CREATE VIEW

Defines a new view.

Synopsis

CREATE [OR REPLACE] [TEMP | TEMPORARY] [RECURSIVE] VIEW <name> [ ( <column_name> [, ...] ) ]
    [ WITH ( view_option_name [= view_option_value] [, ... ] ) ]
    AS <query>
    [ WITH [ CASCADED | LOCAL ] CHECK OPTION ]

Description

CREATE VIEW defines a view of a query. The view is not physically materialized. Instead, Apache Cloudberry runs the query every time the view is referenced in a query.

CREATE OR REPLACE VIEW is similar, but if a view of the same name already exists, it is replaced. The new query must generate the same columns that were generated by the existing view query (that is, the same column names in the same order, and with the same data types), but it may add additional columns to the end of the list. The calculations giving rise to the output columns may be completely different.

If a schema name is given (for example, CREATE VIEW myschema.myview ...) then the view is created in the specified schema. Otherwise it is created in the current schema. Temporary views exist in a special schema, so you may not provide a schema name when creating a temporary view. The name of the view must be distinct from the name of any other view, table, sequence, index, or foreign table in the same schema.

Parameters

TEMPORARY | TEMP

If specified, the view is created as a temporary view. Temporary views are automatically dropped at the end of the current session. Existing permanent relations with the same name are not visible to the current session while the temporary view exists, unless they are referenced with schema-qualified names.

If any of the tables referenced by the view are temporary, the view is created as a temporary view (whether TEMPORARY is specified or not).

RECURSIVE

Creates a recursive view. The syntax

    CREATE RECURSIVE VIEW [ <schema> . ] <view_name> (<column_names>) AS SELECT <...>;

is equivalent to

CREATE VIEW [ <schema> . ] <view_name> AS WITH RECURSIVE <view_name> (<column_names>) AS (SELECT <...>) SELECT <column_names> FROM <view_name>;

A view column name list must be specified for a recursive view.

name

The name (optionally schema-qualified) of a view to be created.

column_name

An optional list of names to be used for columns of the view. If not given, the column names are deduced from the query.

WITH ( view_option_name [= view_option_value] [, ... ] )

This clause specifies optional parameters for a view; the following parameters are supported:

check_option (string)

This parameter may be either local or cascaded, and is equivalent to specifying WITH [ CASCADED | LOCAL ] CHECK OPTION (see below). This option can be changed on existing views using ALTER VIEW.

security_barrier (boolean)

This should be used if the view is intended to provide row-level security. Refer to Rules and Privileges in the PostgreSQL documentation for more information.

query

A SELECT or VALUES command which will provide the columns and rows of the view.

WITH [ CASCADED | LOCAL ] CHECK OPTION

This option controls the behavior of automatically updatable views. When this option is specified, INSERT and UPDATE commands on the view will be checked to ensure that new rows satisfy the view-defining condition (that is, the new rows are checked to ensure that they are visible through the view). If they are not, Apache Cloudberry rejects the update. If the CHECK OPTION is not specified, INSERT and UPDATE commands on the view are allowed to create rows that are not visible through the view. The following check options are supported:

LOCAL

New rows are only checked against the conditions defined directly in the view itself. Any conditions defined on underlying base views are not checked (unless they also specify the CHECK OPTION).

CASCADED

New rows are checked against the conditions of the view and all underlying base views. If the CHECK OPTION is specified, and neither LOCAL nor CASCADED is specified, then CASCADED is assumed.

The CHECK OPTION may not be used with RECURSIVE views.

Notes

Use the DROP VIEW statement to drop views.

Ensure that the names and data types of the view's columns are assigned the way you want. For example:

CREATE VIEW vista AS SELECT 'Hello World';

is bad form in two ways: the column name defaults to ?column?, and the column data type defaults to text, which might not be what you wanted. If you want a string literal in a view's result, use something like:

CREATE VIEW vista AS SELECT text 'Hello World' AS hello;

Access to tables referenced in the view is determined by permissions of the view owner not the current user (even if the current user is a superuser). This can be confusing in the case of superusers, since superusers typically have access to all objects. In the case of a view, even superusers must be explicitly granted access to tables referenced in the view if they are not the owner of the view. In some cases, a view can be used to provide secure but restricted access to the underlying tables. However, not all views are secure against tampering, see Rules and Privileges in the PostgreSQL documentation for details.

Functions called in the view are treated the same as if they had been called directly from the query using the view. Therefore the user of a view must have permissions to call any functions used by the view.

When both CREATE VIEW <view> ... and SELECT ... FROM <view> specify an ORDER BY clause, Apache Cloudberry ignores the ORDER BY clause in the CREATE VIEW statement.

When CREATE OR REPLACE VIEW is used on an existing view, only the view's defining SELECT rule is changed. Other view properties, including ownership, permissions, and non-SELECT rules, remain unchanged. You must own the view to replace it (this includes being a member of the owning role).

Updatable views

Simple views are automatically updatable: Apache Cloudberry allows you to invoke INSERT, UPDATE, and DELETE statements on the view in the same way as on a regular table. A view is automatically updatable if it satisfies all of the following conditions:

• The view must have exactly one entry in its FROM list, which must be a table or another updatable view.

• The view definition must not contain WITH, DISTINCT, GROUP BY, HAVING, LIMIT, or OFFSET clauses at the top level.

• The view definition must not contain set operations (UNION, INTERSECT, or EXCEPT) at the top level.

• The view's select list must not contain any aggregates, window functions, or set-returning functions.

An automatically updatable view may contain a mix of updatable and non-updatable columns. A column is updatable if it is a simple reference to an updatable column of the underlying base relation; otherwise the column is read-only, and Apache Cloudberry raises an error if an INSERT or UPDATE statement attempts to assign a value to it.

If the view is automatically updatable the system will convert any INSERT, UPDATE, or DELETE statement on the view into the corresponding statement on the underlying base relation. INSERT statements that have an ON CONFLICT UPDATE clause are fully supported.

If an automatically updatable view contains a WHERE condition, the condition restricts which rows of the base relation are available to be modified by UPDATE and DELETE statements on the view. However, an UPDATE is permitted to change a row so that it no longer satisfies the WHERE condition, and thus is no longer visible through the view. Similarly, an INSERT command can potentially insert base-relation rows that do not satisfy the WHERE condition and thus are not visible through the view (ON CONFLICT UPDATE may similarly affect an existing row not visible through the view). The CHECK OPTION may be used to prevent INSERT and UPDATE commands from creating such rows that are not visible through the view.

If an automatically updatable view is marked with the security_barrier property, then all of the view's WHERE conditions (and any conditions using operators which are marked as LEAKPROOF) will always be evaluated before any conditions that a user of the view has added. Refer to Rules and Privileges in the PostgreSQL documenatation for full details. Note that, due to this, rows which are not ultimately returned (because they do not pass the user's WHERE conditions) may still end up being locked. You can use the EXPLAIN command to see which conditions are applied at the relation level (and therefore do not lock rows) and which are not.

A more complex view that does not satisfy all of these conditions is read-only by default: the system will not allow an insert, update, or delete on the view. You can get the effect of an updatable view by creating rules (see CREATE RULE).

Note that the user performing the insert, update or delete on the view must have the corresponding insert, update or delete privilege on the view. In addition the view's owner must have the relevant privileges on the underlying base relations, but the user performing the update does not need any permissions on the underlying base relations (again, refer to Rules and Privileges in the PostgreSQL documentation).

Examples

Create a view consisting of all comedy films:

CREATE VIEW comedies AS
    SELECT * FROM films 
    WHERE kind = 'comedy';

This creates a view containing the columns that are in the film table at the time of view creation. Though * was used to create the view, columns added later to the table will not be part of the view.

Create a view with LOCAL CHECK OPTION:

CREATE VIEW universal_comedies AS
    SELECT *
    FROM comedies
    WHERE classification = 'U'
    WITH LOCAL CHECK OPTION;

This creates a view based on the comedies view, showing only films with kind = 'Comedy' and classification = 'U'. Any attempt to INSERT or UPDATE a row in the view is rejected if the new row doesn't have classification = 'U', but the film kind will not be checked.

Create a view with CASCADED CHECK OPTION:

CREATE VIEW pg_comedies AS
    SELECT *
    FROM comedies
    WHERE classification = 'PG'
    WITH CASCADED CHECK OPTION;

This creates a view that checks both the kind and classification of new rows.

Create a view with a mix of updatable and non-updatable columns:

CREATE VIEW comedies AS
    SELECT f.*,
           country_code_to_name(f.country_code) AS country,
           (SELECT avg(r.rating)
            FROM user_ratings r
            WHERE r.film_id = f.id) AS avg_rating
    FROM films f
    WHERE f.kind = 'Comedy';

This view supports INSERT, UPDATE, and DELETE. All the columns from the films table will be updatable, whereas the computed columns country and avg_rating will be read-only.

Create a view that gets the top ten ranked baby names:

CREATE VIEW topten AS 
    SELECT name, rank, gender, year
    FROM names, rank
    WHERE rank < '11' AND names.id=rank.id;

Create a recursive view consisting of the numbers from 1 to 100:

CREATE RECURSIVE VIEW public.nums_1_100 (n) AS
    VALUES (1)
UNION ALL
    SELECT n+1 FROM nums_1_100 WHERE n < 100;

Notice that although the recursive view's name is schema-qualified in this CREATE VIEW command, its internal self-reference is not schema-qualified. This is because the implicitly-created CTE's name cannot be schema-qualified.

Compatibility

CREATE OR REPLACE VIEW is a Apache Cloudberry extension. So is the concept of a temporary view. The WITH ( ... ) clause is an extension as well.

See also

ALTER VIEW, DROP VIEW, CREATE MATERIALIZED VIEW