From 49172d88f9d03d063e680e45fc51dffea218add1 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 30 Apr 2021 15:10:06 -0400
Subject: [PATCH] Doc: update libpq's documentation for PQfn().

Mention specifically that you can't call aggregates, window functions,
or procedures this way (the inability to call SRFs was already
mentioned).

Also, the claim that PQfn doesn't support NULL arguments or results
has been a lie since we invented protocol 3.0.  Not sure why this
text was never updated for that, but do it now.

Discussion: https://postgr.es/m/2039442.1615317309@sss.pgh.pa.us
---
 doc/src/sgml/libpq.sgml | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 9905b2a2899..a955b1b55c4 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -5152,15 +5152,35 @@ typedef struct
 
   <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
    <function>PQclear</function> 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 or window function.
   </para>
 
  </sect1>