add an example of using AUTO_RANDOM with PRE_SPLIT_REGIONS (#18564)

auto-random.md

@@ -47,7 +47,7 @@ When you execute an `INSERT` statement:
 - If you do not explicitly specify the value of the `AUTO_RANDOM` column, TiDB generates a random value and inserts it into the table.
 
 ```sql
-tidb> CREATE TABLE t (a BIGINT PRIMARY KEY AUTO_RANDOM, b VARCHAR(255));
+tidb> CREATE TABLE t (a BIGINT PRIMARY KEY AUTO_RANDOM, b VARCHAR(255)) /*T! PRE_SPLIT_REGIONS=2 */ ;
 Query OK, 0 rows affected, 1 warning (0.01 sec)
 
 tidb> INSERT INTO t(a, b) VALUES (1, 'string');
@@ -76,6 +76,29 @@ tidb> SELECT * FROM t;
 | 4899916394579099651 | string3 |
 +---------------------+---------+
 3 rows in set (0.00 sec)
+
+tidb> SHOW CREATE TABLE t;
++-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| Table | Create Table                                                                                                                                                                                                                                                    |
++-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| t     | CREATE TABLE `t` (
+  `a` bigint(20) NOT NULL /*T![auto_rand] AUTO_RANDOM(5) */,
+  `b` varchar(255) DEFAULT NULL,
+  PRIMARY KEY (`a`) /*T![clustered_index] CLUSTERED */
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin /*T! PRE_SPLIT_REGIONS=2 */ |
++-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+1 row in set (0.00 sec)
+
+tidb> SHOW TABLE t REGIONS;
++-----------+-----------------------------+-----------------------------+-----------+-----------------+---------------------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
+| REGION_ID | START_KEY                   | END_KEY                     | LEADER_ID | LEADER_STORE_ID | PEERS               | SCATTERING | WRITTEN_BYTES | READ_BYTES | APPROXIMATE_SIZE(MB) | APPROXIMATE_KEYS | SCHEDULING_CONSTRAINTS | SCHEDULING_STATE |
++-----------+-----------------------------+-----------------------------+-----------+-----------------+---------------------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
+|     62798 | t_158_                      | t_158_r_2305843009213693952 |     62810 |              28 | 62811, 62812, 62810 |          0 |           151 |          0 |                    1 |                0 |                        |                  |
+|     62802 | t_158_r_2305843009213693952 | t_158_r_4611686018427387904 |     62803 |               1 | 62803, 62804, 62805 |          0 |            39 |          0 |                    1 |                0 |                        |                  |
+|     62806 | t_158_r_4611686018427387904 | t_158_r_6917529027641081856 |     62813 |               4 | 62813, 62814, 62815 |          0 |           160 |          0 |                    1 |                0 |                        |                  |
+|      9289 | t_158_r_6917529027641081856 | 78000000                    |     48268 |               1 | 48268, 58951, 62791 |          0 |         10628 |      43639 |                    2 |             7999 |                        |                  |
++-----------+-----------------------------+-----------------------------+-----------+-----------------+---------------------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
+4 rows in set (0.00 sec)
 ```
 
 The `AUTO_RANDOM(S, R)` column value automatically assigned by TiDB has a total of 64 bits:
@@ -101,6 +124,7 @@ The structure of an `AUTO_RANDOM` value without a signed bit is as follows:
 - The content of the shard bits is obtained by calculating the hash value of the starting time of the current transaction. To use a different length of shard bits (such as 10), you can specify `AUTO_RANDOM(10)` when creating the table.
 - The value of the auto-increment bits is stored in the storage engine and allocated sequentially. Each time a new value is allocated, the value is incremented by 1. The auto-increment bits ensure that the values of `AUTO_RANDOM` are unique globally. When the auto-increment bits are exhausted, an error `Failed to read auto-increment value from storage engine` is reported when the value is allocated again.
 - Value range: the maximum number of bits for the final generated value = shard bits + auto-increment bits. The range of a signed column is `[-(2^(R-1))+1, (2^(R-1))-1]`, and the range of an unsigned column is `[0, (2^R)-1]`.
+- You can use `AUTO_RANDOM` with `PRE_SPLIT_REGIONS`. When a table is created successfully, `PRE_SPLIT_REGIONS` pre-splits data in the table into the number of Regions as specified by `2^(PRE_SPLIT_REGIONS)`.
 
 > **Note:**
 >