AS OF SYSTEM TIME - CockroachDB

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 .

Historical data is available only within the garbage collection window, which is determined by the ttlseconds field in the .

Synopsis

The AS OF SYSTEM TIME clause is supported in multiple SQL contexts, including but not limited to:

In , at the very end of the FROM sub-clause. The locking clause is not allowed with AS OF SYSTEM TIME.
In , after the parameters of the TO sub-clause.
In , after the parameters of the FROM sub-clause.
In , after the BEGIN keyword.
In , after the SET TRANSACTION keyword.

AS OF SYSTEM TIME cannot be used with:

( SELECT... FOR UPDATE and SELECT... FOR SHARE ).
(such as or ).

The preceding statements return an error: cannot execute {SQL STATEMENT} in a read-only transaction.

Parameters

The timestamp argument supports the following formats:

Format	Notes
	Nanoseconds since the Unix epoch.
negative	Added to `statement\_timestamp()`, and thus must be negative.
	A , of nanoseconds, or negative .
`follower\_read\_timestamp()`	A that returns the `statement\_timestamp() - 4.2s`. Using this function will set the time as close as possible to the present time while remaining safe for .
`with\_min\_timestamp(TIMESTAMPTZ, [nearest\_only])`	The minimum at which to perform the . 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 . 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 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 . 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 TIME clauses 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 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 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 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 of your cluster.

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 . 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.

​Synopsis

​Parameters

​Examples

​Select historical data (time-travel)

​Using different timestamp formats

​Selecting from multiple tables

​Using AS OF SYSTEM TIME in subqueries

​Use AS OF SYSTEM TIME in transactions

​Use AS OF SYSTEM TIME to recover recently lost data

​See also

​Tech note

Synopsis

Parameters

Examples

Select historical data (time-travel)

Using different timestamp formats

Selecting from multiple tables

Using `AS OF SYSTEM TIME` in subqueries

Use `AS OF SYSTEM TIME` in transactions

Use `AS OF SYSTEM TIME` to recover recently lost data

See also

Tech note