RE: ALTER SESSION - Mailing list pgsql-hackers

Thanks for a cool feature with nice UI.  Can I expect it to work for background processes?  For troubleshooting, I
wantedto investigate how autovacuum launcher/worker behaves by having them emit DEBUG messages.
 

(My comments follow)


From: Kyotaro HORIGUCHI [mailto:horiguchi.kyotaro@lab.ntt.co.jp]
> .auto.conf is already a kind of such.. My first version signals the change
> via shared memory (in a largely-improvable way) and add the GUC system the
> complex "nontransactional SET" feature, which lets a change persists beyond
> transaction end if any. Holding changes until the session becomes idle seems
> also complex.
> 
> https://www.postgresql.org/message-id/20181127.193622.252197705.horigu
> chi.kyotaro@lab.ntt.co.jp
> 
> The most significant reason for passing-by-file is the affinity with the
> current GUC system.

Regarding the target session specification, do we want to use pid or a session ID like the following?

https://www.postgresql.org/docs/devel/runtime-config-logging.html
--------------------------------------------------
The %c escape prints a quasi-unique session identifier, consisting of two 4-byte hexadecimal numbers (without leading
zeros)separated by a dot. The numbers are the process start time and the process ID, so %c can also be used as a space
savingway of printing those items. For example, to generate the session identifier from pg_stat_activity, use this
query:

SELECT to_hex(trunc(EXTRACT(EPOCH FROM backend_start))::integer) || '.' ||
       to_hex(pid)
FROM pg_stat_activity;

  PRIMARY KEY (session_id, session_line_num)
--------------------------------------------------


pid is easier to type.  However, the session startup needs to try to delete the leftover file.  Is the file system
accessnegligible compared with the total heavy session startup processing?
 

If we choose session ID, the session startup doesn't have to care about the leftover file.  However, the background
processcrash could leave the file for a long time, since the crash may not lead to postmaster restart.  Also, we will
getinclined to add sessionid column in pg_stat_activity (the concept of session ID can be useful for other uses.)
 


I'm OK about passing parameter changes via a file.  But I'm not sure whether using DSM (DSA) is easier with less code.

And considering the multi-threaded server Konstantin is proposing, would it better to take pass-by-memory approach?  I
imaginethat if the server gets multi-threaded, the parameter change would be handled like:
 

1. Allocate memory for one parameter change.
2. Write the change to that memory.
3. Link the memory to a session-specific list.
4. The target session removes the entry from the list, applies the change, and frees the memory.

The code modification may be minimal when we migrate to the multi-threaded server -- only memory allocation and free
functions.


Regards
Takayuki Tsunakawa