diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index d2bf640..fe9cf83 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -1299,22 +1299,12 @@ json_categorize_type(Oid typoid,
 				/* but let's look for a cast to json, if it's not built-in */
 				if (typoid >= FirstNormalObjectId)
 				{
-					HeapTuple	tuple;
+					Oid castfunc = get_cast_func(typoid, JSONOID);
 
-					tuple = SearchSysCache2(CASTSOURCETARGET,
-											ObjectIdGetDatum(typoid),
-											ObjectIdGetDatum(JSONOID));
-					if (HeapTupleIsValid(tuple))
+					if (OidIsValid(castfunc))
 					{
-						Form_pg_cast castForm = (Form_pg_cast) GETSTRUCT(tuple);
-
-						if (castForm->castmethod == COERCION_METHOD_FUNCTION)
-						{
-							*tcategory = JSONTYPE_CAST;
-							*outfuncoid = castForm->castfunc;
-						}
-
-						ReleaseSysCache(tuple);
+						*tcategory = JSONTYPE_CAST;
+						*outfuncoid = castfunc;
 					}
 				}
 			}
diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 552e498..0da08fe 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -21,6 +21,7 @@
 #include "bootstrap/bootstrap.h"
 #include "catalog/pg_amop.h"
 #include "catalog/pg_amproc.h"
+#include "catalog/pg_cast.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_constraint.h"
 #include "catalog/pg_namespace.h"
@@ -2889,3 +2890,36 @@ get_range_subtype(Oid rangeOid)
 	else
 		return InvalidOid;
 }
+
+/*
+ *
+ * get_cast_func(fromtype, totype) -> funcoid
+ *
+ * find a cast function if any from fromtype to totype.
+ *
+ * return the Oid of the fucntion found or InvalidOid otherwise.
+ */
+
+Oid
+get_cast_func(Oid from_type, Oid to_type)
+{
+	HeapTuple	tuple;
+	Oid castfunc = InvalidOid;
+
+	tuple = SearchSysCache2(CASTSOURCETARGET,
+							ObjectIdGetDatum(from_type),
+							ObjectIdGetDatum(to_type));
+	if (HeapTupleIsValid(tuple))
+	{
+		Form_pg_cast castForm = (Form_pg_cast) GETSTRUCT(tuple);
+
+		if (castForm->castmethod == COERCION_METHOD_FUNCTION)
+		{
+			castfunc = castForm->castfunc;
+		}
+
+		ReleaseSysCache(tuple);
+	}
+
+	return castfunc;
+}
diff --git a/src/include/utils/lsyscache.h b/src/include/utils/lsyscache.h
index 07d24d4..0291b96 100644
--- a/src/include/utils/lsyscache.h
+++ b/src/include/utils/lsyscache.h
@@ -151,6 +151,7 @@ extern void free_attstatsslot(Oid atttype,
 				  float4 *numbers, int nnumbers);
 extern char *get_namespace_name(Oid nspid);
 extern Oid	get_range_subtype(Oid rangeOid);
+extern Oid  get_cast_func(Oid from_type, Oid to_type);
 
 #define type_is_array(typid)  (get_element_type(typid) != InvalidOid)
 /* type_is_array_domain accepts both plain arrays and domains over arrays */