KEY_COLUMN_USAGE
KEY_COLUMN_USAGE 表描述列的键约束，例如时间索引键的约束。
sql
USE INFORMATION_SCHEMA;
DESC KEY_COLUMN_USAGE;USE INFORMATION_SCHEMA;
DESC KEY_COLUMN_USAGE;结果如下：
sql
+-------------------------------+--------+------+------+---------+---------------+
| Column | Type | Key | Null | Default | Semantic Type |
+-------------------------------+--------+------+------+---------+---------------+
| constraint_catalog | String | | NO | | FIELD |
| constraint_schema | String | | NO | | FIELD |
| constraint_name | String | | NO | | FIELD |
| table_catalog | String | | NO | | FIELD |
| real_table_catalog | String | | NO | | FIELD |
| table_schema | String | | NO | | FIELD |
| table_name | String | | NO | | FIELD |
| column_name | String | | NO | | FIELD |
| ordinal_position | UInt32 | | NO | | FIELD |
| position_in_unique_constraint | UInt32 | | YES | | FIELD |
| referenced_table_schema | String | | YES | | FIELD |
| referenced_table_name | String | | YES | | FIELD |
| referenced_column_name | String | | YES | | FIELD |
+-------------------------------+--------+------+------+---------+---------------++-------------------------------+--------+------+------+---------+---------------+
| Column | Type | Key | Null | Default | Semantic Type |
+-------------------------------+--------+------+------+---------+---------------+
| constraint_catalog | String | | NO | | FIELD |
| constraint_schema | String | | NO | | FIELD |
| constraint_name | String | | NO | | FIELD |
| table_catalog | String | | NO | | FIELD |
| real_table_catalog | String | | NO | | FIELD |
| table_schema | String | | NO | | FIELD |
| table_name | String | | NO | | FIELD |
| column_name | String | | NO | | FIELD |
| ordinal_position | UInt32 | | NO | | FIELD |
| position_in_unique_constraint | UInt32 | | YES | | FIELD |
| referenced_table_schema | String | | YES | | FIELD |
| referenced_table_name | String | | YES | | FIELD |
| referenced_column_name | String | | YES | | FIELD |
+-------------------------------+--------+------+------+---------+---------------+sql
SELECT * FROM key_column_usage WHERE table_schema='public' and table_name='monitor'\GSELECT * FROM key_column_usage WHERE table_schema='public' and table_name='monitor'\Gsql
*************************** 1. row ***************************
constraint_catalog: def
constraint_schema: public
constraint_name: TIME INDEX
table_catalog: def
real_table_catalog: greptime
table_schema: public
table_name: monitor
column_name: ts
ordinal_position: 1
position_in_unique_constraint: NULL
referenced_table_schema: NULL
referenced_table_name: NULL
referenced_column_name: NULL
*************************** 2. row ***************************
constraint_catalog: def
constraint_schema: public
constraint_name: PRIMARY
table_catalog: def
real_table_catalog: greptime
table_schema: public
table_name: monitor
column_name: host
ordinal_position: 1
position_in_unique_constraint: NULL
referenced_table_schema: NULL
referenced_table_name: NULL
referenced_column_name: NULL
2 rows in set (0.02 sec)*************************** 1. row ***************************
constraint_catalog: def
constraint_schema: public
constraint_name: TIME INDEX
table_catalog: def
real_table_catalog: greptime
table_schema: public
table_name: monitor
column_name: ts
ordinal_position: 1
position_in_unique_constraint: NULL
referenced_table_schema: NULL
referenced_table_name: NULL
referenced_column_name: NULL
*************************** 2. row ***************************
constraint_catalog: def
constraint_schema: public
constraint_name: PRIMARY
table_catalog: def
real_table_catalog: greptime
table_schema: public
table_name: monitor
column_name: host
ordinal_position: 1
position_in_unique_constraint: NULL
referenced_table_schema: NULL
referenced_table_name: NULL
referenced_column_name: NULL
2 rows in set (0.02 sec)KEY_COLUMN_USAGE 表中列的描述如下：
constraint_catalog：约束所属的目录名称。该值始终为def。constraint_schema：约束所属的数据库名称。constraint_name：约束的名称。table_catalog：表所属目录的名称。该值始终为def。real_table_catalog：表所属目录的真实名称。该值始终为greptime。table_schema：表所属的数据库名称。table_name：具有约束的表的名称。column_name：具有约束的列的名称。ordinal_position：列在约束中的位置，而不是在表中的位置。位置编号从1开始。position_in_unique_constraint：唯一约束和主键约束为空。对于外键约束，此列是引用表键的位置。referenced_table_schema：约束引用的数据库名称。目前在 GreptimeDB 中，除外键约束外，所有约束中此列的值均为NULL。referenced_table_name：约束引用的表名称。目前在 GreptimeDB 中，除外键约束外，所有约束中此列的值均为NULL。referenced_column_name：约束引用的列名称。目前在 GreptimeDB 中，除外键约束外，所有约束中此列的值均为NULL。