Views
TiDB supports views. A view acts as a virtual table, whose schema is defined by the SELECT statement that creates the view. Using views has the following benefits:
- Exposing only safe fields and data to users to ensure security of sensitive fields and data stored in the underlying table.
- Defining complex queries that frequently appear as views to make complex queries easier and more convenient.
Query views
Querying a view is similar to querying an ordinary table. However, when TiDB queries a view, it actually queries the SELECT statement associated with the view.
Show metadata
To obtain the metadata of views, choose any of the following methods.
Use the SHOW CREATE TABLE view_name or SHOW CREATE VIEW view_name statement
Usage example:
show create view v;
This statement shows the CREATE VIEW statement corresponding to this view and the value of the character_set_client and collation_connection system variables when the view was created.
+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
| View | Create View | character_set_client | collation_connection |
+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
| v | CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`127.0.0.1` SQL SECURITY DEFINER VIEW `v` (`a`) AS SELECT `s`.`a` FROM `test`.`t` LEFT JOIN `test`.`s` ON `t`.`a`=`s`.`a` | utf8 | utf8_general_ci |
+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
1 row in set (0.00 sec)
Query the INFORMATION_SCHEMA.VIEWS table
Usage example:
select * from information_schema.views;
You can view the relevant meta information of the view by querying this table, such as TABLE_CATALOG, TABLE_SCHEMA, TABLE_NAME, VIEW_DEFINITION, CHECK_OPTION, IS_UPDATABLE, DEFINER, SECURITY_TYPE, CHARACTER_SET_CLIENT, and COLLATION_CONNECTION.
+---------------+--------------+------------+------------------------------------------------------------------------+--------------+--------------+----------------+---------------+----------------------+----------------------+
| TABLE_CATALOG | TABLE_SCHEMA | TABLE_NAME | VIEW_DEFINITION | CHECK_OPTION | IS_UPDATABLE | DEFINER | SECURITY_TYPE | CHARACTER_SET_CLIENT | COLLATION_CONNECTION |
+---------------+--------------+------------+------------------------------------------------------------------------+--------------+--------------+----------------+---------------+----------------------+----------------------+
| def | test | v | SELECT `s`.`a` FROM `test`.`t` LEFT JOIN `test`.`s` ON `t`.`a`=`s`.`a` | CASCADED | NO | root@127.0.0.1 | DEFINER | utf8 | utf8_general_ci |
+---------------+--------------+------------+------------------------------------------------------------------------+--------------+--------------+----------------+---------------+----------------------+----------------------+
1 row in set (0.00 sec)
Use the HTTP APIs
Usage example:
curl http://127.0.0.1:10080/schema/test/v
By visiting http://{TiDBIP}:10080/schema/{db}/{view}, you can get all the metadata for the view.
{
"id": 122,
"name": {
"O": "v",
"L": "v"
},
"charset": "utf8",
"collate": "utf8_general_ci",
"cols": [
{
"id": 1,
"name": {
"O": "a",
"L": "a"
},
"offset": 0,
"origin_default": null,
"default": null,
"default_bit": null,
"default_is_expr": false,
"generated_expr_string": "",
"generated_stored": false,
"dependences": null,
"type": {
"Tp": 0,
"Flag": 0,
"Flen": 0,
"Decimal": 0,
"Charset": "",
"Collate": "",
"Elems": null
},
"state": 5,
"comment": "",
"hidden": false,
"version": 0
}
],
"index_info": null,
"fk_info": null,
"state": 5,
"pk_is_handle": false,
"is_common_handle": false,
"comment": "",
"auto_inc_id": 0,
"auto_id_cache": 0,
"auto_rand_id": 0,
"max_col_id": 1,
"max_idx_id": 0,
"update_timestamp": 416801600091455490,
"ShardRowIDBits": 0,
"max_shard_row_id_bits": 0,
"auto_random_bits": 0,
"pre_split_regions": 0,
"partition": null,
"compression": "",
"view": {
"view_algorithm": 0,
"view_definer": {
"Username": "root",
"Hostname": "127.0.0.1",
"CurrentUser": false,
"AuthUsername": "root",
"AuthHostname": "%"
},
"view_security": 0,
"view_select": "SELECT `s`.`a` FROM `test`.`t` LEFT JOIN `test`.`s` ON `t`.`a`=`s`.`a`",
"view_checkoption": 1,
"view_cols": null
},
"sequence": null,
"Lock": null,
"version": 3,
"tiflash_replica": null
}
Example
The following example creates a view, queries this view, and delete this view:
create table t(a int, b int);
Query OK, 0 rows affected (0.01 sec)
insert into t values(1, 1),(2,2),(3,3);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
create table s(a int);
Query OK, 0 rows affected (0.01 sec)
insert into s values(2),(3);
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
create view v as select s.a from t left join s on t.a = s.a;
Query OK, 0 rows affected (0.01 sec)
select * from v;
+------+
| a |
+------+
| NULL |
| 2 |
| 3 |
+------+
3 rows in set (0.00 sec)
drop view v;
Query OK, 0 rows affected (0.02 sec)
Limitations
Currently, views in TiDB are subject to the following limitations:
- Materialized views are not supported yet.
- Views in TiDB are read-only and do not support write operations such as
UPDATE,INSERT,DELETE, andTRUNCATE. - For created views, the only supported DDL operation is
DROP [VIEW | TABLE]