主要是因为tidb 自动收集统计信息，下面的统计信息参数都设置成1 了，建议大表都手工设置定时任务去收集
查看当前值
mysql> show variables like ‘tidb_build_stats_concurrency’;
±-----------------------------±------+
| Variable_name | Value |
±-----------------------------±------+
| tidb_build_stats_concurrency | 4 |
±-----------------------------±------+
1 row in set (0.00 sec)
mysql> show variables like ‘tidb_distsql_scan_concurrency’;
±------------------------------±------+
| Variable_name | Value |
±------------------------------±------+
| tidb_distsql_scan_concurrency | 15 |
±------------------------------±------+
1 row in set (0.00 sec)
mysql> show variables like ‘tidb_index_serial_scan_concurrency’;
±-----------------------------------±------+
| Variable_name | Value |
±-----------------------------------±------+
| tidb_index_serial_scan_concurrency | 1 |
±-----------------------------------±------+
1 row in set (0.01 sec)