Re: [HACKERS] [PATCH v1] Add and report the new "in_hot_standby"GUC pseudo-variable.

From: pgsql-hackers-owner@postgresql.org
> [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Craig Ringer
> On 13 April 2017 at 14:59, Tsunakawa, Takayuki
> <tsunakawa.takay@jp.fujitsu.com> wrote:
> 
> > 2. Make transaction_read_only GUC_REPORT This is to avoid the added
> > round-trip by SHOW command.  It also benefits client apps that want to
> know when the server gets promoted?  And this may simplify the libpq code.
> > I don't understand yet why we need to provide this feature for older servers
> by using SHOW.  Those who are already using <= 9.6 in production completed
> the system or application, and their business is running.  Why would they
> want to just replace libpq and use this feature?
> 
> I think "transaction_read_only" is a bit confusing for something we're
> expecting to change under us.
> 
> To me, a "read only" xact is one created with
> 
> BEGIN READ ONLY TRANSACTION;
> 
> .... which I would not expect to become read/write under me, since I
> explicitly asked for read-only.
> 
> It's more like "session read only" that we're interested in IMO.

I confirmed that the attached patch successfully provides:

* target_session_attrs=read-only
* If the server is >= 10, avoid the round-trip for SHOW transaction_read_only.

For this, I added a GUC_REPORT variable session_read_only which indicates the session's default read-only status.  The
characteristicsare:
 

* It cannot be changed directly by the user (postgresql.conf, SET, etc.)
* Its value is the same as default_transaction_read_only when not in recovery.
* Its value is false during recovery.

Could you include this in PG 10?  I think these are necessary as the bottom line to meet the average expectation of
users(please don't ask me what's the average; the main reasons are that PostgreSQL provides hot standby, PgJDBC enables
connectionto the standby (targetServerType=slave), and PostgreSQL emphasizes performance.)  Ideally, I wanted to add
otherfeatures of PgJDBC (e.g. targetServerType=preferSlave), but I thought this is the limit not to endanger the
qualityof the final release.
 

Regards
Takayuki Tsunakawa



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