Subject: Re: more detailed description of tup_returned and tup_fetched
To: Masahiro Ikeda <ikedamsh@oss.nttdata.com>, pgsql-docs@lists.postgresql.org
References: <9eeeccdb-5dd7-90f9-2807-a4b5d2b76ca3@oss.nttdata.com>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <19f98fd2-c81b-f77f-d940-2816c565f850@oss.nttdata.com>
Date: Mon, 17 May 2021 15:32:38 +0900
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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