Function getBaseTypeAndTypmod

src/backend/utils/cache/lsyscache.c:3138–3168

* getBaseTypeAndTypmod * If the given type is a domain, return its base type and typmod; * otherwise return the type's own OID, and leave *typmod unchanged. * * Note that the "applied typmod" should be -1 for every domain level * above the bottommost; therefore, if the passed-in typid is indeed * a domain, *typmod should be -1. */

 * a domain, *typmod should be -1.
 */
Oid
getBaseTypeAndTypmod(Oid typid, int32 *typmod)
{
	/*
	 * We loop to find the bottom base type in a stack of domains.
	 */
	for (;;)
	{
		HeapTuple	tup;
		Form_pg_type typTup;

		tup = SearchSysCache1(TYPEOID, ObjectIdGetDatum(typid));
		if (!HeapTupleIsValid(tup))
			elog(ERROR, "cache lookup failed for type %u", typid);
		typTup = (Form_pg_type) GETSTRUCT(tup);
		if (typTup->typtype != TYPTYPE_DOMAIN)
		{
			/* Not a domain, so done */
			ReleaseSysCache(tup);
			break;
		}

		Assert(*typmod == -1);
		typid = typTup->typbasetype;
		*typmod = typTup->typtypmod;

		ReleaseSysCache(tup);
	}

	return typid;
}

/*
 * get_typavgwidth