From: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Subject: Re: more detailed description of tup_returned and tup_fetched
To: Fujii Masao <masao.fujii@oss.nttdata.com>, pgsql-docs@lists.postgresql.org
References: <9eeeccdb-5dd7-90f9-2807-a4b5d2b76ca3@oss.nttdata.com>
 <19f98fd2-c81b-f77f-d940-2816c565f850@oss.nttdata.com>
Message-ID: <ca021203-2c9b-1063-c5fb-503384d781b8@oss.nttdata.com>
Date: Mon, 17 May 2021 18:58:17 +0900
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On 2021/05/17 15:32, Fujii Masao wrote:
> 
> 
> 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.

Oh, Thanks!

I updated the sentences using the descriptions of
"pg_stat_all_tables.seq_tup_read", "pg_stat_all_tables.idx_tup_fetch", and
"pg_stat_all_index.idx_tup_read".

-       Number of rows returned by queries in this database
+       Number of rows returned by queries in this database. The rows
correspond to the live rows fetched by sequential scans and index entries
returned by scans on indexes

-       Number of rows fetched by queries in this database
+       Number of rows fetched by queries in this database. The rows
correspond to the live rows fetched by index scans

Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION