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D.4. Restrictions on Views

View processing is not optimized:

  • It is not possible to create an index on a view.

  • Indexes can be used for views processed using the merge algorithm. However, a view that is processed with the temptable algorithm is unable to take advantage of indexes on its underlying tables (although indexes can be used during generation of the temporary tables).

Subqueries cannot be used in the FROM clause of a view.

There is a general principle that you cannot modify a table and select from the same table in a subquery. See Section D.3, “Restrictions on Subqueries”.

The same principle also applies if you select from a view that selects from the table, if the view selects from the table in a subquery and the view is evaluated using the merge algorithm. Example: 

CREATE VIEW v1 AS
SELECT * FROM t2 WHERE EXISTS (SELECT 1 FROM t1 WHERE t1.a = t2.a);

UPDATE t1, v2 SET t1.a = 1 WHERE t1.b = v2.b;

If the view is evaluated using a temporary table, you can select from the table in the view subquery and still modify that table in the outer query. In this case the view will be stored in a temporary table and thus you are not really selecting from the table in a subquery and modifying it “at the same time.” (This is another reason you might wish to force MySQL to use the temptable algorithm by specifying ALGORITHM = TEMPTABLE in the view definition.)

You can use DROP TABLE or ALTER TABLE to drop or alter a table that is used in a view definition. No warning results from the DROP or ALTER operation, even though this invalidates the view. Instead, an error occurs later, when the view is used. CHECK TABLE can be used to check for views that have been invalidated by DROP or ALTER operations.

A view definition is “frozen” by certain statements:

  • If a statement prepared by PREPARE refers to a view, the view definition seen each time the statement is executed later will be the definition of the view at the time it was prepared. This is true even if the view definition is changed after the statement is prepared and before it is executed. Example: 

    CREATE VIEW v AS SELECT RAND();
PREPARE s FROM 'SELECT * FROM v';
ALTER VIEW v AS SELECT NOW();
EXECUTE s;

    The result returned by the EXECUTE statement is a random number, not the current date and time.

  • If a statement in a stored routine refers to a view, the view definition seen by the statement are its definition the first time that statement is executed. For example, this means that if the statement is executed in a loop, further iterations of the statement see the same view definition, even if the definition is changed later in the loop. Example: 

    CREATE VIEW v AS SELECT 1;
delimiter //
CREATE PROCEDURE p ()
BEGIN
  DECLARE i INT DEFAULT 0;
  WHILE i < 5 DO
    SELECT * FROM v;
    SET i = i + 1;
    ALTER VIEW v AS SELECT 2;
  END WHILE;
END;
//
delimiter ;
CALL p();

    When the procedure p() is called, the SELECT returns 1 each time through the loop, even though the view definition is changed within the loop.

    As of MySQL 5.1.21, ALTER VIEW is prohibited within stored routines, so this restriction does not apply.

With regard to view updatability, the overall goal for views is that if any view is theoretically updatable, it should be updatable in practice. This includes views that have UNION in their definition. Currently, not all views that are theoretically updatable can be updated. The initial view implementation was deliberately written this way to get usable, updatable views into MySQL as quickly as possible. Many theoretically updatable views can be updated now, but limitations still exist:

  • Updatable views with subqueries anywhere other than in the WHERE clause. Some views that have subqueries in the SELECT list may be updatable.

  • You cannot use UPDATE to update more than one underlying table of a view that is defined as a join.

  • You cannot use DELETE to update a view that is defined as a join.

There exists a shortcoming with the current implementation of views. If a user is granted the basic privileges necessary to create a view (the CREATE VIEW and SELECT privileges), that user will be unable to call SHOW CREATE VIEW on that object unless the user is also granted the SHOW VIEW privilege.

That shortcoming can lead to problems backing up a database with mysqldump, which may fail due to insufficient privileges. This problem is described in Bug#22062.

The workaround to the problem is for the administrator to manually grant the SHOW VIEW privilege to users who are granted CREATE VIEW, since MySQL doesn't grant it implicitly when views are created.

Views do not have indexes, so index hints do not apply. Use of index hints when selecting from a view is disallowed.

SHOW CREATE VIEW displays view definitions using an AS alias_name clause for each column. If a column is created from an expression, the default alias is the expression text, which can be quite long. As of MySQL 5.1.23, aliases for column names in CREATE VIEW statements are checked against the maximum column length of 64 characters (not the maximum alias length of 256 characters). As a result, views created from the output of SHOW CREATE VIEW fail if any column alias exceeds 64 characters. This can cause problems in the following circumstances for views with too-long aliases:

  • View definitions fail to replicate to newer slaves that enforce the column-length restriction.

  • Dump files created with mysqldump cannot be loaded into servers that enforce the column-length restriction.

A workaround for either problem is the modify each problematic view definition to use aliases that provide shorter column names. Then the view will replicate properly, and can be dumped and reloaded without causing an error. To modify the definition, drop and create the view again with DROP VIEW and CREATE VIEW, or replace the definition with CREATE OR REPLACE VIEW.

For problems that occur when reloading view definitions in dump files, another workaround is to edit the dump file to modify its CREATE VIEW statements. However, this does not change the original view definitions, which may cause problems for subsequent dump operations.