Subject: Re: more detailed description of tup_returned and tup_fetched
To: Masahiro Ikeda <ikedamsh@oss.nttdata.com>, pgsql-docs@lists.postgresql.org
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 <19f98fd2-c81b-f77f-d940-2816c565f850@oss.nttdata.com>
 <ca021203-2c9b-1063-c5fb-503384d781b8@oss.nttdata.com>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <03d26cbe-cce3-5cdf-89e6-df3769ff7728@oss.nttdata.com>
Date: Mon, 17 May 2021 20:46:38 +0900
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On 2021/05/17 18:58, Masahiro Ikeda wrote:
> 
> 
> 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

This is still not correct because this counter is incremented even when
other scan like TidScan happens?

Regards,

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