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From: Masahiro Ikeda <[email protected]>
To: Fujii Masao <[email protected]>
To: [email protected]
Subject: Re: more detailed description of tup_returned and tup_fetched
Date: Tue, 18 May 2021 13:20:59 +0900
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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	<[email protected]>



On 2021/05/17 20:46, Fujii Masao wrote:
> 
> 
> 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?

Sorry, I couldn't find the way to increment tup_returned by TidScan.
Do you mean that Tid Range Scan increments the counter?

Tid Range Scan increments the tup_returned, and
pg_stat_all_tables.seq_tup_read is also incremented. I thought it's ok because
Tid Range Scan is like sequential scan. That's the reason why the document of
pg_stat_all_tables.seq_tup_read says "Number of live rows fetched by
sequential scans"

Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION

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