MySQL functions are tagged internally as deterministic or nondeterministic. A function is nondeterministic if, given fixed values for its arguments, it can return different results for different invocations. Examples of nondeterministic functions: RAND(), UUID().

If a function is tagged nondeterministic, a reference to it in a WHERE clause is evaluated for every row (when selecting from one table) or combination of rows (when selecting from a multiple-table join).

MySQL also determines when to evaluate functions based on types of arguments, whether the arguments are table columns or constant values. A deterministic function that takes a table column as argument must be evaluated whenever that column changes value.

Nondeterministic functions may affect query performance. For example, some optimizations may not be available, or more locking might be required. The following discussion uses RAND() but applies to other nondeterministic functions as well.

Suppose that a table t has this definition:

CREATE TABLE t (id INT NOT NULL PRIMARY KEY, col_a VARCHAR(100));

Consider these two queries:

SELECT * FROM t WHERE id = POW(1,2);
SELECT * FROM t WHERE id = FLOOR(1 + RAND() * 49);

Both queries appear to use a primary key lookup because of the equality comparison against the primary key, but that is true only for the first of them:

The effects of nondeterminism are not limited to SELECT statements. This UPDATE statement uses a nondeterministic function to select rows to be modified:

UPDATE t SET col_a = some_expr WHERE id = FLOOR(1 + RAND() * 49);

Presumably the intent is to update at most a single row for which the primary key matches the expression. However, it might update zero, one, or multiple rows, depending on the id column values and the values in the RAND() sequence.

The behavior just described has implications for performance and replication:

The difficulties stem from the fact that the RAND() function is evaluated once for every row of the table. To avoid multiple function evaluations, use one of these techniques:

As mentioned previously, a nondeterministic expression in the WHERE clause might prevent optimizations and result in a table scan. However, it may be possible to partially optimize the WHERE clause if other expressions are deterministic. For example:

SELECT * FROM t WHERE partial_key=5 AND some_column=RAND();

If the optimizer can use partial_key to reduce the set of rows selected, RAND() is executed fewer times, which diminishes the effect of nondeterminism on optimization.