Flashback queries

OceanBase Database supports record-specific flashback queries, which allow you to obtain data of a specific historical version. You can use the AS OF SCN and AS OF TIMESTAMP clauses to perform flashback queries in Oracle mode and the AS OF SNAPSHOT clause to perform flashback queries in MySQL mode.

OceanBase Database allows you to set the undo_retention parameter for a tenant to perform a flashback query. After you set the undo_retention parameter, you can use the flashback query feature to obtain data of multiple versions in the time range from T - undo_retention to T. The default value of the undo_retention parameter is 1800, in seconds. For more information about the undo_retention parameter, see undo_retention.

Considerations

When you set the undo_retention parameter to perform a flashback query, note the following:

• If you set the undo_retention parameter to T0, this feature takes effect only on data generated after T - T0.

• If the queried table is moved to the recycle bin, you must restore it from the recycle bin before you can query data in the table.

  For more information, see Restore objects from the recycle bin.

• Assuming that the current time is T, after you set the undo_retention parameter, if a DDL operation is performed in the time range from T - undo_retention to the current time, the system uses the latest data dictionary to explain the data in the table. For data generated before the DDL operation is performed, the deleted column data is changed to the deleted state, and the added column data is set to the default value.

• Flashback queries require additional storage space to retain historical data. If you increase the value of the undo_retention parameter, the storage space required is increased. We recommend that you observe the change of the storage space after you increase the value of the undo_retention parameter.

Prerequisites

Before you set the undo_retention parameter to perform a flashback query, check whether multi-version minor compaction is enabled, namely whether the value of the undo_retention parameter is not 0. The default value of the undo_retention parameter is 1800, in seconds.

1. Log on to the database as a tenant administrator.

Example (the command can vary in practice):

obclient -h10.xx.xx.xx -P2883 -uroot@mysql -p***** -A

For more information, see Database connection overview.

2. Execute the following statement to modify the value of the undo_retention parameter.

   Example:

   obclient [(none)]> ALTER SYSTEM SET undo_retention=900;
   

   For more information about the undo_retention parameter, see undo_retention.

Run a flashback query in MySQL mode

1. Log on to the database as an administrator of a MySQL user tenant.

   Example (the command can vary in practice):

   obclient -h10.xx.xx.xx -P2883 -uinfo@mysql -p***** -A
   

   For more information, see Database connection overview.

2. Perform a flashback query on a table.

   Assume that the value of the undo_retention parameter is 900 and you have a table named table1. Specify a historical point in time by using AS OF SNAPSHOT and query the status and data of the table at this point in time.

   obclient [(none)]> SELECT * FROM table1 AS OF SNAPSHOT 1597306800000000000;
   

   1597306800000000000 is a timestamp, in nanoseconds. The timestamp is the time since 08:00:00 (UTC+8) on January 1, 1970. You can convert the point in time to be queried into a timestamp based on your business needs.

   For example, if you want to perform a flashback query on table data at 16:20:00 on August 13, 2020, you can convert the time as follows:

   obclient [(none)]> SELECT time_to_usec('2020-08-13 16:20:00') * 1000;
   +--------------------------------------------+
   | time_to_usec('2022-01-01 00:00:00') * 1000 |
   +--------------------------------------------+
   |                        1597306800000000000 |
   +--------------------------------------------+
   1 row in set
   

Run a flashback query in Oracle mode

1. Log on to the database as an administrator of an Oracle user tenant.

   Example (the command can vary in practice):

   obclient -h10.xx.xx.xx -P2883 -uinfo@mysql -p***** -A
   

   For more information, see Database connection overview.

2. Perform a flashback query on a table.

   Assume that the value of the undo_retention parameter is 900 and you have three tables: table1, table2, and expr1. The following examples show the sample code for flashback queries in Oracle mode.

   • Specify a historical point in time by using TIMESTAMP and query the status and data of a single table at this point in time:

     obclient [SYS]> SELECT * FROM table1 AS OF TIMESTAMP TO_TIMESTAMP('2020-08-13 16:20:00','yyyy-mm-dd hh24:mi:ss');
     

   • Specify a historical point in time by using TIMESTAMP and query the status and data of the multiple tables at this point in time:

     obclient [SYS]> SELECT * FROM table1 AS OF TIMESTAMP expr1,table2 AS OF TIMESTAMP TO_TIMESTAMP('2020-08-13 16:20:00','yyyy-mm-dd hh24:mi:ss');
     

   • Specify a historical point in time by using SCN and query the status and data of a single table at this point in time:

     obclient [SYS]> SELECT * FROM table1 AS OF SCN 1597306800000000000;
     

     1597306800000000000 is a timestamp, in nanoseconds. The timestamp is the time since 08:00:00 (UTC+8) on January 1, 1970. You can convert the point in time to be queried into a timestamp based on your business needs.

     For example, if you want to perform a flashback query on table data at 16:20:00 on August 13, 2020, you can convert the time as follows:

     obclient [(none)]> SELECT (to_date('2020-08-13 16:20:00','yyyy-mm-dd hh24:mi:ss') - to_date('1970-01-01 08:00:00', 'yyyy-mm-dd hh24:mi:ss')) * 86400 * 1000 * 1000 * 1000 AS unix_nsec_timestamp FROM DUAL;
     +---------------------+
     | UNIX_NSEC_TIMESTAMP |
     +---------------------+
     | 1597306800000000000 |
     +---------------------+
     1 row in set