Add pg_describe_object function

This function is useful to obtain textual descriptions of objects as
stored in pg_depend.

Author: alvherre

doc/src/sgml/func.sgml

@@ -12847,6 +12847,11 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
        <entry><type>setof oid</type></entry>
        <entry>get the set of database OIDs that have objects in the tablespace</entry>
       </row>
+      <row>
+       <entry><literal><function>pg_describe_object(<parameter>catalog_id</parameter>, <parameter>object_id</parameter>, <parameter>object_sub_id</parameter>)</function>)</literal></entry>
+       <entry><type>text</type></entry>
+       <entry>get description of a database object</entry>
+      </row>
       <row>
        <entry><literal><function>pg_typeof(<parameter>any</parameter>)</function></literal></entry>
        <entry><type>regtype</type></entry>
@@ -12940,6 +12945,13 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
    <structname>pg_class</> catalogs.
   </para>
 
+  <para>
+   <function>pg_describe_object</function> returns a description of a database
+   object specified by catalog OID, object OID and a (possibly zero) sub-object ID.
+   This is useful to determine the identity of an object as stored in the
+   <structname>pg_depend</structname> catalog.
+  </para>
+
   <para>
    <function>pg_typeof</function> returns the OID of the data type of the
    value that is passed to it.  This can be helpful for troubleshooting or

src/backend/catalog/dependency.c

@@ -1982,118 +1982,97 @@ free_object_addresses(ObjectAddresses *addrs)
 ObjectClass
 getObjectClass(const ObjectAddress *object)
 {
+	/* only pg_class entries can have nonzero objectSubId */
+	if (object->classId != RelationRelationId &&
+		object->objectSubId != 0)
+		elog(ERROR, "invalid objectSubId 0 for object class %u",
+			 object->classId);
+
 	switch (object->classId)
 	{
 		case RelationRelationId:
 			/* caller must check objectSubId */
 			return OCLASS_CLASS;
 
 		case ProcedureRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_PROC;
 
 		case TypeRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TYPE;
 
 		case CastRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_CAST;
 
 		case ConstraintRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_CONSTRAINT;
 
 		case ConversionRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_CONVERSION;
 
 		case AttrDefaultRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_DEFAULT;
 
 		case LanguageRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_LANGUAGE;
 
 		case LargeObjectRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_LARGEOBJECT;
 
 		case OperatorRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_OPERATOR;
 
 		case OperatorClassRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_OPCLASS;
 
 		case OperatorFamilyRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_OPFAMILY;
 
 		case AccessMethodOperatorRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_AMOP;
 
 		case AccessMethodProcedureRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_AMPROC;
 
 		case RewriteRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_REWRITE;
 
 		case TriggerRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TRIGGER;
 
 		case NamespaceRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_SCHEMA;
 
 		case TSParserRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TSPARSER;
 
 		case TSDictionaryRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TSDICT;
 
 		case TSTemplateRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TSTEMPLATE;
 
 		case TSConfigRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TSCONFIG;
 
 		case AuthIdRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_ROLE;
 
 		case DatabaseRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_DATABASE;
 
 		case TableSpaceRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_TBLSPACE;
 
 		case ForeignDataWrapperRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_FDW;
 
 		case ForeignServerRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_FOREIGN_SERVER;
 
 		case UserMappingRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_USER_MAPPING;
 
 		case DefaultAclRelationId:
-			Assert(object->objectSubId == 0);
 			return OCLASS_DEFACL;
 	}
 
@@ -2807,3 +2786,27 @@ getOpFamilyDescription(StringInfo buffer, Oid opfid)
 	ReleaseSysCache(amTup);
 	ReleaseSysCache(opfTup);
 }
+
+/*
+ * SQL-level callable version of getObjectDescription
+ */
+Datum
+pg_describe_object(PG_FUNCTION_ARGS)
+{
+	Oid			classid = PG_GETARG_OID(0);
+	Oid			objid = PG_GETARG_OID(1);
+	int32		subobjid = PG_GETARG_INT32(2);
+	char	   *description = NULL;
+	ObjectAddress address;
+
+	/* for "pinned" items in pg_depend, return null */
+	if (!OidIsValid(classid) && !OidIsValid(objid))
+		PG_RETURN_NULL();
+
+	address.classId = classid;
+	address.objectId = objid;
+	address.objectSubId = subobjid;
+
+	description = getObjectDescription(&address);
+	PG_RETURN_TEXT_P(cstring_to_text(description));
+}

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201011151
+#define CATALOG_VERSION_NO	201011181
 
 #endif

src/include/catalog/pg_proc.h

@@ -3333,6 +3333,8 @@ DESCR("view system lock information");
 DATA(insert OID = 1065 (  pg_prepared_xact PGNSP PGUID 12 1 1000 0 f f f t t v 0 0 2249 "" "{28,25,1184,26,26}" "{o,o,o,o,o}" "{transaction,gid,prepared,ownerid,dbid}" _null_ pg_prepared_xact _null_ _null_ _null_ ));
 DESCR("view two-phase transactions");
 
+DATA(insert OID = 3537 (  pg_describe_object		PGNSP PGUID 12 1 0 0 f f f t f s 3 0 25 "26 26 23" _null_ _null_ _null_ _null_ pg_describe_object _null_ _null_ _null_ ));
+
 DATA(insert OID = 2079 (  pg_table_is_visible		PGNSP PGUID 12 1 0 0 f f f t f s 1 0 16 "26" _null_ _null_ _null_ _null_ pg_table_is_visible _null_ _null_ _null_ ));
 DESCR("is table visible in search path?");
 DATA(insert OID = 2080 (  pg_type_is_visible		PGNSP PGUID 12 1 0 0 f f f t f s 1 0 16 "26" _null_ _null_ _null_ _null_ pg_type_is_visible _null_ _null_ _null_ ));

src/include/utils/builtins.h

@@ -1045,6 +1045,9 @@ extern Datum window_nth_value(PG_FUNCTION_ARGS);
 /* access/transam/twophase.c */
 extern Datum pg_prepared_xact(PG_FUNCTION_ARGS);
 
+/* catalogs/dependency.c */
+extern Datum pg_describe_object(PG_FUNCTION_ARGS);
+
 /* commands/constraint.c */
 extern Datum unique_key_recheck(PG_FUNCTION_ARGS);