Admin check failed due to memory limit from `tidb_mem_quota_query`

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> select count(1) from sbtest1;
+-----------+
| count(1)  |
+-----------+
| 100000000 |
+-----------+
1 row in set (2.97 sec)

mysql> show create table sbtest1;
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table
                                                         |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| sbtest1 | CREATE TABLE `sbtest1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `k` int(11) NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */,
  KEY `k_1` (`k`),
  KEY `idx` (`c`,`pad`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=100438895 |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

2. What did you expect to see? (Required)

Admin check success.

3. What did you see instead (Required)

mysql> admin check index sbtest1 idx;
ERROR 8175 (HY000): Your query has been cancelled due to exceeding the allowed memory limit for a single SQL query. Please try narrowing your query scope or increase the tidb_mem_quota_query limit and try again.[conn=0]

mysql> set session tidb_mem_quota_query = -1;
Query OK, 0 rows affected (0.00 sec)
mysql> admin check index sbtest1 idx;
ERROR 8175 (HY000): Your query has been cancelled due to exceeding the allowed memory limit for a single SQL query. Please try narrowing your query scope or increase the tidb_mem_quota_query limit and try again.[conn=0]

mysql> set global tidb_mem_quota_query = -1;
Query OK, 0 rows affected (0.03 sec)
mysql> admin check index sbtest1 idx;
ERROR 8175 (HY000): Your query has been cancelled due to exceeding the allowed memory limit for a single SQL query. Please try narrowing your query scope or increase the tidb_mem_quota_query limit and try again.[conn=0]

4. What is your TiDB version? (Required)

be62f7