pg_stat_database的字段tup_returned，tup_fetched含义

--以前一直对tup_returned，tup_fetched的意思有疑惑，不知道两者之间的区别到底是什么，官网解释如下：
tup_returned：Number of rows returned by queries in this database
tup_fetched ：Number of rows fetched by queries in this database


--查postgres数据库的基础数据如下
postgres=# select datname,pg_size_pretty(temp_bytes),temp_files,tup_returned,tup_fetched from pg_stat_database where datname = 'postgres';
 datname  | pg_size_pretty | temp_files | tup_returned | tup_fetched 
----------+----------------+------------+--------------+-------------
 postgres | 0 bytes        |          0 |      2703567 |       70611
(1 row)

--统计一个表的行数
postgres=# select count(*) from test_kenyon ;
 count 
-------
    23
(1 row)


--再次查看postgres数据库的统计，可以发现tup_fetched，是数据库最终呈现给用户的行数，tup_returned是数据库用户为呈现这些数据要返回给客户的端的行数
--听着有占绕，比如上例tup_fetched差值为1，说明count(*)之后数据库呈现给用户的行数为1，tup_returned的差值为24，说明数据库要返回count(*)的值要查询到满
postgres=# select datname,pg_size_pretty(temp_bytes),temp_files,tup_returned,tup_fetched from pg_stat_database where datname = 'postgres';
 datname  | pg_size_pretty | temp_files | tup_returned | tup_fetched 
----------+----------------+------------+--------------+-------------
 postgres | 0 bytes        |          0 |      2703591 |       70612
(1 row)

postgres=# select 2703591-2703567;
 ?column? 
----------
       24