pg_total_relation_size()统计表的大小问题

select schemaname,tablename,round(sum(pg_total_relation_size(schemaname || '.' || partitiontablename)::numeric)/1024/1024,2) "MB" 
  from pg_partitions
  where tablename='a_mro_maxcoverage_lte_cell_d'
  group by 1,2;

使用该sql统计表的大小：

'edw_app','a_mro_maxcoverage_lte_cell_d','5322.85'

insert into edw_dev.tmp_cd_11 或者 create table edw_dev.tmp_cd_11  as
select schemaname,tablename,round(sum(pg_total_relation_size(schemaname || '.' || partitiontablename)::numeric)/1024/1024,2) "MB" 
  from pg_partitions
  where tablename='a_mro_maxcoverage_lte_cell_d'
  group by 1,2;

同样执行把结果插入结构表数据就变了：

'edw_app','a_mro_maxcoverage_lte_cell_d','20.63'

为什么会有这样的结果呢？