Fix pg_prepared_statements.result_types for DML statements

Amendment to 84ad713cf8: Not all
prepared statements have a result descriptor.  As currently coded,
this would crash when reading pg_prepared_statements.  Make those
cases return null for result_types instead.  Also add a test case for
it.

doc/src/sgml/catalogs.sgml

@@ -11511,6 +11511,8 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
        form of an array of <type>regtype</type>. The OID corresponding
        to an element of this array can be obtained by casting the
        <type>regtype</type> value to <type>oid</type>.
+       If the prepared statement does not provide a result (e.g., a DML
+       statement), then this field will be null.
       </para></entry>
      </row>
 

src/backend/commands/prepare.c

@@ -684,15 +684,10 @@ pg_prepared_statement(PG_FUNCTION_ARGS)
 		while ((prep_stmt = hash_seq_search(&hash_seq)) != NULL)
 		{
 			TupleDesc	result_desc;
-			Oid		   *result_types;
 			Datum		values[8];
 			bool		nulls[8];
 
 			result_desc = prep_stmt->plansource->resultDesc;
-			result_types = (Oid *) palloc(result_desc->natts * sizeof(Oid));
-
-			for (int i = 0; i < result_desc->natts; i++)
-				result_types[i] = result_desc->attrs[i].atttypid;
 
 			MemSet(nulls, 0, sizeof(nulls));
 
@@ -701,7 +696,20 @@ pg_prepared_statement(PG_FUNCTION_ARGS)
 			values[2] = TimestampTzGetDatum(prep_stmt->prepare_time);
 			values[3] = build_regtype_array(prep_stmt->plansource->param_types,
 											prep_stmt->plansource->num_params);
-			values[4] = build_regtype_array(result_types, result_desc->natts);
+			if (result_desc)
+			{
+				Oid		   *result_types;
+
+				result_types = (Oid *) palloc(result_desc->natts * sizeof(Oid));
+				for (int i = 0; i < result_desc->natts; i++)
+					result_types[i] = result_desc->attrs[i].atttypid;
+				values[4] = build_regtype_array(result_types, result_desc->natts);
+			}
+			else
+			{
+				/* no result descriptor (for example, DML statement) */
+				nulls[4] = true;
+			}
 			values[5] = BoolGetDatum(prep_stmt->from_sql);
 			values[6] = Int64GetDatumFast(prep_stmt->plansource->num_generic_plans);
 			values[7] = Int64GetDatumFast(prep_stmt->plansource->num_custom_plans);

src/test/regress/expected/prepare.out

@@ -159,6 +159,9 @@ PREPARE q6 AS
     SELECT * FROM tenk1 WHERE unique1 = $1 AND stringu1 = $2;
 PREPARE q7(unknown) AS
     SELECT * FROM road WHERE thepath = $1;
+-- DML statements
+PREPARE q8 AS
+    UPDATE tenk1 SET stringu1 = $2 WHERE unique1 = $1;
 SELECT name, statement, parameter_types, result_types FROM pg_prepared_statements
     ORDER BY name;
  name |                            statement                             |                  parameter_types                   |                                                       result_types                                                       
@@ -177,7 +180,9 @@ SELECT name, statement, parameter_types, result_types FROM pg_prepared_statement
       |     SELECT * FROM tenk1 WHERE unique1 = $1 AND stringu1 = $2;    |                                                    | 
  q7   | PREPARE q7(unknown) AS                                          +| {path}                                             | {text,path}
       |     SELECT * FROM road WHERE thepath = $1;                       |                                                    | 
-(5 rows)
+ q8   | PREPARE q8 AS                                                   +| {integer,name}                                     | 
+      |     UPDATE tenk1 SET stringu1 = $2 WHERE unique1 = $1;           |                                                    | 
+(6 rows)
 
 -- test DEALLOCATE ALL;
 DEALLOCATE ALL;

src/test/regress/sql/prepare.sql

@@ -71,6 +71,10 @@ PREPARE q6 AS
 PREPARE q7(unknown) AS
     SELECT * FROM road WHERE thepath = $1;
 
+-- DML statements
+PREPARE q8 AS
+    UPDATE tenk1 SET stringu1 = $2 WHERE unique1 = $1;
+
 SELECT name, statement, parameter_types, result_types FROM pg_prepared_statements
     ORDER BY name;