schema_unused_indexes
schema_unused_indexes 用于记录自 TiDB 上次启动以来未被使用的索引信息，包括如下列：
OBJECT_SCHEMA：索引所在表的所属数据库的名称。OBJECT_NAME：索引所在表的名称。INDEX_NAME：索引的名称。
USE SYS;
DESC SCHEMA_UNUSED_INDEXES;
输出结果如下：
+---------------+-------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+-------------+------+------+---------+-------+
| object_schema | varchar(64) | YES | | NULL | |
| object_name | varchar(64) | YES | | NULL | |
| index_name | varchar(64) | YES | | NULL | |
+---------------+-------------+------+------+---------+-------+
3 rows in set (0.00 sec)
手动创建 schema_unused_indexes 视图
对于从 v8.0.0 之前版本升级的集群，sys Schema 和其中的视图不会自动创建。你可以通过以下 SQL 语句手动创建：
CREATE DATABASE IF NOT EXISTS sys;
CREATE OR REPLACE VIEW sys.schema_unused_indexes AS
SELECT
table_schema as object_schema,
table_name as object_name,
index_name
FROM information_schema.cluster_tidb_index_usage
WHERE
table_schema not in ('sys', 'mysql', 'INFORMATION_SCHEMA', 'PERFORMANCE_SCHEMA') and
index_name != 'PRIMARY'
GROUP BY table_schema, table_name, index_name
HAVING
sum(last_access_time) is null;