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    PolarDB:Online Copy DDL

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    PolarDB:Online Copy DDL

    Last Updated:Jun 08, 2026

    PolarDB for MySQL Online Copy DDL reduces business disruptions caused by traditional Copy DDL operations—such as MODIFY COLUMN or converting between standard and partitioned tables—that hold metadata locks (MDL) throughout execution.

    Background

    During standard Copy DDL execution, MySQL holds an MDL-SNW (Shared No Write) lock, blocking all write operations (INSERT/UPDATE/DELETE) on the table. This can cause connection buildup and, in severe cases, system collapse. Online Copy DDL optimizes the locking mechanism during the data copy phase, allowing concurrent reads and writes while maintaining data consistency.

    Scope

    PolarDB for MySQL version 8.0.2 and revision 8.0.2.2.33.1 or later. Confirm the cluster version by querying the version number.

    Note

    Online Copy DDL is in phased release. To enable it, go to the Quota Center, find the quota by ID polarm_82_online_copy_ddl, and click **Request** in the **Operation** column. Provide the cluster ID (for example, pc-xxx).

    Limitations

    Supported operations:

    • Converting between standard tables and partitioned tables.

    • MODIFY COLUMN statements. Modifying column position simultaneously is not supported.

    Unsupported operations or objects:

    • Tables that contain JSON-type columns.

    • Tables that contain spatial indexes (SPATIAL INDEX).

    • Tables that contain global secondary indexes (GSI).

    • Hybrid partitioned tables.

    • Multi-master clusters (Limitless).

    • Sessions that have executed LOCK TABLES on the table.

    • DDL statements that explicitly specify ALGORITHM=COPY and LOCK=SHARED or LOCK=EXCLUSIVE.

    Note

    Before executing a DDL operation, run EXPLAIN DDL to check whether Online Copy DDL is supported.

    Usage

    Enable Online Copy DDL by setting the loose_polar_support_online_copy_ddl_algorithm parameter. For MODIFY COLUMN statements, also enable loose_polar_support_online_copy_ddl_algorithm_for_modify. Configure these in Set cluster parameters and node parameters.

    Parameter

    Level

    Description

    loose_polar_support_online_copy_ddl_algorithm

    Global

    Main switch for Online Copy DDL.

    • ON: Enables Online Copy DDL.

    • OFF (default): Disables Online Copy DDL.

    loose_polar_support_online_copy_ddl_algorithm_for_modify

    Global

    Enables Online Copy DDL for MODIFY COLUMN operations.

    • ON: Enabled.

    • OFF (default): Disabled.

    Method 1: Explicitly Specify the Algorithm (Recommended)

    Specify ALGORITHM=COPY, LOCK=NONE to force Online Copy DDL:

    CREATE TABLE t(a int);
ALTER TABLE t MODIFY COLUMN a BIGINT DEFAULT NULL, ALGORITHM=COPY, LOCK=NONE;
    Note

    If the statement returns ERROR 1846 (0A000): LOCK=NONE is not supported. Reason: COPY algorithm requires a lock. Try LOCK=SHARED., the operation does not support Online Copy DDL. Verify that loose_polar_support_online_copy_ddl_algorithm is enabled and review the limitations.

    Method 2: Automatically Select the Optimal Algorithm (Default Behavior)

    If no algorithm is specified or DEFAULT is used, the system selects the optimal execution method automatically:

    -- Method 1: Explicit DEFAULT
ALTER TABLE t MODIFY COLUMN a BIGINT DEFAULT NULL, ALGORITHM=DEFAULT, LOCK=DEFAULT;

-- Method 2: Omit completely (equivalent)
ALTER TABLE t MODIFY COLUMN a BIGINT DEFAULT NULL;
    Note

    The system attempts Online Copy DDL first. If conditions are not met, it falls back to traditional Copy DDL (which locks the table) to ensure the DDL completes.