Re: [Patch] ALTER SYSTEM READ ONLY

On Wed, Jun 17, 2020 at 9:51 AM tushar <tushar.ahuja@enterprisedb.com> wrote:
> 1) ALTER SYSTEM
>
> postgres=# alter system read only;
> ALTER SYSTEM
> postgres=# alter  system reset all;
> ALTER SYSTEM
> postgres=# create table t1(n int);
> ERROR:  cannot execute CREATE TABLE in a read-only transaction
>
> Initially i thought after firing 'Alter system reset all' , it will be
> back to  normal.
>
> can't we have a syntax like - "Alter system set read_only='True' ; "

No, this needs to be separate from the GUC-modification syntax, I
think. It's a different kind of state change. It doesn't, and can't,
just edit postgresql.auto.conf.

> 2)When i connected to postgres in a single user mode , i was not able to
> set the system in read only
>
> [edb@tushar-ldap-docker bin]$ ./postgres --single -D data postgres
>
> PostgreSQL stand-alone backend 14devel
> backend> alter system read only;
> ERROR:  checkpointer is not running
>
> backend>

Hmm, that's an interesting finding. I wonder what happens if you make
the system read only, shut it down, and then restart it in single-user
mode. Given what you see here, I bet you can't put it back into a
read-write state from single user mode either, which seems like a
problem. Either single-user mode should allow changing between R/O and
R/W, or alternatively single-user mode should ignore ALTER SYSTEM READ
ONLY and always allow writes anyway.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company