Hi everyone, I'd like to start a discussion about the tree-to-table view in the
table model.
A tree-to-table view is created over a tree-model path prefix, and exposes
tree-model measurements as table columns. When such a view is created without
explicitly listing Field columns, the system scans the subtree under the given
prefixPath and generates one column
for each measurement that already exists. The structure of the tree-to-table
view is then fixed at creation time.
The scan runs only once, at creation. After that, any new measurement created
under prefixPath does not appear in the tree-to-table view. Users have to run
ALTER VIEW ... ADD COLUMN manually, or drop and recreate the tree-to-table
view. In deployments where
measurements are added continuously, this is hard to maintain and easy to
miss, and the missing columns lead to silently incomplete query results.
We propose adding an optional property, auto_sync_schema, to CREATE VIEW.
When it is set to true, the tree-to-table view keeps tracking measurements
newly created under its prefixPath and extends its schema automatically.
CREATE VIEW view_01 (dev STRING TAG) WITH (auto_sync_schema = true) AS
root.db1.**;
The property defaults to false, so existing behavior is unchanged. It only
applies to tree-to-table views created without explicit Field columns;
specifying both Field columns and auto_sync_schema = true is rejected. It is
orthogonal to the existing RESTRICT option.
Only columns are added, never removed, so if a measurement is dropped the
column remains and queries on it return null. If a new measurement conflicts
with an existing column, either because it has the same name with a different
data type, or a name that differs only
in case, the column is skipped and a warning is logged, while the write
itself is not failed.
A template can also introduce new measurements. Activating a template on a
path, or adding a measurement to a template that is already activated, is
treated as a new measurement, and triggers the same schema extension in the
tree-to-table view.
One point worth calling out: to keep the tree-to-table view consistent with
the tree schema, the extension happens during series creation. If the
tree-to-table view cannot be updated, the series creation itself fails.
On impact, the write path is unchanged when no auto-sync tree-to-table view
matches the path; when one matches, one additional schema update is performed.
Query behavior and existing tree-to-table views are unaffected. SHOW CREATE
VIEW will display the new property.
Best regards,
Yaobin Chen
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主题：[DISCUSS] 树转表视图自动检测新增测点并扩展结构
我想就表模型下的树转表视图发起一个讨论。
树转表视图创建在树模型的某个路径前缀之上，把树模型的测点暴露为表的列。当这样的视图在创建时不显式指定 Field 列时，系统会扫描该 prefixPath
下的子树，为每一个已存在的测点生成一列。树转表视图的结构在创建时即固定。
该扫描只在创建时执行一次。此后，prefixPath 下新增的任何测点都不会出现在该树转表视图中。用户必须手动执行 ALTER VIEW ... ADD
COLUMN，或者删除并重建这棵树转表视图。在测点持续新增的部署场景下，这难以维护且容易遗漏，而缺失的列会导致查询结果静默不完整。
我们建议为 CREATE VIEW 增加一个可选属性 auto_sync_schema。当其设为 true 时，该树转表视图会持续跟踪其
prefixPath 下新增创建的测点，并自动扩展自身结构。
CREATE VIEW view_01 (dev STRING TAG) WITH (auto_sync_schema = true) AS
root.db1.**;
该属性默认为 false，因此现有行为不变。它仅对未显式指定 Field 列的树转表视图生效；同时指定 Field 列与 auto_sync_schema
= true 会被拒绝。它与现有的 RESTRICT 选项正交。只增加列，不删除列，因此若某个测点被删除，对应的列仍然保留，对该列的查询返回
null。若新增测点与已有列冲突，无论是同名但数据类型不同，还是名称仅大小写不同，该列都会被跳过并记录一条告警日志，而写入本身不会失败。
模板同样可能引入新的测点。在某个路径上激活模板，或者向一个已被激活的模板中新增测点，都会被视作新增测点，并触发该树转表视图的同样结构扩展。
有一点值得单独说明：为了让树转表视图与树结构保持一致，扩展动作发生在序列创建过程中。如果该树转表视图无法更新，序列创建本身会失败。
关于影响面，当没有任何开启自动同步的树转表视图匹配该路径时，写入路径不变；当存在匹配时，会额外执行一次结构更新。查询行为与已有的树转表视图不受影响。SHOW
CREATE VIEW 会展示该新增属性。
Best regards,
Yaobin Chen
