27.6.2 View Processing Algorithms

The optional ALGORITHM clause for CREATE VIEW or ALTER VIEW is a MySQL extension to standard SQL. It affects how MySQL processes the view. ALGORITHM takes three values: MERGE, TEMPTABLE, or UNDEFINED.

For MERGE, the text of a statement that refers to the view and the view definition are merged such that parts of the view definition replace corresponding parts of the statement.
For TEMPTABLE, the results from the view are retrieved into a temporary table, which then is used to execute the statement.
For UNDEFINED, MySQL chooses which algorithm to use. It prefers MERGE over TEMPTABLE if possible, because MERGE is usually more efficient and because a view cannot be updated if a temporary table is used.
If no ALGORITHM clause is present, the default algorithm is determined by the value of the derived_merge flag of the optimizer_switch system variable. For additional discussion, see Section 10.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.

A reason to specify TEMPTABLE explicitly is that locks can be released on underlying tables after the temporary table has been created and before it is used to finish processing the statement. This might result in quicker lock release than the MERGE algorithm so that other clients that use the view are not blocked as long.

A view algorithm can be UNDEFINED for three reasons:

No ALGORITHM clause is present in the CREATE VIEW statement.
The CREATE VIEW statement has an explicit ALGORITHM = UNDEFINED clause.
ALGORITHM = MERGE is specified for a view that can be processed only with a temporary table. In this case, MySQL generates a warning and sets the algorithm to UNDEFINED.

As mentioned earlier, MERGE is handled by merging corresponding parts of a view definition into the statement that refers to the view. The following examples briefly illustrate how the MERGE algorithm works. The examples assume that there is a view v_merge that has this definition:

CREATE ALGORITHM = MERGE VIEW v_merge (vc1, vc2) AS
SELECT c1, c2 FROM t WHERE c3 > 100;

Example 1: Suppose that we issue this statement:

SELECT * FROM v_merge;

MySQL handles the statement as follows:

v_merge becomes t
* becomes vc1, vc2, which corresponds to c1, c2
The view WHERE clause is added

The resulting statement to be executed becomes:

SELECT c1, c2 FROM t WHERE c3 > 100;

Example 2: Suppose that we issue this statement:

SELECT * FROM v_merge WHERE vc1 < 100;

This statement is handled similarly to the previous one, except that vc1 < 100 becomes c1 < 100 and the view WHERE clause is added to the statement WHERE clause using an AND connective (and parentheses are added to make sure the parts of the clause are executed with correct precedence). The resulting statement to be executed becomes:

SELECT c1, c2 FROM t WHERE (c3 > 100) AND (c1 < 100);

Effectively, the statement to be executed has a WHERE clause of this form:

WHERE (select WHERE) AND (view WHERE)

If the MERGE algorithm cannot be used, a temporary table must be used instead. Constructs that prevent merging are the same as those that prevent merging in derived tables and common table expressions. Examples are SELECT DISTINCT or LIMIT in the subquery. For details, see Section 10.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.