Add observability for ALTER TABLE ... COMPACT

sql-statements/sql-statement-alter-table-compact.md

@@ -89,11 +89,108 @@ To obtain greater table-level concurrency with higher resource usage, you can mo
 
 </CustomContent>
 
-## MySQL compatibility
+## Observe data compaction progress
+
+You can observe the progress of data compaction or determine whether to initiate compaction for a table by checking the `TOTAL_DELTA_ROWS` column in the `INFORMATION_SCHEMA.TIFLASH_TABLES` table. The larger the value of `TOTAL_DELTA_ROWS`, the more data that can be compacted. If `TOTAL_DELTA_ROWS` is `0`, all data in the table is in the best state and does not need to be compacted.
+
+<details>
+  <summary>Example: Check the compaction state of a common table</summary>
+
+```sql
+USE test;
+CREATE TABLE foo(id INT);
+ALTER TABLE foo SET TIFLASH REPLICA 1;
+SELECT TOTAL_DELTA_ROWS, TOTAL_STABLE_ROWS FROM INFORMATION_SCHEMA.TIFLASH_TABLES
+    WHERE IS_TOMBSTONE = 0 AND
+    `TIDB_DATABASE` = "test" AND `TIDB_TABLE` = "foo";
++------------------+-------------------+
+| TOTAL_DELTA_ROWS | TOTAL_STABLE_ROWS |
++------------------+-------------------+
+|                0 |                 0 |
++------------------+-------------------+
+INSERT INTO foo VALUES (1), (3), (7);
+SELECT TOTAL_DELTA_ROWS, TOTAL_STABLE_ROWS FROM INFORMATION_SCHEMA.TIFLASH_TABLES
+    WHERE IS_TOMBSTONE = 0 AND
+    `TIDB_DATABASE` = "test" AND `TIDB_TABLE` = "foo";
++------------------+-------------------+
+| TOTAL_DELTA_ROWS | TOTAL_STABLE_ROWS |
++------------------+-------------------+
+|                3 |                 0 |
++------------------+-------------------+
+-- Newly written data can be compacted
+ALTER TABLE foo COMPACT TIFLASH REPLICA;
+SELECT TOTAL_DELTA_ROWS, TOTAL_STABLE_ROWS FROM INFORMATION_SCHEMA.TIFLASH_TABLES
+    WHERE IS_TOMBSTONE = 0 AND
+    `TIDB_DATABASE` = "test" AND `TIDB_TABLE` = "foo";
++------------------+-------------------+
+| TOTAL_DELTA_ROWS | TOTAL_STABLE_ROWS |
++------------------+-------------------+
+|                0 |                 3 |
++------------------+-------------------+
+-- All data is in the best state and no compaction is required
+```
+
+</details>
+
+<details>
+  <summary>Example: Check the compaction state of partitioned tables</summary>
+
+```sql
+USE test;
+CREATE TABLE employees
+    (id INT NOT NULL, store_id INT)
+    PARTITION BY LIST (store_id) (
+        PARTITION pNorth VALUES IN (1, 2, 3, 4, 5),
+        PARTITION pEast VALUES IN (6, 7, 8, 9, 10),
+        PARTITION pWest VALUES IN (11, 12, 13, 14, 15),
+        PARTITION pCentral VALUES IN (16, 17, 18, 19, 20)
+    );
+ALTER TABLE employees SET TIFLASH REPLICA 1;
+INSERT INTO employees VALUES (1, 1), (6, 6), (10, 10);
+SELECT PARTITION_NAME, TOTAL_DELTA_ROWS, TOTAL_STABLE_ROWS
+    FROM INFORMATION_SCHEMA.TIFLASH_TABLES t, INFORMATION_SCHEMA.PARTITIONS p
+    WHERE t.IS_TOMBSTONE = 0 AND t.TABLE_ID = p.TIDB_PARTITION_ID AND
+    p.TABLE_SCHEMA = "test" AND p.TABLE_NAME = "employees";
++----------------+------------------+-------------------+
+| PARTITION_NAME | TOTAL_DELTA_ROWS | TOTAL_STABLE_ROWS |
++----------------+------------------+-------------------+
+| pNorth         |                1 |                 0 |
+| pEast          |                2 |                 0 |
+| pWest          |                0 |                 0 |
+| pCentral       |                0 |                 0 |
++----------------+------------------+-------------------+
+-- Some partitioned tables can be compacted
+ALTER TABLE employees COMPACT TIFLASH REPLICA;
+SELECT PARTITION_NAME, TOTAL_DELTA_ROWS, TOTAL_STABLE_ROWS
+    FROM INFORMATION_SCHEMA.TIFLASH_TABLES t, INFORMATION_SCHEMA.PARTITIONS p
+    WHERE t.IS_TOMBSTONE = 0 AND t.TABLE_ID = p.TIDB_PARTITION_ID AND
+    p.TABLE_SCHEMA = "test" AND p.TABLE_NAME = "employees";
++----------------+------------------+-------------------+
+| PARTITION_NAME | TOTAL_DELTA_ROWS | TOTAL_STABLE_ROWS |
++----------------+------------------+-------------------+
+| pNorth         |                0 |                 1 |
+| pEast          |                0 |                 2 |
+| pWest          |                0 |                 0 |
+| pCentral       |                0 |                 0 |
++----------------+------------------+-------------------+
+-- Data in al partitions is in the best state and no compaction is required
+```
+
+</details>
+
+> **Note:**
+>
+> - If data is updated during compaction, `TOTAL_DELTA_ROWS` might still be a non-zero value after compaction is done. This is normal and indicates that these updates have not been compacted. To compact these updates, execute the `ALTER TABLE ... COMPACT` statement again.
+>
+> - `TOTAL_DELTA_ROWS` indicates the number of data versions, not the number of rows. For example, if you insert a row and then delete it, `TOTAL_DELTA_ROWS` will increase by 2.
+
+## Compatibility
+
+### MySQL compatibility
 
 The `ALTER TABLE ... COMPACT` syntax is TiDB specific, which is an extension to the standard SQL syntax. Although there is no equivalent MySQL syntax, you can still execute this statement by using MySQL clients or various database drivers that comply with the MySQL protocol.
 
-## TiDB Binlog and TiCDC compatibility
+### TiDB Binlog and TiCDC compatibility
 
 The `ALTER TABLE ... COMPACT` statement does not result in logical data changes and are therefore not replicated to downstream by TiDB Binlog or TiCDC.