pg_get_partkeydef: return NULL for non-partitions

Our general rule for pg_get_X(oid) functions is to simply return NULL
when passed an invalid or inappropriate OID.  Teach pg_get_partkeydef to
do this also, making it easier for users to use this function when
querying against tables with both partitions and non-partitions (such as
pg_class).

As a concrete example, this makes pg_dump's life a little easier.

Author: Amit Langote

src/backend/utils/adt/ruleutils.c

@@ -320,7 +320,7 @@ static char *pg_get_indexdef_worker(Oid indexrelid, int colno,
 					   int prettyFlags, bool missing_ok);
 static char *pg_get_statisticsext_worker(Oid statextid, bool missing_ok);
 static char *pg_get_partkeydef_worker(Oid relid, int prettyFlags,
-						 bool attrsOnly);
+						 bool attrsOnly, bool missing_ok);
 static char *pg_get_constraintdef_worker(Oid constraintId, bool fullCommand,
 							int prettyFlags, bool missing_ok);
 static text *pg_get_expr_worker(text *expr, Oid relid, const char *relname,
@@ -1555,10 +1555,14 @@ Datum
 pg_get_partkeydef(PG_FUNCTION_ARGS)
 {
 	Oid			relid = PG_GETARG_OID(0);
+	char	   *res;
+
+	res = pg_get_partkeydef_worker(relid, PRETTYFLAG_INDENT, false, true);
+
+	if (res == NULL)
+		PG_RETURN_NULL();
 
-	PG_RETURN_TEXT_P(string_to_text(pg_get_partkeydef_worker(relid,
-														PRETTYFLAG_INDENT,
-															 false)));
+	PG_RETURN_TEXT_P(string_to_text(res));
 }
 
 /* Internal version that just reports the column definitions */
@@ -1568,7 +1572,7 @@ pg_get_partkeydef_columns(Oid relid, bool pretty)
 	int			prettyFlags;
 
 	prettyFlags = pretty ? PRETTYFLAG_PAREN | PRETTYFLAG_INDENT : PRETTYFLAG_INDENT;
-	return pg_get_partkeydef_worker(relid, prettyFlags, true);
+	return pg_get_partkeydef_worker(relid, prettyFlags, true, false);
 }
 
 /*
@@ -1576,7 +1580,7 @@ pg_get_partkeydef_columns(Oid relid, bool pretty)
  */
 static char *
 pg_get_partkeydef_worker(Oid relid, int prettyFlags,
-						 bool attrsOnly)
+						 bool attrsOnly, bool missing_ok)
 {
 	Form_pg_partitioned_table form;
 	HeapTuple	tuple;
@@ -1594,7 +1598,11 @@ pg_get_partkeydef_worker(Oid relid, int prettyFlags,
 
 	tuple = SearchSysCache1(PARTRELID, ObjectIdGetDatum(relid));
 	if (!HeapTupleIsValid(tuple))
+	{
+		if (missing_ok)
+			return NULL;
 		elog(ERROR, "cache lookup failed for partition key of %u", relid);
+	}
 
 	form = (Form_pg_partitioned_table) GETSTRUCT(tuple);
 

src/test/regress/expected/rules.out

@@ -3212,6 +3212,12 @@ SELECT pg_get_function_arg_default('pg_class'::regclass, 0);
  
 (1 row)
 
+SELECT pg_get_partkeydef(0);
+ pg_get_partkeydef 
+-------------------
+ 
+(1 row)
+
 -- test rename for a rule defined on a partitioned table
 CREATE TABLE parted_table (a int) PARTITION BY LIST (a);
 CREATE TABLE parted_table_1 PARTITION OF parted_table FOR VALUES IN (1);

src/test/regress/sql/rules.sql

@@ -1163,6 +1163,7 @@ SELECT pg_get_function_identity_arguments(0);
 SELECT pg_get_function_result(0);
 SELECT pg_get_function_arg_default(0, 0);
 SELECT pg_get_function_arg_default('pg_class'::regclass, 0);
+SELECT pg_get_partkeydef(0);
 
 -- test rename for a rule defined on a partitioned table
 CREATE TABLE parted_table (a int) PARTITION BY LIST (a);