Re: more detailed description of tup_returned and tup_fetched

On 2021/05/14 17:00, Masahiro Ikeda wrote:
> Hi,
>
> I worried the difference between "tup_returned" and "tup_fetched" in
> pg_stat_database. I assumed that "tup_returned" means the number of tuples
> that returned to clients. Of course, this is wrong.

- Number of rows returned by queries in this database
+ Number of live rows returned by sequential scans of queries in this database

- Number of rows fetched by queries in this database
+ Number of live rows fetched by index scan of queries in this database

I found the following comments in pgstat.h. So maybe even these
new descriptions are incorrect?

* Note: for a table, tuples_returned is the number of tuples successfully
* fetched by heap_getnext, while tuples_fetched is the number of tuples
* successfully fetched by heap_fetch under the control of bitmap indexscans.
* For an index, tuples_returned is the number of index entries returned by
* the index AM, while tuples_fetched is the number of tuples successfully
* fetched by heap_fetch under the control of simple indexscans for this index.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION