diff --git a/src/backend/executor/functions.c b/src/backend/executor/functions.c
index 3630f5d9668..22c167574be 100644
--- a/src/backend/executor/functions.c
+++ b/src/backend/executor/functions.c
@@ -902,6 +902,7 @@ postquel_sub_params(SQLFunctionCachePtr fcache,
 	if (nargs > 0)
 	{
 		ParamListInfo paramLI;
+		Oid		   *argtypes = fcache->pinfo->argtypes;
 		int			i;
 
 		if (fcache->paramLI == NULL)
@@ -928,10 +929,24 @@ postquel_sub_params(SQLFunctionCachePtr fcache,
 		{
 			ParamExternData *prm = &paramLI->params[i];
 
-			prm->value = fcinfo->arg[i];
+			/*
+			 * If an incoming parameter value is a R/W expanded datum, we
+			 * force it to R/O.  We'd be perfectly entitled to scribble on it,
+			 * but the problem is that if the parameter is referenced more
+			 * than once in the function, earlier references might mutate the
+			 * value seen by later references, which won't do at all.  We
+			 * could do better if we could be sure of the number of Param
+			 * nodes in the function's plans; but we might not have planned
+			 * all the statements yet, nor do we have plan tree walker
+			 * infrastructure.  (Examining the parse trees is not good enough,
+			 * because of possible function inlining during planning.)
+			 */
 			prm->isnull = fcinfo->argnull[i];
+			prm->value = MakeExpandedObjectReadOnly(fcinfo->arg[i],
+													prm->isnull,
+													get_typlen(argtypes[i]));
 			prm->pflags = 0;
-			prm->ptype = fcache->pinfo->argtypes[i];
+			prm->ptype = argtypes[i];
 		}
 	}
 	else
diff --git a/src/test/regress/expected/create_function_3.out b/src/test/regress/expected/create_function_3.out
index b5e19485e56..e43aebb148a 100644
--- a/src/test/regress/expected/create_function_3.out
+++ b/src/test/regress/expected/create_function_3.out
@@ -227,9 +227,25 @@ ERROR:  could not find a function named "functest_b_1"
 DROP FUNCTION functest_b_2;  -- error, ambiguous
 ERROR:  function name "functest_b_2" is not unique
 HINT:  Specify the argument list to select the function unambiguously.
--- Cleanups
+-- Regression tests for bugs:
+-- Check that arguments that are R/W expanded datums aren't corrupted by
+-- multiple uses.  This test knows that array_append() returns a R/W datum
+-- and will modify a R/W array input in-place.  We use SETOF to prevent
+-- inlining of the SQL function.
+CREATE FUNCTION double_append(anyarray, anyelement) RETURNS SETOF anyarray
+LANGUAGE SQL IMMUTABLE AS
+$$ SELECT array_append($1, $2) || array_append($1, $2) $$;
+SELECT double_append(array_append(ARRAY[q1], q2), q3)
+  FROM (VALUES(1,2,3), (4,5,6)) v(q1,q2,q3);
+ double_append 
+---------------
+ {1,2,3,1,2,3}
+ {4,5,6,4,5,6}
+(2 rows)
+
+-- Cleanup
 DROP SCHEMA temp_func_test CASCADE;
-NOTICE:  drop cascades to 16 other objects
+NOTICE:  drop cascades to 17 other objects
 DETAIL:  drop cascades to function functest_a_1(text,date)
 drop cascades to function functest_a_2(text[])
 drop cascades to function functest_a_3()
@@ -246,5 +262,6 @@ drop cascades to function functext_f_2(integer)
 drop cascades to function functext_f_3(integer)
 drop cascades to function functext_f_4(integer)
 drop cascades to function functest_b_2(bigint)
+drop cascades to function double_append(anyarray,anyelement)
 DROP USER regress_unpriv_user;
 RESET search_path;
diff --git a/src/test/regress/sql/create_function_3.sql b/src/test/regress/sql/create_function_3.sql
index 0a0e407aaba..645102b178c 100644
--- a/src/test/regress/sql/create_function_3.sql
+++ b/src/test/regress/sql/create_function_3.sql
@@ -166,8 +166,20 @@ DROP FUNCTION functest_b_1;
 DROP FUNCTION functest_b_1;  -- error, not found
 DROP FUNCTION functest_b_2;  -- error, ambiguous
 
+-- Regression tests for bugs:
 
--- Cleanups
+-- Check that arguments that are R/W expanded datums aren't corrupted by
+-- multiple uses.  This test knows that array_append() returns a R/W datum
+-- and will modify a R/W array input in-place.  We use SETOF to prevent
+-- inlining of the SQL function.
+CREATE FUNCTION double_append(anyarray, anyelement) RETURNS SETOF anyarray
+LANGUAGE SQL IMMUTABLE AS
+$$ SELECT array_append($1, $2) || array_append($1, $2) $$;
+
+SELECT double_append(array_append(ARRAY[q1], q2), q3)
+  FROM (VALUES(1,2,3), (4,5,6)) v(q1,q2,q3);
+
+-- Cleanup
 DROP SCHEMA temp_func_test CASCADE;
 DROP USER regress_unpriv_user;
 RESET search_path;