视图
TiDB 支持视图，视图是一张虚拟表，该虚拟表的结构由创建视图时的 SELECT 语句定义。使用视图一方面可以对用户只暴露安全的字段及数据，进而保证底层表的敏感字段及数据的安全。另一方面，将频繁出现的复杂查询定义为视图，可以使复杂查询更加简单便捷。
查询视图
查询一个视图和查询一张普通表类似。但是 TiDB 在真正执行查询视图时，会将视图展开成创建视图时定义的 SELECT 语句，进而执行展开后的查询语句。
查看视图的相关信息
通过以下方式，可以查看 view 相关的信息。
使用 SHOW CREATE TABLE view_name 或 SHOW CREATE VIEW view_name 语句
示例：
show create view v;
使用该语句可以查看 view 对应的创建语句，及创建 view 时对应的 character_set_client 及 collation_connection 系统变量值。
+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
| 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)
查询 INFORMATION_SCHEMA.VIEWS 表
示例：
select * from information_schema.views;
通过查询该表可以查看 view 的相关元信息，如 TABLE_CATALOG、TABLE_SCHEMA、TABLE_NAME、VIEW_DEFINITION、CHECK_OPTION、IS_UPDATABLE、DEFINER、SECURITY_TYPE、CHARACTER_SET_CLIENT、COLLATION_CONNECTION 等。
+---------------+--------------+------------+------------------------------------------------------------------------+--------------+--------------+----------------+---------------+----------------------+----------------------+
| 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)
查询 HTTP API
示例：
curl http://127.0.0.1:10080/schema/test/v
通过访问 http://{TiDBIP}:10080/schema/{db}/{view} 可以得到对应 view 的所有元信息。
{
"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
}
示例
以下例子将创建一个视图，并在该视图上进行查询，最后删除该视图。
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)
局限性
目前 TiDB 中的视图有以下局限性：
- 不支持物化视图。
- TiDB 中视图为只读视图，不支持对视图进行
UPDATE、INSERT、DELETE、TRUNCATE等写入操作。 - 对已创建的视图仅支持
DROP的 DDL 操作，即DROP [VIEW | TABLE]。