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docs: clarify TiCDC index DDL ordering #23336

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docs: clarify TiCDC index DDL ordering
doc-claw-bot Jul 22, 2026
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docs: sync TiCDC DDL wording with docs-cn review
doc-claw-bot Jul 22, 2026
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qiancai Jul 22, 2026
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2 changes: 1 addition & 1 deletion ticdc/ticdc-ddl.md
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Expand Up @@ -61,7 +61,7 @@ The following is the allow list of DDL statements supported by TiCDC. The abbrev

### Asynchronous execution of `ADD INDEX` and `CREATE INDEX` DDLs

When the downstream is TiDB, TiCDC executes `ADD INDEX` and `CREATE INDEX` DDL operations asynchronously to minimize the impact on changefeed replication latency. This means that, after replicating `ADD INDEX` and `CREATE INDEX` DDLs to the downstream TiDB for execution, TiCDC returns immediately without waiting for the completion of the DDL execution. This avoids blocking subsequent DML executions.
Although TiCDC usually replicates DDL statements in order, when the downstream is TiDB, TiCDC executes DDL operations for creating and adding indexes asynchronously. This means that, after replicating `ADD INDEX` and `CREATE INDEX` DDLs to the downstream TiDB cluster for execution, TiCDC returns immediately without waiting for DDL execution to complete. This minimizes the impact on changefeed replication latency and avoids blocking subsequent DML executions.

During the execution of the `ADD INDEX` or `CREATE INDEX` DDL operation in the downstream, when TiCDC executes the next DDL operation of the same table, this DDL operation might be blocked in the `queueing` state for a long time. This can cause TiCDC to repeatedly execute this DDL operation, and if retries take too long, it might lead to replication task failure. Starting from v8.4.0, if TiCDC has the `SUPER` permission of the downstream database, it periodically runs `ADMIN SHOW DDL JOBS` to check the status of asynchronously executed DDL tasks. TiCDC will wait for index creation to complete before proceeding with replication. Although this might increase replication latency, it avoids replication task failure.

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8 changes: 5 additions & 3 deletions ticdc/ticdc-faq.md
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Expand Up @@ -413,11 +413,13 @@ For TiCDC versions earlier than v6.5.2, it is recommended that you deploy TiCDC

## What is the order of executing DML and DDL statements?

Currently, TiCDC adopts the following order:
For most DDL statements, TiCDC adopts the following order:

1. TiCDC blocks the replication progress of the tables affected by DDL statements until the DDL `commitTS`. This ensures that DML statements executed before DDL `commitTS` can be successfully replicated to the downstream first.
2. TiCDC continues with the replication of DDL statements. If there are multiple DDL statements, TiCDC replicates them in a serial manner.
3. After the DDL statements are executed in the downstream, TiCDC will continue with the replication of DML statements executed after DDL `commitTS`.
2. TiCDC continues with the replication of DDL statements. If there are multiple DDL statements, TiCDC usually replicates them in a serial manner.
3. After the DDL statements are executed in the downstream, TiCDC continues with the replication of DML statements executed after DDL `commitTS`.

For `ADD INDEX` and `CREATE INDEX`, when the downstream is TiDB, TiCDC executes these DDLs asynchronously and does not wait for them to finish executing in the downstream before returning, to minimize the impact on changefeed replication latency. For more information, see [Asynchronous execution of `ADD INDEX` and `CREATE INDEX` DDLs](/ticdc/ticdc-ddl.md#asynchronous-execution-of-add-index-and-create-index-ddls).

## How should I check whether the upstream and downstream data is consistent?

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