Re: Statistics updates is delayed when using `commit and chain`

On Mon, 12 Jul 2021 at 23:30, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> John Naylor <john.naylor@enterprisedb.com> writes:
>> I understand this functionality to be tracking what happens at the physical
>> level, in which case it is correct. In any case, the bug reported is clear
>> and changing that behavior is the focus here. About the patch:
>
> I do not think this is a bug at all.  The behavior is, and always has
> been, that we report stats when we are about to wait for client input
> and are not inside a transaction.  Likewise for NOTIFY.  The proposed
> patch randomly changes that in a way that is very likely to break
> clients.
>
Sorry, I'm not very clearly why we can not process NOTIFY when a transaction is
end.  For example:

------------------------------------------------------------------------------
Step 1. Session 1:
postgres# LISTEN tn;
LISTEN

Step 2. Session 2:
postgers# CREATE TABLE tbl (id int)
CREATE TABLE
postgres=# BEGIN;
BEGIN
postgres=*# INSERT INTO tbl values (1);
INSERT 0 1
postgres=*# NOTIFY tn, 'hello';
NOTIFY

Step 3. Session 1:
postgres=# SELECT * FROM tbl;
 id
----
(0 rows)

Step 4. Session 2:
postgres=*# COMMIT AND CHAIN;
COMMIT

Step 5. Session 1:
postgres=# SELECT * FROM tbl;
 id
----
  1
(1 row)

Step 6. Session 2:
postgres=*# COMMIT;
COMMIT

Step 7. Session 1:
postgres=# SELECT * FROM tbl;
 id
----
  1
(1 row)

Asynchronous notification "tn" with payload "hello" received from server process with PID 16348.

------------------------------------------------------------------------------

Since we commit the data in step 4, however the notify doesn't handle, I think
this is a bit confused.  Why we must wait session is in idle to handle the notify?

> Maybe you can persuade me that there's a reason to move the responsibility
> for stats reporting to some other place, but please keep your hands
> OFF of NOTIFY.  You clearly haven't the faintest idea what the client
> contract for that is.
>
>             regards, tom lane


-- 
Regrads,
Japin Li.
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