DDL Fast Timeout

For some specific DDL operations, you can configure their metadata lock (MDL) waiting time, preventing subsequent DML operations from being blocked.

The kernel version is 2.0.45.230900 or later.
Currently, the following DDL operations are supported: ALTER TABLE, CREATE INDEX, and DROP INDEX.

**Table 1** Parameter description
Parameter	Level	Description
rds_ddl_lock_wait_timeout	Global, Session	Defines how long that a DDL operation waits for a lock in the current session or global sessions. Value range: 1 to 31536000 (s). Default value: 31536000, indicating that the function is disabled. The actual lock wait timeout for DDL operations is the smaller value between lock_wait_timeout and this parameter value. The actual table lock timeout during DDL execution at the InnoDB layer is the minimum value of innodb_lock_wait_timeout and this parameter value. Row locks are not considered.

Start a client and add a lock for tables.
Figure 1 Adding a lock
Run the following command to check the status of the DDL fast timeout function
show variables like "%rds_ddl_lock_wait_timeout%";

Figure 2 Querying the status of the DDL fast timeout function

As shown in the preceding figure, the value of rds_ddl_lock_wait_timeout is 31536000 (default value). The function is disabled. The subsequent operations will wait for a long time.

To enable function, referring to 3.
Run the following command to set rds_ddl_lock_wait_timeout.
set rds_ddl_lock_wait_timeout=1;

Figure 3 Configuring parameters
Run the following command to create an index. It is found that the DDL operation times out quickly.
alter table lzk.t_lzk drop index indexa;
Figure 4 Creating an index