Function RelationInitTableAccessMethod

src/backend/utils/cache/relcache.c:1854–1904

* Initialize table access method support for a table like relation */

 * Initialize table access method support for a table like relation
 */
void
RelationInitTableAccessMethod(Relation relation)
{
	HeapTuple	tuple;
	Form_pg_am	aform;

	if (relation->rd_rel->relkind == RELKIND_SEQUENCE)
	{
		/*
		 * Sequences are currently accessed like heap tables, but it doesn't
		 * seem prudent to show that in the catalog. So just overwrite it
		 * here.
		 */
		relation->rd_amhandler = F_HEAP_TABLEAM_HANDLER;
	}
	else if (IsCatalogRelation(relation))
	{
		/*
		 * Avoid doing a syscache lookup for catalog tables.
		 */
		Assert(relation->rd_rel->relam == HEAP_TABLE_AM_OID);
		relation->rd_amhandler = F_HEAP_TABLEAM_HANDLER;
	}
	else
	{
		/*
		 * Look up the table access method, save the OID of its handler
		 * function.
		 */
		Assert(relation->rd_rel->relam != InvalidOid);
		tuple = SearchSysCache1(AMOID,
								ObjectIdGetDatum(relation->rd_rel->relam));
		if (!HeapTupleIsValid(tuple))
			elog(ERROR, "cache lookup failed for access method %u",
				 relation->rd_rel->relam);
		aform = (Form_pg_am) GETSTRUCT(tuple);
		relation->rd_amhandler = aform->amhandler;
		ReleaseSysCache(tuple);
		/*
		 * Cloudberry: append-optimized relations should not have a valid
		 * relfrozenxid.
		 */
		Assert (!RelationStorageIsAO(relation) ||
				!TransactionIdIsValid(relation->rd_rel->relfrozenxid));
	}

	/*
	 * Now we can fetch the table AM's API struct
	 */
	InitTableAmRoutine(relation);
}

/*
 *		formrdesc