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<refentry id="SQL-CREATEFUNCTION">

<listitem>

<para>

These attributes inform the system whether it is safe to

replace multiple evaluations of the function with a single

evaluation, for run-time optimization. At most one choice

These attributes inform the query optimizer about the behavior

of the function. At most one choice

may be specified. If none of these appear,

<literal>VOLATILE</literal> is the default assumption.

</para>

<para>

<literal>IMMUTABLE</literal> indicates that the function always

<literal>IMMUTABLE</literal> indicates that the function

cannot modify the database and always

returns the same result when given the same argument values; that

is, it does not do database lookups or otherwise use information not

directly present in its argument list. If this option is given,

</para>

<para>

<literal>STABLE</literal> indicates that within a single table scan

the function will consistently

<literal>STABLE</literal> indicates that the function

cannot modify the database,

and that within a single table scan it will consistently

return the same result for the same argument values, but that its

result could change across SQL statements. This is the appropriate

selection for functions whose results depend on database lookups,