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PS-10085 Update the online DDL limitations (8.0) #592

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Oct 29, 2025
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PS-10085 Update the online DDL limitations (8.0) #592

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Update the online DDL limitations
alina-derkach-oaza Oct 28, 2025
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Update limitations.md
alina-derkach-oaza Oct 29, 2025
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6 changes: 6 additions & 0 deletions docs/glossary.md
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Expand Up @@ -12,6 +12,12 @@ A pair of keys used for cryptographic purposes, consisting of a private key and

Atomicity means that database operations are applied following a “all or nothing” rule. A transaction is either fully applied or not at all.

## Atomic DDL (Data Definition Language)

Atomic DDL refers to the concept where DDL operations such as `CREATE`, `ALTER`, `DROP`, or `RENAME` statements are executed in a way that ensures atomicity, meaning they either fully succeed or have no effect at all.

This means that if a DDL statement fails, the database automatically rolls back any partial changes, leaving the schema in a consistent state.

## Consistency

Consistency means that each transaction that modifies the database takes it from one consistent state to another.
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33 changes: 17 additions & 16 deletions docs/limitations.md
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Expand Up @@ -2,7 +2,7 @@

## Online DDL limitations

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.
MyRocks has limited support for [Online DDL operations](https://dev.mysql.com/doc/refman/{{vers}}/en/innodb-online-ddl.html) due to the lack of [atomic DDL](./glossary.md#atomic-ddl-data-definition-language). As a result the schema changes are more restricted compared to InnoDB.

### Traditional MyRocks DDL behavior

Expand All @@ -12,7 +12,7 @@ MyRocks has limited support for [Online DDL operations](https://dev.mysql.com/do
| 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.
**Note:** MyRocks does not support [atomic DDL](./glossary.md#atomic-ddl-data-definition-language). Even metadata-only operations may require a full table rebuild, depending on the nature of the change.

### Partition management support

Expand All @@ -22,27 +22,26 @@ As of `Percona Server for MySQL 8.0.25-15`, MyRocks supports `INPLACE` partition
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.
These 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.
As of `Percona Server for MySQL 8.0.42-33`, MyRocks provides limited Instant DDL support that is disabled by default, and you can activate the specific instant operations you need by setting the appropriate configuration variables.

To enable specific types of instant operations, use the following configuration options:
| Configuration variable | Enables Instant DDL for |
|------------------------|--------------------------|
| [`rocksdb_enable_instant_ddl_for_append_column=ON`](variables.md#rocksdb_enable_instant_ddl_for_append_column) | `ALTER TABLE ... ADD COLUMN` |
| [`rocksdb_enable_instant_ddl_for_column_default_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_column_default_changes) | `ALTER/MODIFY COLUMN … DEFAULT` |
| [`rocksdb_enable_instant_ddl_for_drop_index_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_drop_index_changes) | `ALTER TABLE ... DROP INDEX` |
| [`rocksdb_enable_instant_ddl_for_table_comment_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_table_comment_changes) | `ALTER TABLE ... COMMENT` |

| Configuration variable | Enables Instant DDL for | ALGORITHM required |
|------------------------|--------------------------|----------------------------|
| [`rocksdb_enable_instant_ddl_for_append_column=ON`](variables.md#rocksdb_enable_instant_ddl_for_append_column) | `ALTER TABLE ... ADD COLUMN` | `ALGORITHM=INSTANT` |
| [`rocksdb_enable_instant_ddl_for_column_default_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_column_default_changes) | `ALTER/MODIFY COLUMN … DEFAULT` | `ALGORITHM=INSTANT` |
| [`rocksdb_enable_instant_ddl_for_drop_index_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_drop_index_changes) | `ALTER TABLE ... DROP INDEX` | `ALGORITHM=INSTANT` |
| [`rocksdb_enable_instant_ddl_for_table_comment_changes=ON`](variables.md#rocksdb_enable_instant_ddl_for_table_comment_changes) | `ALTER TABLE ... COMMENT` | `ALGORITHM=INSTANT` |
**Note:** Instant DDL in MyRocks is applied only when **both** of the following conditions are met:

**Note:** Instant DDL in MyRocks is only triggered when **both** of the following conditions are met:
* The configuration variable is set to `ON`.

1. The configuration variable is set to `ON`,
2. The `ALTER TABLE` statement explicitly includes `ALGORITHM=INSTANT`.
* The `ALTER TABLE` statement explicitly includes `ALGORITHM=INSTANT`.

For example:

Expand All @@ -51,9 +50,11 @@ SET GLOBAL rocksdb_enable_instant_ddl_for_table_comment_changes = ON;
ALTER TABLE my_table COMMENT = 'New comment', ALGORITHM=INSTANT;
```

If the configuration variable is enabled (`ON`) but `ALGORITHM=INSTANT` is not specified, MyRocks will fall back to the default algorithm.
If either condition is missing:

Conversely, if the configuration variable is disabled (`OFF`), attempting to use `ALGORITHM=INSTANT` will result in an error.
* When the variable is `ON` but `ALGORITHM=INSTANT` is omitted, MyRocks falls back to the default (non‑instant) algorithm.

* When the variable is `OFF`, any `ALTER TABLE … ALGORITHM=INSTANT` statement fails with an error.

## Unsupported InnoDB features in MyRocks

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