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CREATE VIEW

Name

CREATE VIEW -- define a new view

Synopsis

CREATE [ OR REPLACE ] [ TEMP | TEMPORARY ] [ RECURSIVE ] VIEW name [ ( column_name [, ...] ) ]
    [ WITH ( view_option_name [= view_option_value] [, ... ] ) ]
    AS query

Description

CREATE VIEW defines a view of a query. The view is not physically materialized. Instead, the query is run 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 a schema name cannot be given 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 or 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 name (columns) AS SELECT ...;

is equivalent to

CREATE VIEW name AS WITH RECURSIVE name (columns) AS (SELECT ...) SELECT columns FROM name;

A view column 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; currently, the only supported parameter name is security_barrier, which should be enabled when a view is intended to provide row-level security. See Section 38.5 for full details.

query

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

Notes

Use the DROP VIEW statement to drop views.

Be careful that the names and types of the view's columns will be 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 unknown. 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. In some cases, this can be used to provide secure but restricted access to the underlying tables. However, not all views are secure against tampering; see Section 38.5 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 all functions used by the view.

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: the system will allow INSERT, UPDATE and DELETE statements to be used 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.

  • All columns in the view's select list must be simple references to columns of the underlying relation. They cannot be expressions, literals or functions. System columns cannot be referenced, either.

  • No column of the underlying relation can appear more than once in the view's select list.

  • The view must not have the security_barrier property.

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.

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 allowed 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.

A more complex view that does not satisfy all 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 INSTEAD OF triggers on the view, which must convert attempted inserts, etc. on the view into appropriate actions on other tables. For more information see CREATE TRIGGER. Another possibility is to create rules (see CREATE RULE), but in practice triggers are easier to understand and use correctly.

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 (see Section 38.5).

Examples

Create a view consisting of all comedy films:

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

This will create 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 recursive view consisting of the numbers from 1 to 100:

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

Compatibility

The SQL standard specifies some additional capabilities for the CREATE VIEW statement:

CREATE VIEW name [ ( column_name [, ...] ) ]
    AS query
    [ WITH [ CASCADED | LOCAL ] CHECK OPTION ]

The optional clauses for the full SQL command are:

CHECK OPTION

This option controls the behavior of automatically updatable views. When given, INSERT and UPDATE commands on the view will be checked to ensure new rows satisfy the view-defining condition (that is, the new rows would be visible through the view). If they do not, the update will be rejected. Without CHECK OPTION, INSERT and UPDATE commands on the view are allowed to create rows that are not visible through the view. (The latter behavior is the only one currently provided by PostgreSQL.)

LOCAL

Check for integrity on this view.

CASCADED

Check for integrity on this view and on any dependent view. CASCADED is assumed if neither CASCADED nor LOCAL is specified.

CREATE OR REPLACE VIEW is a PostgreSQL language extension. So is the concept of a temporary view. The WITH clause is an extension as well.

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