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
