大数据计算MaxCompute访问 holo外表 , 没有使用到直读是为什么？

大数据计算MaxCompute中，用maxcompute访问 holo外表 , 没有使用到直读是为什么, 文档上没有这个fallback的原因。

CREATE EXTERNAL TABLE IF NOT EXISTS fenghuo_db_dev.fenghuo_tag_mark_relation(
tenant_id STRING,
tag_definition_id STRING,
tag_mark_object_id STRING,
tag_mark_object STRING,
profile_id STRING,
campaign_id STRING,
top_parent_asin STRING,
ad_group_id STRING,
marketplace_id STRING,
ds STRING,
tag_store_source_table STRING,
tag_store_source_id STRING)
STORED BY 'com.aliyun.odps.jdbc.JdbcStorageHandler'
WITH SERDEPROPERTIES(
'odps.properties.rolearn'='acs:ram::x:role/aliyunodpsholorole')
LOCATION 'jdbc:postgresql://hgpostcn-cn-lbj3g07hc001-cn-hangzhou-internal.hologres.aliyuncs.com:80/fenghuo?ApplicationName=MaxCompute&currentSchema=public&preferQueryMode=simple&useSSL=false&table=fenghuo_tag_mark_relation/'
TBLPROPERTIES ('mcfed.mapreduce.jdbc.driver.class'='org.postgresql.Driver',
'odps.federation.jdbc.target.db.type'='holo',
'odps.federation.jdbc.colmapping'='tenant_id:tenant_id,tag_definition_id:tag_definition_id,tag_mark_object_id:tag_mark_object_id,profile_id:profile_id,campaign_id:campaign_id,top_parent_asin:top_parent_asin,ad_group_id:ad_group_id,marketplace_id:marketplace_id,ds:ds,tag_store_source_table:tag_store_source_table,tag_store_source_id:tag_store_source_id');