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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From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <dc21f9fd-52de-ba5d-24c2-c87069986f6f@oss.nttdata.com>
Date: Tue, 18 May 2021 20:10:56 +0900
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On 2021/05/18 18:23, Masahiro Ikeda wrote:
> 
> 
> On 2021/05/18 16:01, Fujii Masao wrote:
>> On 2021/05/18 13:20, Masahiro Ikeda wrote:
>>> 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.
>>
>> Yes, you're right. One interesting thing I found is;
>> when Tid Range Scan happens, seq_tup_read is incremented
>> but seq_scan is not. I'm not sure if this is expected behavior or not.
> 
> The following comment says that this behavior is expected. But, I agree it's
> odd and it's natural both seq_tup_read and seq_scan are incremented at the
> same time or not...
> 
> /*
>   * Currently, we only have a stats counter for sequential heap scans (but
>   * e.g for bitmap scans the underlying bitmap index scans will be counted,
>   * and for sample scans we update stats for tuple fetches).
>   */
> if (scan->rs_base.rs_flags & SO_TYPE_SEQSCAN)
> 	pgstat_count_heap_scan(scan->rs_base.rs_rd);
> 
> 
>>> That's the reason why the document of
>>> pg_stat_all_tables.seq_tup_read says "Number of live rows fetched by
>>> sequential scans"
>>
>> Regarding the original issue, as far as I understand correctly,
>>
>> * pg_stat_database.tup_returned = sum(pg_stat_all_tables.seq_tup_read) +
>> sum(pg_stat_all_indexes.idx_tup_read)
>> * pg_stat_database.tup_fetched = sum(pg_stat_all_tables.idx_tup_fetch)
>>
>> But the counters for some system catalogs like pg_database shared
>> across all databases of a cluster are excluded from that calculation.
>> Is this my understanding right? If right, probably we can reuse
>> the existing descriptions for those counters to document
>> pg_stat_database counters. For example,
> 
> Yes, my understanding is same now.
> 
> 
>> pg_stat_database.tup_returned:> Number of live rows fetched by sequential and index scans in this database
> 
> I wonder "live rows fetched by index scans" may mislead. I think "live" means
> it's not dead tuple and "rows" mean the tuple user want to get.
> 
> But, pg_stat_all_indexes.idx_tup_read says that "index entires returned by
> scans on this index". There is no meaning of "live" and "rows", so I thought
> it's better to distinguish them.
> 
> So, why don't you change to "Number of live rows fetched by sequential scans
> and index entries returned by index scans in this database"?

Yes, LGTM.


>> pg_stat_database.tup_fetched:
>> Number of index entries returned by scans on indexes in this database
> Is this the sum of pg_stat_all_indexes.idx_tup_read? This is accounted to
> pg_stat_database.tup_returned.

I was thinking that pg_stat_database.tup_fetched is the same as
the sum of pg_stat_all_tables.idx_tup_fetch. Because they both
are incremented by bitmap index scans, but pg_stat_all_indexes.idx_tup_read
is not.

Regards,

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