Periodically Delete Expired Data Using TTL (Time to Live)
Time to live (TTL) is a feature that allows you to manage TiDB data lifetime at the row level. For a table with the TTL attribute, TiDB automatically checks data lifetime and deletes expired data at the row level. This feature can effectively save storage space and enhance performance in some scenarios.
The following are some common scenarios for TTL:
- Regularly delete verification codes and short URLs.
- Regularly delete unnecessary historical orders.
- Automatically delete intermediate results of calculations.
TTL is designed to help users clean up unnecessary data periodically and in a timely manner without affecting the online read and write workloads. TTL concurrently dispatches different jobs to different TiDB nodes to delete data in parallel in the unit of table. TTL does not guarantee that all expired data is deleted immediately, which means that even if some data is expired, the client might still read that data some time after the expiration time until that data is deleted by the background TTL job.
You can configure the TTL attribute of a table using the
CREATE TABLE or
ALTER TABLE statement.
Create a table with a TTL attribute
Create a table with a TTL attribute:
CREATE TABLE t1 ( id int PRIMARY KEY, created_at TIMESTAMP ) TTL = `created_at` + INTERVAL 3 MONTH;
The preceding example creates a table
created_atas the TTL timestamp column, which indicates the creation time of the data. The example also sets the longest time that a row is allowed to live in the table to 3 months through
INTERVAL 3 MONTH. Data that lives longer than this value will be deleted later.
TTL_ENABLEattribute to enable or disable the feature of cleaning up expired data:
CREATE TABLE t1 ( id int PRIMARY KEY, created_at TIMESTAMP ) TTL = `created_at` + INTERVAL 3 MONTH TTL_ENABLE = 'OFF';
TTL_ENABLEis set to
OFF, even if other TTL options are set, TiDB does not automatically clean up expired data in this table. For a table with the TTL attribute,
To be compatible with MySQL, you can set a TTL attribute using a comment:
CREATE TABLE t1 ( id int PRIMARY KEY, created_at TIMESTAMP ) /*T![ttl] TTL = `created_at` + INTERVAL 3 MONTH TTL_ENABLE = 'OFF'*/;
In TiDB, using the table TTL attribute or using comments to configure TTL is equivalent. In MySQL, the comment is ignored and an ordinary table is created.
Modify the TTL attribute of a table
Modify the TTL attribute of a table:
ALTER TABLE t1 TTL = `created_at` + INTERVAL 1 MONTH;
You can use the preceding statement to modify a table with an existing TTL attribute or to add a TTL attribute to a table without a TTL attribute.
Modify the value of
TTL_ENABLEfor a table with the TTL attribute:
ALTER TABLE t1 TTL_ENABLE = 'OFF';
To remove all TTL attributes of a table:
ALTER TABLE t1 REMOVE TTL;
TTL and the default values of data types
You can use TTL together with default values of the data types. The following are two common usage examples:
DEFAULT CURRENT_TIMESTAMPto specify the default value of a column as the current creation time and use this column as the TTL timestamp column. Records that were created 3 months ago are expired:
CREATE TABLE t1 ( id int PRIMARY KEY, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ) TTL = `created_at` + INTERVAL 3 MONTH;
Specify the default value of a column as the creation time or the latest update time and use this column as the TTL timestamp column. Records that have not been updated for 3 months are expired:
CREATE TABLE t1 ( id int PRIMARY KEY, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, ) TTL = `created_at` + INTERVAL 3 MONTH;
TTL and generated columns
You can use TTL together with generated columns (experimental feature) to configure complex expiration rules. For example:
CREATE TABLE message ( id int PRIMARY KEY, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP, image bool, expire_at TIMESTAMP AS (IF(image, created_at + INTERVAL 5 DAY, created_at + INTERVAL 30 DAY )) ) TTL = `expire_at` + INTERVAL 0 DAY;
The preceding statement uses the
expire_at column as the TTL timestamp column and sets the expiration time according to the message type. If the message is an image, it expires in 5 days. Otherwise, it expires in 30 days.
You can use TTL together with the JSON type. For example:
CREATE TABLE orders ( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, order_info JSON, created_at DATE AS (JSON_EXTRACT(order_info, '$.created_at')) VIRTUAL ) TTL = `created_at` + INTERVAL 3 month;
For each table with a TTL attribute, TiDB internally schedules a background job to clean up expired data. You can customize the execution period of these jobs by setting the
tidb_ttl_job_run_interval global variable. The following example sets the background cleanup jobs to run once every 24 hours:
SET @@global.tidb_ttl_job_run_interval = '24h';
To disable the execution of TTL jobs, in addition to setting the
TTL_ENABLE='OFF' table option, you can also disable the execution of TTL jobs in the entire cluster by setting the
tidb_ttl_job_enable global variable:
SET @@global.tidb_ttl_job_enable = OFF;
In some scenarios, you might want to allow TTL jobs to run only in a certain time window. In this case, you can set the
tidb_ttl_job_schedule_window_end_time global variables to specify the time window. For example:
SET @@global.tidb_ttl_job_schedule_window_start_time = '01:00 +0000'; SET @@global.tidb_ttl_job_schedule_window_end_time = '05:00 +0000';
The preceding statement allows TTL jobs to be scheduled only between 1:00 and 5:00 UTC. By default, the time window is set to
00:00 +0000 to
23:59 +0000, which allows the jobs to be scheduled at any time.
Monitoring metrics and charts
TiDB collects runtime information about TTL periodically and provides visualized charts of these metrics in Grafana. You can see these metrics in the TiDB -> TTL panel in Grafana.
For details of the metrics, see the TTL section in TiDB Monitoring Metrics.
Compatibility with TiDB tools
As an experimental feature, the TTL feature is not compatible with data import and export tools, including BR, TiDB Lightning, and TiCDC.
Currently, the TTL feature has the following limitations:
- The TTL attribute cannot be set on temporary tables, including local temporary tables and global temporary tables.
- A table with the TTL attribute does not support being referenced by other tables as the primary table in a foreign key constraint.
- It is not guaranteed that all expired data is deleted immediately. The time when expired data is deleted depends on the scheduling interval and scheduling window of the background cleanup job.
- Currently, a single table can only run a cleanup job on a single TiDB node at a given time. This might cause performance bottlenecks in some scenarios (for example, when the table is extremely large). This issue will be optimized in future releases.