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_nameSHOW 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_clientcollation_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

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_CATALOGTABLE_SCHEMATABLE_NAMEVIEW_DEFINITIONCHECK_OPTIONIS_UPDATABLEDEFINERSECURITY_TYPECHARACTER_SET_CLIENTCOLLATION_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 UPDATEINSERTDELETETRUNCATE 等写入操作。
  • 对已创建的视图仅支持 DROP 的 DDL 操作,即 DROP [VIEW | TABLE]

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