single FROM clause, the AS
OF SYSTEM TIME clause must appear at the very end and applies to the
entire SELECT clause.
For example:
> SELECT * FROM t, u, v AS OF SYSTEM TIME '-4h';
> SELECT * FROM t JOIN u ON t.x = u.y AS OF SYSTEM TIME '-4h';
> SELECT * FROM (SELECT * FROM t), (SELECT * FROM u) AS OF SYSTEM TIME '-4h';
Using AS OF SYSTEM TIME in subqueries
To enable time travel, the AS OF SYSTEM TIME clause must appear in
at least the top-level statement. It is not valid to use it only in a
subquery.
For example, the following is invalid:
SELECT * FROM (SELECT * FROM t AS OF SYSTEM TIME '-4h'), u
To facilitate the composition of larger queries from simpler queries,
CockroachDB allows AS OF SYSTEM TIME in sub-queries under the
following conditions:
- The top level query also specifies
AS OF SYSTEM TIME. - All the
AS OF SYSTEM TIMEclauses specify the same timestamp.
For example:
> SELECT * FROM (SELECT * FROM t AS OF SYSTEM TIME '-4h') tp
JOIN u ON tp.x = u.y
AS OF SYSTEM TIME '-4h' -- same timestamp as above - OK.
WHERE x < 123;
Use AS OF SYSTEM TIME in transactions
You can use the BEGIN statement to execute the transaction using the database contents "as of" a specified time in the past.
> BEGIN AS OF SYSTEM TIME '2019-04-09 18:02:52.0+00:00';
> SELECT * FROM orders;
> SELECT * FROM products;
> COMMIT;
Alternatively, you can use the SET statement to execute the transaction using the database contents "as of" a specified time in the past.
> BEGIN;
> SET TRANSACTION AS OF SYSTEM TIME '2019-04-09 18:02:52.0+00:00';
> SELECT * FROM orders;
> SELECT * FROM products;
> COMMIT;
Use AS OF SYSTEM TIME to recover recently lost data
It is possible to recover lost data as a result of an online schema change prior to when garbage collection begins:
> CREATE DATABASE foo;
CREATE DATABASE
Time: 3ms total (execution 3ms / network 0ms)
> CREATE TABLE foo.bar (id INT PRIMARY KEY);
CREATE TABLE
Time: 4ms total (execution 3ms / network 0ms)
> INSERT INTO foo.bar VALUES (1), (2);
INSERT 2
Time: 5ms total (execution 5ms / network 0ms)
> SELECT now();
now
--------------------------------
2022-02-01 21:11:53.63771+00
(1 row)
Time: 1ms total (execution 0ms / network 0ms)
> DROP TABLE foo.bar;
DROP TABLE
Time: 45ms total (execution 45ms / network 0ms)
> SELECT * FROM foo.bar AS OF SYSTEM TIME '2022-02-01 21:11:53.63771+00';
id
------
1
2
(2 rows)
Time: 2ms total (execution 2ms / network 0ms)
> SELECT * FROM foo.bar;
ERROR: relation "foo.bar" does not exist
SQLSTATE: 42P01
Once garbage collection has occurred, AS OF SYSTEM TIME will no longer be able to recover lost data. For more long-term recovery solutions, consider taking either a full or incremental backup of your cluster.
See also
Tech note
Although the following format is supported, it is not intended to be used by most users.
HLC timestamps can be specified using a DECIMAL. The
integer part is the wall time in nanoseconds. The fractional part is
the logical counter, a 10-digit integer. This is the same format as
produced by the cluster_logical_timestamp() function.
Was this helpful?
