CREATE VIEW — define a new view
CREATE [ OR REPLACE ] [ TEMP | TEMPORARY ] [ RECURSIVE ] VIEWname
[ (column_name
[, ...] ) ] [ WITH (view_option_name
[=view_option_value
] [, ... ] ) ] ASquery
[ WITH [ CASCADED | LOCAL ] CHECK OPTION ]
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.
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 [schema
. ]view_name
(column_names
) AS SELECT...
;
is equivalent to
CREATE VIEW [schema
. ]view_name
AS WITH RECURSIVEview_name
(column_names
) AS (SELECT...
) SELECTcolumn_names
FROMview_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
(enum
)
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. See Section 39.5 for full details.
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, the update will be
rejected. 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.
Note that the CHECK OPTION
is only supported on views that
are automatically updatable, and do not have INSTEAD OF
triggers or INSTEAD
rules. If an automatically updatable
view is defined on top of a base view that has INSTEAD OF
triggers, then the LOCAL CHECK OPTION
may be used to check
the conditions on the automatically updatable view, but the conditions
on the base view with INSTEAD OF
triggers will not be
checked (a cascaded check option will not cascade down to a
trigger-updatable view, and any check options defined directly on a
trigger-updatable view will be ignored). If the view or any of its base
relations has an INSTEAD
rule that causes the
INSERT
or UPDATE
command to be rewritten, then
all check options will be ignored in the rewritten query, including any
checks from automatically updatable views defined on top of the relation
with the INSTEAD
rule.
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 because the column name defaults to ?column?
;
also, the column data type defaults to text
, which might not
be what you wanted. Better style for a string literal in a view's
result is 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 39.5 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).
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.
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 an error will be raised 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 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 (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 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. See Section 39.5 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.
EXPLAIN
can be used 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 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 39.5).
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 view with LOCAL CHECK OPTION
:
CREATE VIEW universal_comedies AS SELECT * FROM comedies WHERE classification = 'U' WITH LOCAL CHECK OPTION;
This will create 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 will be 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 will create 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 will support 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 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
, its internal self-reference is not schema-qualified.
This is because the implicitly-created CTE's name cannot be
schema-qualified.
CREATE OR REPLACE VIEW
is a
LightDB language extension.
So is the concept of a temporary view.
The WITH ( ... )
clause is an extension as well.