Message-ID: <32cd9a82-9221-f081-87d4-1f4fa6359a4d@gmail.com>
Date: Tue, 28 Mar 2023 07:49:45 +0200
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Subject: Re: Reconcile stats in find_tabstat_entry() and get rid of
 PgStat_BackendFunctionEntry
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To: Michael Paquier <michael@paquier.xyz>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: melanieplageman@gmail.com, andres@anarazel.de,
 pgsql-hackers@lists.postgresql.org
References: <ZBuEPgVYzigWcldN@paquier.xyz>
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 <ZCIa4alHwix6o0if@paquier.xyz>
 <20230328.123615.1049160234680207474.horikyota.ntt@gmail.com>
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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
In-Reply-To: <ZCJ53/VggExTFoO+@paquier.xyz>
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Hi,

On 3/28/23 7:23 AM, Michael Paquier wrote:
> On Tue, Mar 28, 2023 at 12:36:15PM +0900, Kyotaro Horiguchi wrote:
>> I found that commit ddfc2d9a37 removed the descriptions for
>> pg_stat_get_blocks_fetched and pg_stat_get_blocks_hit. Right before
>> that commit, monitoring.sgml had these lines:
>>
>> -     <function>pg_stat_get_blocks_fetched</function> minus
>> -     <function>pg_stat_get_blocks_hit</function> gives the number of kernel
>> -     <function>read()</> calls issued for the table, index, or
>> -     database; the number of actual physical reads is usually
>> -     lower due to kernel-level buffering.  The <literal>*_blks_read</>
>> -     statistics columns use this subtraction, i.e., fetched minus hit.
>>
>> The commit then added the following sentence to the description for
>> pg_statio_all_tables.heap_blks_read.
>>
>> Later, in 5f2b089387 it twas revised as:
>> +     <entry>Number of disk blocks read in this database</entry>
> 
> Yeah, maybe adding something like that at the bottom of the table for
> stat functions, telling that the difference is the number of read()
> calls, may help.  Perhaps also adding a mention that these are used in
> none of the existing system views..
> 
>> The confusion stems from the inconsistency between the views and
>> underlying functions related to block reads and hits. If we add
>> descriptions for the two functions, we should also explain their
>> relationship. 

I agree that adding more explanation would help and avoid confusion.

What about something like?

for pg_stat_get_xact_blocks_fetched(): "block read requests for table or index, in the current
transaction. This number minus pg_stat_get_xact_blocks_hit() gives the number of kernel
read() calls."

pg_stat_get_xact_blocks_hit(): "block read requests for table or index, in the current
transaction, found in cache (not triggering kernel read() calls)".

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com