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Read Historical Data Using the tidb_external_ts Variable

To support reading the historical data, TiDB v6.4.0 introduces a system variable tidb_external_ts. This document describes how to read historical data through this system variable, including detailed usage examples.

Scenarios

Read historical data from a specified point in time is very useful for data replication tools such as TiCDC. After the data replication tool completes the data replication before a certain point in time, you can set the tidb_external_ts system variable of the downstream TiDB to read the data before that point in time. This prevents the data inconsistency caused by data replication.

Feature description

The system variable tidb_external_ts specifies the timestamp of the historical data to be read when tidb_enable_external_ts_read is enabled.

The system variable tidb_enable_external_ts_read controls whether to read historical data in the current session or globally. The default value is OFF, which means the feature of reading historical data is disabled, and the tidb_external_ts value is ignored. When tidb_enable_external_ts_read is set to ON globally, all queries read historical data before the time specified by tidb_external_ts. If tidb_enable_external_ts_read is set to ON only for a certain session, only queries in that session read historical data.

When the tidb_enable_external_ts_read is enabled, TiDB becomes read-only. All write queries will fail with an error like ERROR 1836 (HY000): Running in read-only mode.

Usage examples

This section describes how to use the tidb_external_ts variable to read historical data with examples.

  1. Create a table and insert some rows into the table:

    CREATE TABLE t (c INT);
    
    Query OK, 0 rows affected (0.01 sec)
    
    INSERT INTO t VALUES (1), (2), (3);
    
    Query OK, 3 rows affected (0.00 sec)
    
  2. View the data in the table:

    SELECT * FROM t;
    
    +------+
    | c    |
    +------+
    |    1 |
    |    2 |
    |    3 |
    +------+
    3 rows in set (0.00 sec)
    
  3. Set tidb_external_ts to @@tidb_current_ts:

    START TRANSACTION;
    SET GLOBAL tidb_external_ts=@@tidb_current_ts;
    COMMIT;
    
  4. Insert a new row and confirm that it is inserted:

    INSERT INTO t VALUES (4);
    
    Query OK, 1 row affected (0.001 sec)
    
    SELECT * FROM t;
    
    +------+
    | id   |
    +------+
    |    1 |
    |    2 |
    |    3 |
    |    4 |
    +------+
    4 rows in set (0.00 sec)
    
  5. Set tidb_enable_external_ts_read to ON and then view data in the table:

    SET tidb_enable_external_ts_read=ON;
    SELECT * FROM t;
    
    +------+
    | c    |
    +------+
    |    1 |
    |    2 |
    |    3 |
    +------+
    3 rows in set (0.00 sec)
    

    Because tidb_external_ts is set to the timestamp before the new row is inserted, the newly inserted row is not returned after the tidb_enable_external_ts_read is enabled.

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