ticdc: add event filter example (#20186)

ticdc/ticdc-ddl.md

@@ -79,12 +79,12 @@ TiCDC processes this type of DDL as follows:
 | `RENAME TABLE test.t1 TO test.t2` | Replicate | `test.t1` matches the filter rule |
 | `RENAME TABLE test.t1 TO ignore.t1` | Replicate | `test.t1` matches the filter rule |
 | `RENAME TABLE ignore.t1 TO ignore.t2` | Ignore | `ignore.t1` does not match the filter rule |
-| `RENAME TABLE test.n1 TO test.t1` | Report an error and exit the replication | `test.n1` does not match the filter rule, but `test.t1` matches the filter rule. This operation is illegal. In this case, refer to the error message for handling. |
+| `RENAME TABLE test.n1 TO test.t1` | Report an error and exit the replication | The old table name `test.n1` does not match the filter rule, but the new table name `test.t1` matches the filter rule. This operation is illegal. In this case, refer to the error message for handling. |
 | `RENAME TABLE ignore.t1 TO test.t1` | Report an error and exit the replication | Same reason as above. |
 
 #### Rename multiple tables in a DDL statement
 
-If a DDL statement renames multiple tables, TiCDC only replicates the DDL statement when the old database name, old table names, and the new database name all match the filter rule.
+If a DDL statement renames multiple tables, TiCDC replicates the DDL statement only when the **old database name**, **old table names**, and **new database name** all match the filter rule.
 
 In addition, TiCDC does not support the `RENAME TABLE` DDL that swaps the table names. The following is an example.
 
@@ -130,3 +130,32 @@ CREATE TABLE "t1" ("a" int PRIMARY KEY);
 Because in the default SQL mode of TiDB, double quotation marks are treated as strings rather than identifiers, TiCDC fails to parse the DDL statement correctly.
 
 Therefore, when creating a replication task, it is recommended that you specify the SQL mode used by the upstream TiDB cluster in the configuration file.
+
+### Notes on using event filter rules to filter DDL events
+
+If a filtered DDL statement involves table creation or deletion, TiCDC only filters out the DDL statement without affecting the replication behavior of DML statements. The following is an example.
+
+Assume that the configuration file of your changefeed is as follows:
+
+```toml
+[filter]
+rules = ['test.t*']
+
+matcher = ["test.t1"] # This filter rule applies only to the t1 table in the test database.
+ignore-event = ["create table", "drop table", "truncate table", "rename table"]
+```
+
+| DDL | DDL behavior | DML behavior | Explanation |
+| --- | --- | --- | --- |
+| `CREATE TABLE test.t1 (id INT, name VARCHAR(50));` | Ignore | Replicate | `test.t1` matches the event filter rule, so the `CREATE TABLE` event is ignored. The replication of DML events remains unaffected. |
+| `CREATE TABLE test.t2 (id INT, name VARCHAR(50));` | Replicate | Replicate | `test.t2` does not match the event filter rule. |
+| `CREATE TABLE test.ignore (id INT, name VARCHAR(50));` | Ignore | Ignore | `test.ignore` matches the event filter rule, so both DDL and DML events are ignored. |
+| `DROP TABLE test.t1;` | Ignore | - | `test.t1` matches the event filter rule, so the `DROP TABLE` event is ignored. Because the table is deleted, TiCDC no longer replicates DML events for `t1`. |
+| `TRUNCATE TABLE test.t1;` | Ignore | Replicate | `test.t1` matches the event filter rule, so the `TRUNCATE TABLE` event is ignored. The replication of DML events remains unaffected. |
+| `RENAME TABLE test.t1 TO test.t2;` | Ignore | Replicate | `test.t1` matches the event filter rule, so the `RENAME TABLE` event is ignored. The replication of DML events remains unaffected. |
+| `RENAME TABLE test.t1 TO test.ignore;` | Ignore | Ignore | `test.t1` matches the event filter rule, so the `RENAME TABLE` event is ignored. `test.ignore` matches the event filter rule, so both DDL and DML events are ignored. |
+
+> **Note:**
+>
+> - When replicating data to a database, use the event filter to filter DDL events with caution. Ensure that the upstream and downstream database schemas remain consistent during replication. Otherwise, TiCDC might report errors or cause undefined replication behavior.
+> - For versions earlier than v6.5.8, v7.1.4, and v7.5.1, using the event filter to filter DDL events involving table creation or deletion affects DML replication. It is not recommended to use this feature in these versions.