COLUMNS
COLUMNS 提供了关于表中每列的详细信息。
USE INFORMATION_SCHEMA;
DESC COLUMNS;
结果如下：
+--------------------------+--------+------+------+---------+---------------+
| Column | Type | Key | Null | Default | Semantic Type |
+--------------------------+--------+------+------+---------+---------------+
| table_catalog | String | | NO | | FIELD |
| table_schema | String | | NO | | FIELD |
| table_name | String | | NO | | FIELD |
| column_name | String | | NO | | FIELD |
| ordinal_position | Int64 | | NO | | FIELD |
| character_maximum_length | Int64 | | YES | | FIELD |
| character_octet_length | Int64 | | YES | | FIELD |
| numeric_precision | Int64 | | YES | | FIELD |
| numeric_scale | Int64 | | YES | | FIELD |
| datetime_precision | Int64 | | YES | | FIELD |
| character_set_name | String | | YES | | FIELD |
| collation_name | String | | YES | | FIELD |
| column_key | String | | NO | | FIELD |
| extra | String | | NO | | FIELD |
| privileges | String | | NO | | FIELD |
| generation_expression | String | | NO | | FIELD |
| greptime_data_type | String | | NO | | FIELD |
| data_type | String | | NO | | FIELD |
| semantic_type | String | | NO | | FIELD |
| column_default | String | | YES | | FIELD |
| is_nullable | String | | NO | | FIELD |
| column_type | String | | NO | | FIELD |
| column_comment | String | | YES | | FIELD |
| srs_id | Int64 | | YES | | FIELD |
+--------------------------+--------+------+------+---------+---------------+
24 rows in set (0.00 sec)
创建表 public.t1 并查询其在 COLUMNS 中的信息：
CREATE TABLE public.t1 (h STRING, v FLOAT64, ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP() TIME INDEX, PRIMARY KEY(h));
SELECT * FROM COLUMNS WHERE table_schema='public' AND TABLE_NAME='t1'\G
结果如下：
*************************** 1. row ***************************
table_catalog: greptime
table_schema: public
table_name: t1
column_name: h
ordinal_position: 1
character_maximum_length: 2147483647
character_octet_length: 2147483647
numeric_precision: NULL
numeric_scale: NULL
datetime_precision: NULL
character_set_name: utf8
collation_name: utf8_bin
column_key: PRI
extra:
privileges: select,insert
generation_expression:
greptime_data_type: String
data_type: string
semantic_type: TAG
column_default: NULL
is_nullable: Yes
column_type: string
column_comment: NULL
srs_id: NULL
*************************** 2. row ***************************
table_catalog: greptime
table_schema: public
table_name: t1
column_name: v
ordinal_position: 2
character_maximum_length: NULL
character_octet_length: NULL
numeric_precision: 22
numeric_scale: NULL
datetime_precision: NULL
character_set_name: NULL
collation_name: NULL
column_key:
extra:
privileges: select,insert
generation_expression:
greptime_data_type: Float64
data_type: double
semantic_type: FIELD
column_default: NULL
is_nullable: Yes
column_type: double
column_comment: NULL
srs_id: NULL
*************************** 3. row ***************************
table_catalog: greptime
table_schema: public
table_name: t1
column_name: ts
ordinal_position: 3
character_maximum_length: NULL
character_octet_length: NULL
numeric_precision: NULL
numeric_scale: NULL
datetime_precision: 3
character_set_name: NULL
collation_name: NULL
column_key: TIME INDEX
extra:
privileges: select,insert
generation_expression:
greptime_data_type: TimestampMillisecond
data_type: timestamp(3)
semantic_type: TIMESTAMP
column_default: current_timestamp()
is_nullable: No
column_type: timestamp(3)
column_comment: NULL
srs_id: NULL
3 rows in set (0.03 sec)
COLUMNS 表中列的描述如下：
table_catalog：列所属的目录的名称。在 OSS 项目中该值始终为greptime。table_schema：包含列的表所属的数据库的名称。table_name：包含列的表的名称。column_name：列的名称。ordinal_position：列在表中的位置。character_maximum_length：对于字符串列，以字符为单位的最大长度。character_octet_length：对于字符串列，以字节为单位的最大长度。numeric_precision：对于数值数据类型，列的精度。numeric_scale：对于数值数据类型，列的标度。datetime_precision：对于日期时间数据类型，列的小数秒精度。character_set_name：字符串列的字符集的名称。collation_name：字符串列的排序规 则的名称。column_key：列的键类型。可以是以下之一：PRI、TIME INDEX或空字符串。extra：关于列的附加信息。privileges：当前用户对该列的权限。generation_expression：对于生成的列，此值显示用于计算列值的表达式。对于非生成的列，该值为空。greptime_data_type：列的 GreptimeDB 数据类型。data_type：列中的数据类型。semantic_type：列的类型。可以是以下之一：TAG、FIELD或TIMESTAMP。column_default：列的默认值。如果默认值被明确设定为NULL，或者列定义中不包含default子句，则该值为NULL。is_nullable：列是否可为空。如果列可以存储空值，则该值为YES；否则，为NO。column_type：列的数据类型。与DATA_TYPE列相同。column_comment：列定义中包含的注释。srs_id：列的空间参考系统（SRS）的 ID。
相应的 SHOW 语句如下：
SHOW COLUMNS FROM t1 FROM public;
结果如下：
+-------+--------------+------+------------+---------------------+-------+----------------------+
| Field | Type | Null | Key | Default | Extra | Greptime_type |
+-------+--------------+------+------------+---------------------+-------+----------------------+
| h | string | Yes | PRI | NULL | | String |
| ts | timestamp(3) | No | TIME INDEX | current_timestamp() | | TimestampMillisecond |
| v | double | Yes | | NULL | | Float64 |
+-------+--------------+------+------------+---------------------+-------+----------------------+
3 rows in set (0.01 sec)