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PS-10085 MyRocks limitations section still showing the Online DDL limitation part (8.0) #557

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PS-10085 MyRocks limitations section still showing the Online DDL lim…
alina-derkach-oaza Jul 31, 2025
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alina-derkach-oaza Sep 24, 2025
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alina-derkach-oaza Sep 24, 2025
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42 changes: 38 additions & 4 deletions docs/limitations.md
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# MyRocks limitations

The MyRocks storage engine lacks the following features compared to InnoDB:
## Online DDL limitations

* [Online DDL](https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl.html) is not supported due to the lack of atomic DDL support.
MyRocks has limited support for [Online DDL operations](https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl.html) due to the lack of atomic DDL. As a result the schema changes are more restricted compared to InnoDB.

* There is no `ALTER TABLE ... ALGORITHM=INSTANT` functionality
### Traditional MyRocks DDL behavior

* A partition management operation only supports the `COPY` algorithms, which rebuilds the partition table and moves the data based on the new `PARTITION ... VALUE` definition. In the case of `DROP PARTITION`, the data not moved to another partition is deleted.
| Operation type | Examples | ALGORITHM |
|---------------------|--------------------------------------------------|-----------------------------|
| Index operations | `ADD INDEX`, `DROP INDEX`, `RENAME INDEX` | `INPLACE` (always) |
| Column changes | `ADD COLUMN`, `DROP COLUMN`, `MODIFY COLUMN` | `COPY` (full table rebuild) |
| Metadata changes | `RENAME TABLE`, some `RENAME COLUMN` operations | May be `INSTANT` |

**Note:** MyRocks does not support atomic DDL. Even metadata-only operations may require a full table rebuild, depending on the nature of the change.

### Partition management support

As of `Percona Server for MySQL 8.0.25-15`, MyRocks supports `INPLACE` partition management for certain operations:

```sql
ALTER TABLE t1 DROP PARTITION p1, ALGORITHM=INPLACE;
ALTER TABLE t1 ADD PARTITION (PARTITION p2 VALUES LESS THAN (MAXVALUE)), ALGORITHM=INPLACE;
```
The aforementioned operations no longer require a full table rebuild. However, operations that modify partitioning schemes, such as changing `VALUES LESS THAN`, still fall back to the `COPY` algorithm.

**Note:** Dropping a partition permanently deletes any data stored in it unless that data is reassigned to another partition.

### Instant DDL support

As of `Percona Server for MySQL 8.0.42-33`, MyRocks introduces limited support for Instant DDL, which is disabled by default and controlled via configuration variables.

To enable specific types of instant operations, use the following configuration options:

| Configuration variable | Enables Instant DDL for | ALGORITHM (internal only) |
|------------------------|--------------------------|----------------------------|
| [`rocksdb_enable_instant_ddl_for_append_column=ON`](variables.md#rocksdb_enable_instant_ddl_for_append_column) | `ALTER TABLE ... ADD COLUMN` | `INSTANT` |
| [`rocksdb_enable_instant_ddl_for_column_default_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_column_default_changes) | `ALTER/MODIFY COLUMN … DEFAULT` | `INSTANT` |
| [`rocksdb_enable_instant_ddl_for_drop_index_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_drop_index_changes) | `ALTER TABLE ... DROP INDEX` | `INSTANT` |
| [`rocksdb_enable_instant_ddl_for_table_comment_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_table_comment_changes) | `ALTER TABLE ... COMMENT` | `INSTANT` |

**Note:** MyRocks does **not support `ALGORITHM=INSTANT`** in SQL syntax. These operations behave like Instant DDL **internally**, but only if the respective configuration variables are enabled.

## Unsupported InnoDB features in MyRocks

* [ALTER TABLE .. EXCHANGE PARTITION](https://dev.mysql.com/doc/refman/8.0/en/partitioning-management-exchange.html).

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