<para>

<function>PQfn</function> always returns a valid

<structname>PGresult</structname> pointer. The result status should be

<structname>PGresult</structname> pointer, with

status <literal>PGRES_COMMAND_OK</literal> for success

or <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.

The result status should be

checked before the result is used. The caller is responsible for

freeing the <structname>PGresult</structname> with

<xref linkend="libpq-PQclear"/> when it is no longer needed.

</para>

<para>

Note that it is not possible to handle null arguments, null results,

nor set-valued results when using this interface.

To pass a NULL argument to the function, set

the <parameter>len</parameter> field of that parameter structure

to <literal>-1</literal>; the <parameter>isint</parameter>

and <parameter>u</parameter> fields are then irrelevant.

(But this works only in protocol 3.0 and later connections.)

</para>

<para>

If the function returns NULL, <parameter>*result_len</parameter> is set

to <literal>-1</literal>, and <parameter>*result_buf</parameter> is not

modified. (This works only in protocol 3.0 and later connections; in

protocol 2.0, neither <parameter>*result_len</parameter>

nor <parameter>*result_buf</parameter> are modified.)

</para>

<para>

Note that it is not possible to handle set-valued results when using

this interface. Also, the function must be a plain function, not an

aggregate, window function, or procedure.

</para>

</sect1>