From 296b3cc6010bbe1670ef02706ef90d21df667f92 Mon Sep 17 00:00:00 2001
From: Joel Jacobson <joel@compiler.org>
Date: Sat, 30 Aug 2025 12:13:35 +0200
Subject: [PATCH] Add error_on_null() to produce an error if the input is null

This polymorphic function produces an error if the input value is null,
otherwise it returns the input value unchanged.
---
 doc/src/sgml/func/func-comparison.sgml       | 20 ++++++++++++++++++++
 src/backend/utils/adt/misc.c                 | 15 +++++++++++++++
 src/include/catalog/pg_proc.dat              |  3 +++
 src/test/regress/expected/misc_functions.out | 17 +++++++++++++++++
 src/test/regress/sql/misc_functions.sql      |  8 ++++++++
 5 files changed, 63 insertions(+)

diff --git a/doc/src/sgml/func/func-comparison.sgml b/doc/src/sgml/func/func-comparison.sgml
index c1205983f8b..4986442551c 100644
--- a/doc/src/sgml/func/func-comparison.sgml
+++ b/doc/src/sgml/func/func-comparison.sgml
@@ -631,6 +631,26 @@ SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in
         <returnvalue>1</returnvalue>
        </para></entry>
       </row>
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>error_on_null</primary>
+        </indexterm>
+        <function>error_on_null</function> ( <type>anyelement</type> )
+        <returnvalue>anyelement | error</returnvalue>
+       </para>
+       <para>
+        Produces an error if the input is null, and returns the input otherwise.
+       </para>
+       <para>
+        <literal>error_on_null(42)</literal>
+        <returnvalue>42</returnvalue>
+       </para>
+       <para>
+        <literal>error_on_null(null)</literal>
+        <returnvalue>does not return</returnvalue>
+       </para></entry>
+      </row>
      </tbody>
     </tgroup>
    </table>
diff --git a/src/backend/utils/adt/misc.c b/src/backend/utils/adt/misc.c
index 6c5e3438447..c59b0126697 100644
--- a/src/backend/utils/adt/misc.c
+++ b/src/backend/utils/adt/misc.c
@@ -185,6 +185,21 @@ pg_num_nonnulls(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32(nargs - nulls);
 }
 
+/*
+ * error_on_null()
+ *	Produces an error if the input is null, and returns the input otherwise
+ */
+Datum
+error_on_null(PG_FUNCTION_ARGS)
+{
+	if (PG_ARGISNULL(0))
+		ereport(ERROR,
+				(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+				 errmsg("null value not allowed")));
+
+	PG_RETURN_DATUM(PG_GETARG_DATUM(0));
+}
+
 
 /*
  * current_database()
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 118d6da1ace..8ffc001bf95 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -12518,6 +12518,9 @@
 { oid => '6292', descr => 'aggregate transition function',
   proname => 'any_value_transfn', prorettype => 'anyelement',
   proargtypes => 'anyelement anyelement', prosrc => 'any_value_transfn' },
+{ oid => '8488', descr => 'error on null input; otherwise returns input unchanged',
+  proname => 'error_on_null', proisstrict => 'f', prorettype => 'anyelement',
+  proargtypes => 'anyelement', prosrc => 'error_on_null' },
 
 { oid => '6321', descr => 'list of available WAL summary files',
   proname => 'pg_available_wal_summaries', prorows => '100', proretset => 't',
diff --git a/src/test/regress/expected/misc_functions.out b/src/test/regress/expected/misc_functions.out
index c3b2b9d8603..f7c1daac05d 100644
--- a/src/test/regress/expected/misc_functions.out
+++ b/src/test/regress/expected/misc_functions.out
@@ -177,6 +177,23 @@ ERROR:  function num_nulls() does not exist
 LINE 1: SELECT num_nulls();
                ^
 HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
+--
+-- error_on_null()
+--
+SELECT error_on_null(1);
+ error_on_null 
+---------------
+             1
+(1 row)
+
+SELECT error_on_null(NULL::int);
+ERROR:  null value not allowed
+SELECT error_on_null(ROW(1,NULL::int));
+ error_on_null 
+---------------
+ (1,)
+(1 row)
+
 --
 -- canonicalize_path()
 --
diff --git a/src/test/regress/sql/misc_functions.sql b/src/test/regress/sql/misc_functions.sql
index 23792c4132a..d5595504fb8 100644
--- a/src/test/regress/sql/misc_functions.sql
+++ b/src/test/regress/sql/misc_functions.sql
@@ -77,6 +77,14 @@ SELECT num_nulls(VARIADIC '{}'::int[]);
 SELECT num_nonnulls();
 SELECT num_nulls();
 
+--
+-- error_on_null()
+--
+
+SELECT error_on_null(1);
+SELECT error_on_null(NULL::int);
+SELECT error_on_null(ROW(1,NULL::int));
+
 --
 -- canonicalize_path()
 --
-- 
2.50.1