Version: 0.9

COLUMNS

The COLUMNS provides detailed information about columns in tables.

USE INFORMATION_SCHEMA;
DESC COLUMNS;

The output is as follows:

+--------------------------+--------+------+------+---------+---------------+
| 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)

Create a table public.t1 and query the information in the COLUMNS table:

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

The output is as follows:

*************************** 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)

The description of columns in the COLUMNS table is as follows:

table_catalog: The name of the catalog to which the table with the column belongs. The value is always greptime in OSS project.
table_schema: The name of the database in which the table with the column is located.
table_name: The name of the table with the column.
column_name: The name of the column.
ordinal_position: The position of the column in the table.
character_maximum_length: For string columns, the maximum length in characters.
character_octet_length: For string columns, the maximum length in bytes.
numeric_precision: The precision of the column for numeric data types.
numeric_scale: The scale of the column for numeric data types.
datetime_precision: The fractional seconds precision of the column for datetime data types.
character_set_name: The name of the character set of a string column.
collation_name: The name of the collation of a string column.
column_key: The key type of the column. It can be one of the following: PRI, TIME INDEX, or an empty string.
extra: Additional information about the column.
privileges: The privilege that the current user has on this column.
generation_expression: For generated columns, this value displays the expression used to calculate the column value. For non-generated columns, the value is empty.
greptime_data_type: The GreptimeDB data type of the column.
data_type: The type of data in the column.
semantic_type: The type of the column. It can be one of the following: TAG, FIELD, or TIMESTAMP.
column_default: The default value of the column. If the explicit default value is NULL, or if the column definition does not include the default clause, this value is NULL.
is_nullable: Whether the column is nullable. If the column can store null values, this value is YES; otherwise, it is NO.
column_type: The data type of the column. It is the same as the DATA_TYPE column.
column_comment: Comments contained in the column definition.
srs_id: The ID of the spatial reference system (SRS) of the column.

The corresponding SHOW statement is as follows:

SHOW COLUMNS FROM t1 FROM public;

The output is as follows:

+-------+--------------+------+------------+---------------------+-------+----------------------+
| 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)