postgres pg_stat_all_indexes 中 idx_tup_read 和 idx_tup_fetch 的差异

我已经阅读了文档和以前的 stackoverflow 问题，但没有发现那些有用的问题。Postgres 版本 10.6

即使对于仅索引扫描，idx_tup_fetch 的值也在增加。所以我无法理解 idx_tup_read 和 idx_tup_fetch 之间的区别。

最初的理解是：idx_tup_read：从索引中读取的匹配记录。

idx_tup_fetch：记录在索引上匹配后从表中获取的实时行。所以我的理解是，只扫描索引，因为没有从表中获取数据，所以这不应该增加。

但是当我进行测试时，这种理解是不正确的。

postgres=> select * from pg_stat_all_indexes where indexrelname like 'test_monika_rank';
 relid | indexrelid | schemaname |   relname   |   indexrelname   | idx_scan | idx_tup_read | idx_tup_fetch 
-------+------------+------------+-------------+------------------+----------+--------------+---------------
 16413 |      16418 | public     | test_monika | test_monika_rank |        4 |            2 |             2
(1 row)

postgres=> select * from test_monika where rank=100;
 id | name | rank 
----+------+------
 95 |      |  100
(1 row)

postgres=> select * from pg_stat_all_indexes where indexrelname like 'test_monika_rank';
 relid | indexrelid | schemaname |   relname   |   indexrelname   | idx_scan | idx_tup_read | idx_tup_fetch 
-------+------------+------------+-------------+------------------+----------+--------------+---------------
 16413 |      16418 | public     | test_monika | test_monika_rank |        5 |            3 |             3
(1 row)

postgres=> select rank from test_monika where rank=100;
 rank 
------
  100
(1 row)

postgres=> select * from pg_stat_all_indexes where indexrelname like 'test_monika_rank';
 relid | indexrelid | schemaname |   relname   |   indexrelname   | idx_scan | idx_tup_read | idx_tup_fetch 
-------+------------+------------+-------------+------------------+----------+--------------+---------------
 16413 |      16418 | public     | test_monika | test_monika_rank |        6 |            4 |             4
(1 row)

postgres=> select id from test_monika where rank=100;
 id 
----
 95
(1 row)

postgres=> select * from pg_stat_all_indexes where indexrelname like 'test_monika_rank';
 relid | indexrelid | schemaname |   relname   |   indexrelname   | idx_scan | idx_tup_read | idx_tup_fetch 
-------+------------+------------+-------------+------------------+----------+--------------+---------------
 16413 |      16418 | public     | test_monika | test_monika_rank |        7 |            5 |             5
(1 row)

postgres=> select 1 from test_monika where rank=100;
 ?column? 
----------
        1
(1 row)

postgres=> select * from pg_stat_all_indexes where indexrelname like 'test_monika_rank';
 relid | indexrelid | schemaname |   relname   |   indexrelname   | idx_scan | idx_tup_read | idx_tup_fetch 
-------+------------+------------+-------------+------------------+----------+--------------+---------------
 16413 |      16418 | public     | test_monika | test_monika_rank |        8 |            6 |             6
(1 row)