`tidb_replica_read` looks not available on TiDB Cloud Essential needs documented

[yahonda]

File: /release-8.5/system-variables.md

As of right now, tidb_replica_read document does not mention any restriction on TiDB Cloud Essential, Actually this sysvar cannot be mofified as follows.

• Connect to TiDB Cloud Essential cluster as root user.

mysql> select version();
+-------------------------------+
| version()                     |
+-------------------------------+
| 8.0.11-TiDB-v7.5.6-serverless |
+-------------------------------+
1 row in set (0.01 sec)

mysql> select user();
+-------------------------------------+
| user()                              |
+-------------------------------------+
| KfZ5tZ8fbf2UGaZ.root@153.240.206.13 |
+-------------------------------------+
1 row in set (0.01 sec)

• set global gets the Access denied; you need (at least one of) the RESTRICTED_VARIABLES_ADMIN privilege(s) for this operation warning and the value is not changed.

mysql> select @@session.tidb_replica_read, @@global.tidb_replica_read;
+-----------------------------+----------------------------+
| @@session.tidb_replica_read | @@global.tidb_replica_read |
+-----------------------------+----------------------------+
| leader                      | leader                     |
+-----------------------------+----------------------------+
1 row in set (0.01 sec)

mysql> set global tidb_replica_read = 'closest-adaptive';
Query OK, 0 rows affected, 1 warning (0.02 sec)

mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                  |
+---------+------+----------------------------------------------------------------------------------------------------------+
| Warning | 1227 | Access denied; you need (at least one of) the RESTRICTED_VARIABLES_ADMIN privilege(s) for this operation |
+---------+------+----------------------------------------------------------------------------------------------------------+
1 row in set (0.02 sec)

mysql> select @@session.tidb_replica_read, @@global.tidb_replica_read;
+-----------------------------+----------------------------+
| @@session.tidb_replica_read | @@global.tidb_replica_read |
+-----------------------------+----------------------------+
| leader                      | leader                     |
+-----------------------------+----------------------------+
1 row in set (0.01 sec)

• set session gets the Access denied; you need (at least one of) the RESTRICTED_VARIABLES_ADMIN privilege(s) for this operation warning and the value is not changed.

mysql> set session tidb_replica_read = 'closest-adaptive';
Query OK, 0 rows affected, 1 warning (0.02 sec)

mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                  |
+---------+------+----------------------------------------------------------------------------------------------------------+
| Warning | 1227 | Access denied; you need (at least one of) the RESTRICTED_VARIABLES_ADMIN privilege(s) for this operation |
+---------+------+----------------------------------------------------------------------------------------------------------+
1 row in set (0.02 sec)

mysql> select @@session.tidb_replica_read, @@global.tidb_replica_read;
+-----------------------------+----------------------------+
| @@session.tidb_replica_read | @@global.tidb_replica_read |
+-----------------------------+----------------------------+
| leader                      | leader                     |
+-----------------------------+----------------------------+
1 row in set (0.01 sec)

mysql>