add system variable 'tidb_enable_clustered_index' for v5.0

system-variables.md

@@ -285,6 +285,22 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - Default value: 0
 - This variable is used to control whether to enable the cascades planner, which is currently considered experimental.
 
+### tidb_enable_clustered_index <!-- New in v5.0 -->
+
+- Scope: SESSION | GLOBAL
+- Default value: 1
+- This variable is used to control whether to enable the clustered index feature.
+    - This feature is only applicable to newly created tables and does not affect the existing old tables.
+    - This feature is only applicable to tables whose primary key is the single-column non-integer type or the multi-column type. It does not affect the tables without a primary key or tables with the primary key of the single-column non-integer type.
+    - You can execute `select tidb_pk_type from information_schema.tables where table_name ='{table_name}'` to check whether the clustered index feature has been enabled on a table.
+- After you enable this feature, rows are stored directly on the primary key instead of on the internally allocated `rows_id` to which the extra primary key index is created to point.
+
+    This feature impacts performance in the following aspects:
+    - For each `INSERT` operation, there is one less index key written into each row.
+    - When you make a query using the primary key as the equivalent condition, one read request can be saved.
+    - When you make a query using the primary key as the range condition, multiple read requests can be saved.
+    - When you make a query using the prefix of the multi-column primary key as the equivalent condition or range condition, multiple read requests can be saved.
+
 ### tidb_enable_chunk_rpc <span class="version-mark">New in v4.0</span>
 
 - Scope: SESSION