23.9 Restrictions on Views
The maximum number of tables that can be referenced in the definition of a view is 61.
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).
Before MySQL 5.7.7, 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 184.108.40.206, “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
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
With regard to view updatability, the overall goal for views is that if any view is theoretically updatable, it should be updatable in practice. MySQL as quickly as possible. Many theoretically updatable views can be updated now, but limitations still exist. For details, see Section 23.5.3, “Updatable and Insertable Views”.
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.
Views do not have indexes, so index hints do not apply. Use of index hints when selecting from a view is not permitted.
SHOW CREATE VIEW displays view definitions using an
AS clause for each column. If a column is created from an expression, the default alias is the expression text, which can be quite long. 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 replicas 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 to 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.