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RDS for MySQL

Feature

Online extension of varchar field length

This topic describes the feature for extending varchar field lengths online in RDS for MySQL.

Supported version

MySQL 5.7 version, and the kernel minor version is MySQL 5.7.44_20250815 and above.

MySQL 8.0 version, and the kernel minor version is MySQL 8.0.32_20250815 and above.

MySQL 8.4 version, and the kernel minor version is MySQL 8.4.7_20260415 and above.

Background

Native MySQL does not allow using the inplace algorithm when extending a varchar field's length beyond 256 bytes. Only the copy algorithm is permitted. During this copy phase, users cannot modify the old table. All write requests are blocked, and only read requests are allowed.

RDS for MySQL supports extending varchar fields online across the 256-byte boundary without blocking write operations during DDL execution. Additionally, when extending a field's length from 127 bytes or less to 256 bytes or more, the extension can be completed in seconds by modifying only the metadata, not the data.

Feature description

This feature implements Online DDL by changing the algorithm for extending varchar fields from copy to inplace. During a DDL process, you can continue writing to the old table. Subsequent write operations are replayed to the new table through the row log.

By using the properties of varchar fields, it can extend a field from 127 bytes or less to 256 bytes or more in seconds, without modifying the data.

Instructions

Parameter description

A new parameter is added to manage the online extension of varchar fields.

Parameter name	Description	Effective scope	Value range	Default value
loose_extend_varchar_enhance	Specifies whether to enable the extended varchar enhancement feature.	GLOBAL/SESSION	ON/OFF	OFF

When the parameter is enabled, the extensions are classified as follows:

Varchar field length	Whether to support online expansion	Whether to support modifying only metadata
Extensions within 256 bytes	Support	Support
Expansion from 0–127 bytes to 256 bytes or more	Support	Support
Extends from 128-255 bytes to 256 bytes or more	Support	Not support
Expansion for lengths of more than 256 bytes	Support	Support

Examples

After you set the loose_extend_varchar_enhance parameter to ON, the online varchar field extension feature is enabled. For detailed information on modifying parameters, please refer to Modifying parameters.

Extensions within 256 bytes

create table t(c1 varchar(100))charset ascii;
Query OK, 0 rows affected (0.017 sec)

alter table t modify column c1 varchar(170),algorithm=inplace,LOCK=NONE;
Query OK, 0 rows affected (0.007 sec)
Records: 0 Duplicates: 0 Warnings: 0

Expansion from 0–127 bytes to 256 bytes or more

create table t(c1 varchar(100))charset ascii;
Query OK, 0 rows affected (0.011 sec)

alter table t modify column c1 varchar(300),algorithm=inplace,LOCK=NONE;
Query OK, 0 rows affected (0.006 sec)
Records: 0 Duplicates: 0 Warnings: 0

Expansion from 128–255 bytes to 256 bytes or more

create table t(c1 varchar(170))charset ascii;
Query OK, 0 rows affected (0.013 sec)

alter table t modify column c1 varchar(300),algorithm=inplace,LOCK=NONE;
Query OK, 0 rows affected (0.020 sec)
Records: 0 Duplicates: 0 Warnings: 0

Expansion for lengths of more than 256 bytes

create table t(c1 varchar(300))charset ascii;
Query OK, 0 rows affected (0.012 sec)

alter table t modify column c1 varchar(400),algorithm=inplace,LOCK=NONE;
Query OK, 0 rows affected (0.005 sec)
Records: 0 Duplicates: 0 Warnings: 0

Last updated: 2026.05.28 15:17:55