Extend DDL/MDL info (#20329)

TOC-tidb-cloud.md

@@ -622,6 +622,7 @@
     - System Tables
       - `mysql` Schema
         - [Overview](/mysql-schema/mysql-schema.md)
+        - [`tidb_mdl_view`](/mysql-schema/mysql-schema-tidb-mdl-view.md)
         - [`user`](/mysql-schema/mysql-schema-user.md)
       - INFORMATION_SCHEMA
         - [Overview](/information-schema/information-schema.md)

TOC.md

@@ -960,6 +960,7 @@
     - System Tables
       - `mysql` Schema
         - [Overview](/mysql-schema/mysql-schema.md)
+        - [`tidb_mdl_view`](/mysql-schema/mysql-schema-tidb-mdl-view.md)
         - [`user`](/mysql-schema/mysql-schema-user.md)
       - INFORMATION_SCHEMA
         - [Overview](/information-schema/information-schema.md)

ddl-introduction.md

@@ -184,6 +184,11 @@ When TiDB is adding an index, the phase of backfilling data will cause read and
 
     You can only resume a paused DDL task. Otherwise, the `Job 3 can't be resumed` error is shown in the `RESULT` column.
 
+## DDL-related tables
+
+- [`information_schema.DDL_JOBS`](/information-schema/information-schema-ddl-jobs.md): Information about currently running and finished DDL jobs.
+- [`mysql.tidb_mdl_view`](/mysql-schema/mysql-schema-tidb-mdl-view.md): Information about [metadata lock](/metadata-lock.md) views. It can help identify what query is blocking the DDL from making progress.
+
 ## Common questions
 
 For common questions about DDL execution, see [SQL FAQ - DDL execution](https://docs.pingcap.com/tidb/stable/sql-faq).

metadata-lock.md

@@ -77,29 +77,33 @@ TiDB v6.3.0 introduces the `mysql.tidb_mdl_view` view to help you obtain the inf
 The following takes adding an index for table `t` as an example. Assume that there is a DDL statement `ALTER TABLE t ADD INDEX idx(a)`:
 
 ```sql
-SELECT * FROM mysql.tidb_mdl_view\G
+TABLE mysql.tidb_mdl_view\G
+```
+
+```
 *************************** 1. row ***************************
-    JOB_ID: 141
-   DB_NAME: test
-TABLE_NAME: t
-     QUERY: ALTER TABLE t ADD INDEX idx(a)
-SESSION ID: 2199023255957
-  TxnStart: 08-30 16:35:41.313(435643624013955072)
+     job_id: 118
+    db_name: test
+ table_name: t
+      query: ALTER TABLE t ADD COLUMN c INT
+ session_id: 1547698182
+ start_time: 2025-03-19 09:52:36.509000
 SQL_DIGESTS: ["begin","select * from `t`"]
-1 row in set (0.02 sec)
+1 row in set (0.00 sec)
+
 ```
 
-From the preceding output, you can see that the transaction whose `SESSION ID` is `2199023255957` blocks the `ADD INDEX` DDL. `SQL_DIGEST` shows the SQL statements executed by this transaction, which is ``["begin","select * from `t`"]``. To make the blocked DDL continue to execute, you can use the following global `KILL` statement to kill the `2199023255957` transaction:
+From the preceding output, you can see that the transaction whose `SESSION ID` is `1547698182` blocks the `ADD COLUMN` DDL. `SQL_DIGEST` shows the SQL statements executed by this transaction, which is ``["begin","select * from `t`"]``. To make the blocked DDL continue to execute, you can use the following global `KILL` statement to kill the `1547698182` transaction:
 
 ```sql
-mysql> KILL 2199023255957;
+mysql> KILL 1547698182;
 Query OK, 0 rows affected (0.00 sec)
 ```
 
 After killing the transaction, you can select the `mysql.tidb_mdl_view` view again. At this time, the preceding transaction is not shown in the output, which means the DDL is not blocked.
 
 ```sql
-SELECT * FROM mysql.tidb_mdl_view\G
+TABLE mysql.tidb_mdl_view\G
 Empty set (0.01 sec)
 ```
 

mysql-schema/mysql-schema-tidb-mdl-view.md

@@ -0,0 +1,39 @@
+---
+title: mysql.tidb_mdl_view
+summary: Learn about the `tidb_mdl_view` table in the `mysql` schema.
+---
+
+# `mysql.tidb_mdl_view`
+
+This table shows the information about the [metadata lock](/metadata-lock.md) views.
+
+```sql
+DESC mysql.tidb_mdl_view;
+```
+
+The output is as follows:
+
+```
++-------------+-----------------+------+------+---------+-------+
+| Field       | Type            | Null | Key  | Default | Extra |
++-------------+-----------------+------+------+---------+-------+
+| job_id      | bigint          | NO   | PRI  | NULL    |       |
+| db_name     | longtext        | YES  |      | NULL    |       |
+| table_name  | longtext        | YES  |      | NULL    |       |
+| query       | longtext        | YES  |      | NULL    |       |
+| session_id  | bigint unsigned | YES  |      | NULL    |       |
+| start_time  | timestamp(6)    | YES  |      | NULL    |       |
+| SQL_DIGESTS | varchar(5)      | YES  |      | NULL    |       |
++-------------+-----------------+------+------+---------+-------+
+7 rows in set (0.00 sec)
+```
+
+## Fields
+
+* `job_id`: The identifier of the job.
+* `db_name`: The database name.
+* `table_name`: The table name.
+* `query`: The query.
+* `session_id`: The identifier of the session.
+* `start_time`: The start time. This column was called `TxnStart` in earlier versions.
+* `SQL_DIGESTS`: The digests of the SQL statements.

mysql-schema/mysql-schema.md

@@ -91,7 +91,7 @@ Currently, the `help_topic` is NULL.
 
 ## System tables related to metadata locks
 
-* `tidb_mdl_view`: a view of metadata locks. You can use it to view information about the currently blocked DDL statements. See also [Metadata Lock](/metadata-lock.md).
+* [`tidb_mdl_view`](/mysql-schema/mysql-schema-tidb-mdl-view.md): a view of metadata locks. You can use it to view the information about the currently blocked DDL statements. See also [Metadata Lock](/metadata-lock.md).
 * `tidb_mdl_info`: used internally by TiDB to synchronize metadata locks across nodes.
 
 ## System tables related to DDL statements