The table-maintenance statements CHECK TABLES and OPTIMIZE TABLES. Note: This restriction is lifted beginning with MySQL 5.0.17.

The locking statements LOCK TABLES, UNLOCK TABLES.

LOAD DATA and LOAD TABLE.

SQL prepared statements (PREPARE, EXECUTE, DEALLOCATE PREPARE). Implication: You cannot use dynamic SQL within stored routines (where you construct dynamically statements as strings and then execute them). This restriction is lifted as of MySQL 5.0.13 for stored procedures; it still applies to stored functions and triggers.

Statements that do explicit or implicit commit or rollback.

Statements that return a result set. This includes SELECT statements that do not have an INTO var_list clause and SHOW statements. A function can process a result set either with SELECT ... INTO var_list or by using a cursor and FETCH statements. See Section 17.2.7.3, “SELECT ... INTO Statement”.

FLUSH statements.

Note: Before MySQL 5.0.10, stored functions created with CREATE FUNCTION must not contain references to tables, with limited exceptions. They may include some SET statements that contain table references, for example SET a:= (SELECT MAX(id) FROM t), and SELECT statements that fetch values directly into variables, for example SELECT i INTO var1 FROM t.

Recursive statements. That is, stored functions cannot be used recursively.

Within a stored function or trigger, it is not allowable to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.

A local variable takes precedence over a routine parameter or table column

A routine parameter takes precedence over a table column

A local variable in an inner block takes precedence over a local variable in an outer block

Known bug to be fixed later: If you compare a NULL value to a subquery using ALL, ANY, or SOME, and the subquery returns an empty result, the comparison might evaluate to the non-standard result of NULL rather than to TRUE or FALSE.

A subquery's outer statement can be any one of: SELECT, INSERT, UPDATE, DELETE, SET, or DO.

Subquery optimization for IN is not as effective as for the = operator or for IN(value_list) constructs.

A typical case for poor IN subquery performance is when the subquery returns a small number of rows but the outer query returns a large number of rows to be compared to the subquery result.

The problem is that, for a statement that uses an IN subquery, the optimizer rewrites it as a correlated subquery. Consider the following statement that uses an uncorrelated subquery:

SELECT ... FROM t1 WHERE t1.a IN (SELECT b FROM t2);

The optimizer rewrites the statement to a correlated subquery:

SELECT ... FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t2.b = t1.a);

If the inner and outer queries return M and N rows, respectively, the execution time becomes on the order of O(M×N), rather than O(M+N) as it would be for an uncorrelated subquery.

An implication is that an IN subquery can be much slower than a query written using an IN(value_list) construct that lists the same values that the subquery would return.

In general, you cannot modify a table and select from the same table in a subquery. For example, this limitation applies to statements of the following forms:

DELETE FROM t WHERE ... (SELECT ... FROM t ...);
UPDATE t ... WHERE col = (SELECT ... FROM t ...);
{INSERT|REPLACE} INTO t (SELECT ... FROM t ...);

Exception: The preceding prohibition does not apply if you are using a subquery for the modified table in the FROM clause. Example:

UPDATE t ... WHERE col = (SELECT (SELECT ... FROM t...) AS _t ...);

Here the prohibition does not apply because a subquery in the FROM clause is materialized as a temporary table, so the relevant rows in t have already been selected by the time the update to t takes place.

Row comparison operations are only partially supported:

In other words, for a subquery that returns rows of n-tuples, this is supported:

(val_1, ..., val_n) IN (subquery)

But this is not supported:

(val_1, ..., val_n) op {ALL|ANY|SOME} (subquery)

The reason for supporting row comparisons for IN but not for the others is that IN is implemented by rewriting it as a sequence of = comparisons and AND operations. This approach cannot be used for ALL, ANY, or SOME.

Row constructors are not well optimized. The following two expressions are equivalent, but only the second can be optimized:

(col1, col2, ...) = (val1, val2, ...)
col1 = val1 AND col2 = val2 AND ...

Subqueries in the FROM clause cannot be correlated subqueries. They are materialized (executed to produce a result set) before evaluating the outer query, so they cannot be evaluated per row of the outer query.

The optimizer is more mature for joins than for subqueries, so in many cases a statement that uses a subquery can be executed more efficiently if you rewrite it as a join.

An exception occurs for the case where an IN subquery can be rewritten as a SELECT DISTINCT join. Example:

SELECT col FROM t1 WHERE id_col IN (SELECT id_col2 FROM t2 WHERE condition);

That statement can be rewritten as follows:

SELECT DISTINCT col FROM t1, t2 WHERE t1.id_col = t2.id_col AND condition;

But in this case, the join requires an extra DISTINCT operation and is not more efficient than the subquery.

Possible future optimization: MySQL does not rewrite the join order for subquery evaluation. In some cases, a subquery could be executed more efficiently if MySQL rewrote it as a join. This would give the optimizer a chance to choose between more execution plans. For example, it could decide whether to read one table or the other first.

Example:

SELECT a FROM outer_table AS ot
WHERE a IN (SELECT a FROM inner_table AS it WHERE ot.b = it.b);

For that query, MySQL always scans outer_table first and then executes the subquery on inner_table for each row. If outer_table has a lot of rows and inner_table has few rows, the query probably will not be as fast as it could be.

The preceding query could be rewritten like this:

SELECT a FROM outer_table AS ot, inner_table AS it
WHERE ot.a = it.a AND ot.b = it.b;

In this case, we can scan the small table (inner_table) and look up rows in outer_table, which will be fast if there is an index on (ot.a,ot.b).

Possible future optimization: A correlated subquery is evaluated for each row of the outer query. A better approach is that if the outer row values do not change from the previous row, do not evaluate the subquery again. Instead, use its previous result.

Possible future optimization: A subquery in the FROM clause is evaluated by materializing the result into a temporary table, and this table does not use indexes. This does not allow the use of indexes in comparison with other tables in the query, although that might be useful.

Possible future optimization: If a subquery in the FROM clause resembles a view to which the merge algorithm can be applied, rewrite the query and apply the merge algorithm so that indexes can be used. The following statement contains such a subquery:

SELECT * FROM (SELECT * FROM t1 WHERE t1.t1_col) AS _t1, t2 WHERE t2.t2_col;

The statement can be rewritten as a join like this:

SELECT * FROM t1, t2 WHERE t1.t1_col AND t2.t2_col;

This type of rewriting would provide two benefits:

Possible future optimization: For IN, = ANY, <> ANY, = ALL, and <> ALL with non-correlated subqueries, use an in-memory hash for a result result or a temporary table with an index for larger results. Example:

SELECT a FROM big_table AS bt
WHERE non_key_field IN (SELECT non_key_field FROM table WHERE condition)

In this case, we could create a temporary table:

CREATE TABLE t (key (non_key_field))
(SELECT non_key_field FROM table WHERE condition)

Then, for each row in big_table, do a key lookup in t based on bt.non_key_field.

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).

If a statement prepared by PREPARE refers to a view, the view contents seen each time the statement is executed later will be the contents 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 1;
PREPARE s FROM 'SELECT * FROM v';
ALTER VIEW v AS SELECT 2;
EXECUTE s;

The result returned by the EXECUTE statement is 1, not 2.

If a statement in a stored routine refers to a view, the view contents seen by the statement are its contents 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 contents, even if the view 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.

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.