Known issues and limitations for Amazon RDS for MySQL

Known issues and limitations for working with Amazon RDS for MySQL are as follows.

InnoDB reserved word

InnoDB is a reserved word for RDS for MySQL. You can't use this name for a MySQL database.

Storage-full behavior for Amazon RDS for MySQL

When storage becomes full for a MySQL DB instance, there can be metadata inconsistencies, dictionary mismatches, and orphan tables. To prevent these issues, Amazon RDS automatically stops a DB instance that reaches the storage-full state.

A MySQL DB instance reaches the storage-full state in the following cases:

• The DB instance has less than 20,000 MiB of storage, and available storage reaches 200 MiB or less.

• The DB instance has more than 102,400 MiB of storage, and available storage reaches 1024 MiB or less.

• The DB instance has between 20,000 MiB and 102,400 MiB of storage, and has less than 1% of storage available.

After Amazon RDS stops a DB instance automatically because it reached the storage-full state, you can still modify it. To restart the DB instance, complete at least one of the following:

• Modify the DB instance to enable storage autoscaling.

  For more information about storage autoscaling, see Managing capacity automatically with Amazon RDS storage autoscaling.

• Modify the DB instance to increase its storage capacity.

  For more information about increasing storage capacity, see Increasing DB instance storage capacity.

After you make one of these changes, the DB instance is restarted automatically. For information about modifying a DB instance, see Modifying an Amazon RDS DB instance.

Inconsistent InnoDB buffer pool size

For MySQL 5.7, there is currently a bug in the way that the InnoDB buffer pool size is managed. MySQL 5.7 might adjust the value of the innodb_buffer_pool_size parameter to a large value that can result in the InnoDB buffer pool growing too large and using up too much memory. This effect can cause the MySQL database engine to stop running or can prevent it from starting. This issue is more common for DB instance classes that have less memory available.

To resolve this issue, set the value of the innodb_buffer_pool_size parameter to a multiple of the product of the innodb_buffer_pool_instances parameter value and the innodb_buffer_pool_chunk_size parameter value. For example, you might set the innodb_buffer_pool_size parameter value to a multiple of eight times the product of the innodb_buffer_pool_instances and innodb_buffer_pool_chunk_size parameter values, as shown in the following example.

innodb_buffer_pool_chunk_size = 536870912
innodb_buffer_pool_instances = 4
innodb_buffer_pool_size = (536870912 * 4) * 8 = 17179869184

For details on this MySQL 5.7 bug, see https://bugs.mysql.com/bug.php?id=79379 in the MySQL documentation.

Index merge optimization returns incorrect results

Queries that use index merge optimization might return incorrect results due to a bug in the MySQL query optimizer that was introduced in MySQL 5.5.37. When you issue a query against a table with multiple indexes, the optimizer scans ranges of rows based on the multiple indexes, but does not merge the results together correctly. For more information on the query optimizer bug, see http://bugs.mysql.com/bug.php?id=72745 and http://bugs.mysql.com/bug.php?id=68194 in the MySQL bug database.

For example, consider a query on a table with two indexes where the search arguments reference the indexed columns.

SELECT * FROM table1
WHERE indexed_col1 = 'value1' AND indexed_col2 = 'value2';

In this case, the search engine will search both indexes. However, due to the bug, the merged results are incorrect.

To resolve this issue, you can do one of the following:

• Set the optimizer_switch parameter to index_merge=off in the DB parameter group for your MySQL DB instance. For information on setting DB parameter group parameters, see Working with parameter groups.

• Upgrade your MySQL DB instance to MySQL version 5.7 or 8.0. For more information, see Upgrading the MySQL DB engine.

• If you cannot upgrade your instance or change the optimizer_switch parameter, you can work around the bug by explicitly identifying an index for the query, for example:

  SELECT * FROM table1
  USE INDEX covering_index
  WHERE indexed_col1 = 'value1' AND indexed_col2 = 'value2'; 

For more information, see Index merge optimization in the MySQL documentation.

MySQL parameter exceptions for Amazon RDS DB instances

Some MySQL parameters require special considerations when used with an Amazon RDS DB instance.

lower_case_table_names

Because Amazon RDS uses a case-sensitive file system, setting the value of the lower_case_table_names server parameter to 2 (names stored as given but compared in lowercase) is not supported. The following are the supported values for Amazon RDS for MySQL DB instances:

• 0 (names stored as given and comparisons are case-sensitive) is supported for all RDS for MySQL versions.

• 1 (names stored in lowercase and comparisons are not case-sensitive) is supported for RDS for MySQL version 5.7 and version 8.0.28 and higher 8.0 versions.

Set the lower_case_table_names parameter in a custom DB parameter group before creating a DB instance. Then, specify the custom DB parameter group when you create the DB instance.

When a parameter group is associated with a MySQL DB instance with a version lower than 8.0, we recommend that you avoid changing the lower_case_table_names parameter in the parameter group. Changing it could cause inconsistencies with point-in-time recovery backups and read replica DB instances.

When a parameter group is associated with a version 8.0 MySQL DB instance, you can't modify the lower_case_table_names parameter in the parameter group.

Read replicas should always use the same lower_case_table_names parameter value as the source DB instance.

long_query_time

You can set the long_query_time parameter to a floating point value so that you can log slow queries to the MySQL slow query log with microsecond resolution. You can set a value such as 0.1 seconds, which would be 100 milliseconds, to help when debugging slow transactions that take less than one second.

MySQL file size limits in Amazon RDS

For MySQL DB instances, the maximum provisioned storage limit constrains the size of a table to a maximum size of 16 TB when using InnoDB file-per-table tablespaces. This limit also constrains the system tablespace to a maximum size of 16 TB. InnoDB file-per-table tablespaces (with tables each in their own tablespace) is set by default for MySQL DB instances.

Note

Some existing DB instances have a lower limit. For example, MySQL DB instances created before April 2014 have a file and table size limit of 2 TB. This 2 TB file size limit also applies to DB instances or read replicas created from DB snapshots taken before April 2014, regardless of when the DB instance was created.

There are advantages and disadvantages to using InnoDB file-per-table tablespaces, depending on your application. To determine the best approach for your application, see File-per-table tablespaces in the MySQL documentation.

We don't recommend allowing tables to grow to the maximum file size. In general, a better practice is to partition data into smaller tables, which can improve performance and recovery times.

One option that you can use for breaking up a large table into smaller tables is partitioning. Partitioning distributes portions of your large table into separate files based on rules that you specify. For example, if you store transactions by date, you can create partitioning rules that distribute older transactions into separate files using partitioning. Then periodically, you can archive the historical transaction data that doesn't need to be readily available to your application. For more information, see Partitioning in the MySQL documentation.

Because there is no single system table or view that provides the size of all the tables and the InnoDB system tablespace, you must query multiple tables to determine the size of the tablespaces.

To determine the size of the InnoDB system tablespace and the data dictionary tablespace

• Use the following SQL command to determine if any of your tablespaces are too large and are candidates for partitioning.

  Note

  The data dictionary tablespace is specific to MySQL 8.0.

  select FILE_NAME,TABLESPACE_NAME, ROUND(((TOTAL_EXTENTS*EXTENT_SIZE)
  /1024/1024/1024), 2)  as "File Size (GB)" from information_schema.FILES
  where tablespace_name in ('mysql','innodb_system');

To determine the size of InnoDB user tables outside of the InnoDB system tablespace (for MySQL 5.7 versions)

• Use the following SQL command to determine if any of your tables are too large and are candidates for partitioning.

  SELECT SPACE,NAME,ROUND((ALLOCATED_SIZE/1024/1024/1024), 2)
  as "Tablespace Size (GB)"
  FROM information_schema.INNODB_SYS_TABLESPACES ORDER BY 3 DESC;

To determine the size of InnoDB user tables outside of the InnoDB system tablespace (for MySQL 8.0 versions)

• Use the following SQL command to determine if any of your tables are too large and are candidates for partitioning.

  SELECT SPACE,NAME,ROUND((ALLOCATED_SIZE/1024/1024/1024), 2)
  as "Tablespace Size (GB)"
  FROM information_schema.INNODB_TABLESPACES ORDER BY 3 DESC;

To determine the size of non-InnoDB user tables

• Use the following SQL command to determine if any of your non-InnoDB user tables are too large.

  SELECT TABLE_SCHEMA, TABLE_NAME, round(((DATA_LENGTH + INDEX_LENGTH+DATA_FREE)
  / 1024 / 1024/ 1024), 2) As "Approximate size (GB)" FROM information_schema.TABLES
  WHERE TABLE_SCHEMA NOT IN ('mysql', 'information_schema', 'performance_schema')
  and ENGINE<>'InnoDB';

To enable InnoDB file-per-table tablespaces

• Set the innodb_file_per_table parameter to 1 in the parameter group for the DB instance.

To disable InnoDB file-per-table tablespaces

• Set the innodb_file_per_table parameter to 0 in the parameter group for the DB instance.

For information on updating a parameter group, see Working with parameter groups.

When you have enabled or disabled InnoDB file-per-table tablespaces, you can issue an ALTER TABLE command to move a table from the global tablespace to its own tablespace, or from its own tablespace to the global tablespace as shown in the following example:

ALTER TABLE table_name ENGINE=InnoDB;

MySQL Keyring Plugin not supported

Currently, Amazon RDS for MySQL doesn't support the MySQL keyring_aws Amazon Web Services Keyring Plugin.

Custom ports

Amazon RDS blocks connections to custom port 33060 for the MySQL engine. Choose a different port for your MySQL engine.

MySQL stored procedure limitations

The mysql.rds_kill and mysql.rds_kill_query stored procedures can't terminate sessions or queries owned by MySQL users with usernames longer than 16 characters on the following RDS for MySQL versions:

• 8.0.32 and lower 8 versions

• 5.7.41 and lower 5.7 versions

GTID-based replication with an external source instance

Amazon RDS doesn't support replication based on global transaction identifiers (GTIDs) from an external MySQL instance into an Amazon RDS for MySQL DB instance that requires setting GTID_PURGED during configuration.

MySQL default authentication plugin

RDS for MySQL version 8.0.34 and higher use the mysql_native_password plugin. You can't change the default_authentication_plugin setting.