update the doc of view (#3179)

views.md

@@ -12,7 +12,143 @@ TiDB 支持视图，视图是一张虚拟表，该虚拟表的结构由创建视
 
 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。
 
-## 样例
+## 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：
+
+{{< copyable "sql" >}}
+
+```sql
+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。
+
+```sql
++------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
+| 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:
+
+{{< copyable "sql" >}}
+
+```sql
+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。
+
+```sql
++---------------+--------------+------------+------------------------------------------------------------------------+--------------+--------------+----------------+---------------+----------------------+----------------------+
+| 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:
+
+{{< copyable "" >}}
+
+```
+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。
 
@@ -100,7 +236,7 @@ Query OK, 0 rows affected (0.02 sec)
 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` 等写入操作。
+- 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]`。
 
 ## 扩展阅读