138 lines
8.2 KiB
Markdown
138 lines
8.2 KiB
Markdown
# Proposal: Implement View Feature
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- Author(s): AndrewDi
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- Last updated: 2018-12-03
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- Discussion at: https://github.com/pingcap/tidb/issues/7974
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## Abstract
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This proposal proposes to implement basic VIEW feature in TiDB, aimed to make SQL easier to write. VIEW's advanced feature would be considered later.
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## Background
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A database view is a searchable object in a database that is defined by a query. Though a view doesn’t store data, some refer to a VIEW as "virtual tables", and you can query a view like you can query a table. A view can combine data from two or more tables, using joins, and also just contain a subset of information. This makes them convenient to abstract, or hide, complicated queries.
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Below is a visual depiction of a view:
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reference from https://www.essentialsql.com/what-is-a-relational-database-view/
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A view is created from a query using the "`CREATE OR REPLACE VIEW`" command. In the example below we are creating a PopularBooks view based on a query which selects all Books that have the IsPopular field checked. Following is the query:
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```sql
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CREATE OR replace VIEW popularbooks
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AS
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SELECT isbn,
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title,
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author,
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publishdate
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FROM books
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WHERE ispopular = 1
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```
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Once a view is created, you can use it as you query any table in a `SELECT` statement. For example, to list all the popular book titles ordered by an author, you could write:
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```sql
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SELECT Author, Title FROM PopularBooks ORDER BY Author
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```
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In general you can use any of the SELECT clauses, such as GROUP BY, in a select statement containing a view.
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## Proposal
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This proposal is prepared to implement the basic VIEW feature, which contains following ddl operations:
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* `CREATE OR REPLACE VIEW`
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* `SELECT FROM VIEW`
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* `DROP VIEW`
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* `SHOW TABLE STATUS`
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All other unimplemented features will be listed for compatibility and discussed later. And we introduce `ViewInfo` to store the metadata for view and add an attribute `*ViewInfo` which named `View` to TableInfo. If `TableInfo.View` != nil, then this `TableInfo` is a view, otherwise this `TableInfo` is a base table:
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```
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type ViewInfo struct {
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Algorithm ViewAlgorithm `json:"view_algorithm"`
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Definer UserIdentity `json:"view_definer"`
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Security ViewSecurity `json:"view_security"`
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SelectStmt string `json:"view_select"`
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CheckOption ViewCheckOption `json:"view_checkoption"`
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Cols []model.CIStr `json:"view_cols"`
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}
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```
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* [Algorithm](https://dev.mysql.com/doc/refman/5.7/en/view-algorithms.html)
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The view SQL `AlGORITHM` characteristic. The value should be one of `UNDEFINED`, `MERGE` OR `TEMPTABLE`. If no `ALGORITHM` clause is present, `UNDEFINED` is the default algorithm.
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Currently, we will only implement the `MERGE` algorithm.
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* [Definer](https://dev.mysql.com/doc/refman/5.7/en/create-view.html)
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The account of the user who created the view, in `'user_name'@'host_name'` format.
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* [Security](https://dev.mysql.com/doc/refman/5.7/en/create-view.html)
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The view SQL `SECURITY` characteristic. The value is one of `DEFINER` or `INVOKER`.
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* [CheckOption](https://dev.mysql.com/doc/refman/5.7/en/view-check-option.html)
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The `WITH CHECK OPTION` clause can be given to an updatable view to prevent inserts to rows for which the `WHERE` clause in the select_statement is not true. It also prevents updates to rows for which the `WHERE` clause is true but the update would cause it to be not true (in other words, it prevents visible rows from being updated to nonvisible rows).
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In a `WITH CHECK OPTION` clause for an updatable view, the `LOCAL` and `CASCADED` keywords determine the scope of check testing when the view is defined in terms of another view. When neither keyword is given, the default is `CASCADED`.
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* SelectStmt
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This string is the origin `SELECT` SQL statement.
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* Cols
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This `model.CIStr` array stores view's column alias names.
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* TableInfo.Columns
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`TableInfo.Columns` only stores view's column origin names.
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## Rationale
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1. Create VIEW
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This proposal only supports the following grammar to create view:
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```sql
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CREATE
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[OR REPLACE]
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[ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}]
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[DEFINER = { user | CURRENT_USER }]
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[SQL SECURITY { DEFINER | INVOKER }]
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VIEW view_name [(column_list)]
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AS select_statement
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[WITH [CASCADED | LOCAL] CHECK OPTION]
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```
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1. Parse the create view statement and build a logical plan for select clause part. If any grammar error occurs, return errors to parser.
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2. Examine view definer's privileges. Definer should own both `CREATE_VIEW_PRIV` privilege and base table's `SELECT` privilege. We will reuse `CREATE_PRIV` privilege when implement `CREATE VIEW` feature and will support `CREATE_VIEW_PRIV` check later.
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3. Examine create view statement. If the `ViewFieldList` clause part is empty, then we should generate view column names from SelectStmt clause. Otherwise check len(ViewFieldList) == len(Columns from SelectStmt). And then we save column names to `TableInfo.Columns`.
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2. DROP VIEW
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Implement `DROP VIEW` grammar, and delete the existing view tableinfo object. This function should reuse `DROP TABLE` code logic.
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3. SELECT FROM VIEW
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In function `func (b *PlanBuilder) buildDataSource(tn *ast.TableName) (LogicalPlan, error)`, if `tn *ast.TableName` is a view, then we build a select `LogicalPlan` from view's `SelectStmt` string. But this solution meets a problem, and here is the example:
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```sql
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create table t(a int,b int);
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create view v like select * from t;
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select * from t;
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```
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Once we query from view `v`, database will rewrite view's `SelectStmt` from `select * from t` into **`select a as a,b as b from t`**.
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But, if we alter the schema of t like this:
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```sql
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drop table t;
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create table t(c int,d int);
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```
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Executing `select * from v` now is equivalent to **`select c as c, d as d from t`**. The result of this query will be incompatible with the character of VIEW that a VIEW should be "frozen" after being defined.
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In order to solve the problem described above, we must build a `Projection` at the top of original select's `LogicalPlan`, just like we rewrite view's `SelectStmt` from `select * from t` into **`select a as a,b as b from (select * from t)`**.
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This is a temporary fix and we will implement TiDB to rewrite SQL with replacing all wildcard finally.
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4. Show table status
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Modify `SHOW TABLE STATUS` function to support show view status, and we use this command to check if `CREATE VIEW` and `DROP VIEW` operation is successful. To reuse `SHOW TABLE STATUS` code logic is preferred.
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## Compatibility
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Support the basic VIEW feature without affecting other existing functions, and make TiDB more compatible with MySQL.
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## Implementation
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Here is the work items list:
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|Action |Priority|Deadline|Notes|
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| ------ | ------ | ------ |-----|
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|Extract ViewAlgorithm\ViewDefiner\ViewSQLSecurity\CheckOption to CreateViewStmt struct|P1|2019/01/15|--|
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|Add ViewInfo attribute to TableInfo and add ActionCreateView to ddl actionMap|P1|2019/01/15|--|
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|CREATE [OR REPLACE] VIEW view_name [ALGORITHM = {UNDEFINED \| MERGE \| TEMPTABLE}] [(column_list)] AS select_statement|P1|2019/01/15|This task must be done before any other tasks.|
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|SHOW TABLE STATUS|P1|2019/01/30|--|
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|DROP VIEW View|P1|2019/01/30|--|
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|SELECT … FROM VIEW|P1|2019/03/10|--|
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|Add some test cases for CreateView and Select … From View|P1|2019/03/30|Port from MySQL test case|
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|Support CREATE_VIEW_PRIV check|P2| | |
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|UPDATE VIEW|P2| |Difficult|
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|INSERT VIEW|P2| |Difficult, dependent on UPDATE VIEW|
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|SHOW CREATE [VIEW \| TABLE]|P2| | |
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|ALTER VIEW|P2| | |
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|ALTER \| DROP TABLE|P2| |Check if table is a View|
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|Add test cases for UPDATE \| INSERT INTO View|P2| | |
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|Add INFORMATION_SCHEMA.VIEWS view|P3| | |
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|CREATE [OR REPLACE] VIEW [DEFINER = { user \| CURRENT_USER }] [SQL SECURITY { DEFINER \| INVOKER }] AS SELECT_STATEMENT|P3| | |
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|CREATE [OR REPLACE] VIEW [ALGORITHM = {TEMPTABLE}] AS select_statement|P3| | |
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|CREATE [OR REPLACE] VIEW AS select_statement [WITH [CASCADED \| LOCAL] CHECK OPTION]|P3| | |
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|Support global sql_mode system variable|P3| | |
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## Open issues (if applicable)
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https://github.com/pingcap/tidb/issues/7974
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