A subprogram has side effects if it changes anything except the values of its own local variables. For example, a subprogram that changes any of the following has side effects:

• Its own OUT or IN OUT parameter

• A global variable

• A public variable in a package

• A database table

• The database

• The external state (by invoking DBMS_OUTPUT or sending e‐mail, for example)

Side effects can prevent the parallelization of a query, yield order-dependent (and therefore, indeterminate) results, or require that package state be maintained across user sessions.

Minimizing side effects is especially important when defining a result-cached function or a stored function for SQL statements to invoke.