docs: improve table sharding repartition flow

docs/user-guide/deployments-administration/manage-data/table-sharding.md

@@ -1,6 +1,6 @@
 ---
 keywords: [table sharding, GreptimeDB, partitioning methods, distributed tables, data management, SQL, query performance]
-description: Explains table sharding in GreptimeDB, including when to shard a table, partitioning methods, creating distributed tables, inserting data, querying data, and inspecting sharded tables.
+description: Explains table sharding in GreptimeDB, including when to shard a table, partitioning methods, creating and partitioning distributed tables, inserting data, querying data, and inspecting sharded tables.
 ---
 
 # Table Sharding
@@ -69,9 +69,7 @@ We recommend partitioning on Tag columns (the primary key columns). This matters
 The parentheses in `PARTITION ON COLUMNS (...)` only accept column names (e.g., `device_id`, `area`), not expressions like `device_id + 1`. GreptimeDB does not support MySQL's `PARTITION BY RANGE` syntax.
 :::
 
-### Example
-
-## Create a distributed table
+## Create a partitioned table
 
 You can use the MySQL CLI to [connect to GreptimeDB](/user-guide/protocols/mysql.md) and create a distributed table.
 The following example creates a table and partitions it based on the `device_id` column.
@@ -112,9 +110,51 @@ PARTITION ON COLUMNS (device_id, area) (
 
 The following content uses the `sensor_readings` table with two partition columns as an example.
 
-## Repartition a sharded table
+## Add partitions to an existing table
+
+You can also partition an existing unpartitioned table with `ALTER TABLE ... PARTITION ON COLUMNS`.
+The syntax is similar to creating a partitioned table, except that the partition rule is added to an existing table:
+
+```sql
+ALTER TABLE sensor_readings PARTITION ON COLUMNS (device_id, area) (
+  device_id < 100 AND area < 'South',
+  device_id < 100 AND area >= 'South',
+  device_id >= 100 AND area <= 'East',
+  device_id >= 100 AND area > 'East'
+);
+```
+
+This is useful when a table starts as a single-partition table and later needs to be distributed across multiple regions.
+
+:::warning Warning
+Partitioning an existing table is a repartitioning operation. It is only supported in distributed clusters and requires shared object storage and GC. For the full prerequisites, see [Repartition](/user-guide/deployments-administration/manage-data/repartition.md#prerequisites).
+:::
+
+## Adjust partition rules
+
+After a table is partitioned, you can keep tuning its partition layout by splitting hot partitions or merging small cold partitions.
+
+Use `SPLIT PARTITION` to split an existing partition into multiple partitions:
+
+```sql
+ALTER TABLE sensor_readings SPLIT PARTITION (
+  device_id < 100 AND area < 'South'
+) INTO (
+  device_id < 50 AND area < 'South',
+  device_id >= 50 AND device_id < 100 AND area < 'South'
+);
+```
+
+Use `MERGE PARTITION` to merge multiple partitions into one:
+
+```sql
+ALTER TABLE sensor_readings MERGE PARTITION (
+  device_id < 100 AND area < 'South',
+  device_id < 100 AND area >= 'South'
+);
+```
 
-If you need to modify partition rules for an existing table, refer to the separate [Repartition](/user-guide/deployments-administration/manage-data/repartition.md) page.
+For prerequisites, hotspot diagnosis, and more step-by-step examples, see [Repartition](/user-guide/deployments-administration/manage-data/repartition.md).
 
 ## Insert data into the table
 

i18n/zh/docusaurus-plugin-content-docs/current/user-guide/deployments-administration/manage-data/table-sharding.md

@@ -1,6 +1,6 @@
 ---
 keywords: [表分片, 分区规则, 分布式表, 插入数据, 分布式查询]
-description: 介绍表分片技术及其在 GreptimeDB 中的应用，包括分片时机、分区规则、创建分布式表、插入数据和分布式查询等内容。
+description: 介绍表分片技术及其在 GreptimeDB 中的应用，包括分片时机、分区规则、创建和调整分布式表、插入数据和分布式查询等内容。
 ---
 
 # 表分片
@@ -69,9 +69,7 @@ PARTITION ON COLUMNS (<COLUMN LIST>) (
 `PARTITION ON COLUMNS` 括号里只能写列名（如 `device_id`、`area`），不支持表达式。GreptimeDB 不支持 MySQL 的 `PARTITION BY RANGE` 语法。
 :::
 
-### 示例
-
-## 创建分布式表
+## 创建分区表
 
 你可以使用 MySQL CLI [连接到 GreptimeDB](/user-guide/protocols/mysql.md) 并创建一个分布式表。
 下方的示例创建了一个表并基于 `device_id` 列进行分区。
@@ -112,9 +110,51 @@ PARTITION ON COLUMNS (device_id, area) (
 
 以下内容以具有两个分区列的 `sensor_readings` 表为例。
 
-## 重分区
+## 为已有表添加分区
+
+你也可以使用 `ALTER TABLE ... PARTITION ON COLUMNS` 为已有的未分区表创建分区。
+它的语法和创建分区表类似，只是把分区规则添加到已有表上：
+
+```sql
+ALTER TABLE sensor_readings PARTITION ON COLUMNS (device_id, area) (
+  device_id < 100 AND area < 'South',
+  device_id < 100 AND area >= 'South',
+  device_id >= 100 AND area <= 'East',
+  device_id >= 100 AND area > 'East'
+);
+```
+
+当一张表最初是单分区表，后续需要分布到多个 region 上时，可以使用这种方式。
+
+:::warning 警告
+为已有表创建分区属于重分区操作，仅支持分布式集群，并且需要共享对象存储和 GC。完整前置条件请参考[重分区](/user-guide/deployments-administration/manage-data/repartition.md#前置条件)。
+:::
+
+## 调整分区规则
+
+表分区后，你可以继续通过拆分热点分区或合并较小的冷分区来调整分区布局。
+
+使用 `SPLIT PARTITION` 将已有分区拆分为多个分区：
+
+```sql
+ALTER TABLE sensor_readings SPLIT PARTITION (
+  device_id < 100 AND area < 'South'
+) INTO (
+  device_id < 50 AND area < 'South',
+  device_id >= 50 AND device_id < 100 AND area < 'South'
+);
+```
+
+使用 `MERGE PARTITION` 将多个分区合并为一个分区：
+
+```sql
+ALTER TABLE sensor_readings MERGE PARTITION (
+  device_id < 100 AND area < 'South',
+  device_id < 100 AND area >= 'South'
+);
+```
 
-如果你需要修改已创建表的分区规则，请参考单独的 [重分区](/user-guide/deployments-administration/manage-data/repartition.md) 页面。
+如需了解前置条件、热点分区判断方法和更多分步示例，请参考[重分区](/user-guide/deployments-administration/manage-data/repartition.md)。
 
 ## 向表中插入数据