Re: pgsql_fdw, FDW for PostgreSQL server

2012/4/9 Shigeru HANADA <shigeru(dot)hanada(at)gmail(dot)com>:
>    1) connect to the server at the beginning of the local query
>    2) execute EXPLAIN for foreign table foo
>    3) execute EXPLAIN for foreign table bar
>    4) execute actual query for foreign table foo
>    5) execute actual query for foreign table bar
>    6) disconnect from the server at the end of the local query
>
> If the connection has broken between 4) and 5), and immediate reconnect
> succeeded, retrieved results for foo and bar might be inconsistent from
> the viewpoint of transaction isolation.
>
> In current implementation, next local query which contains foreign table
> of failed foreign table tries to reconnect to the server.

How would this apply to the scenario where you haven't even begun a
transaction yet? There's no risk of inconsistency if the connection
is lost before the first command can execute, so why fail in such a
case? Isn't there a line in the sand we can draw where we say that if
we have passed it, we just die, otherwise we try to reconnect as
there's no risk of undesirable results?

>> Also I'm not particularly keen on the message provided to the user in
>> this event:
>>
>> ERROR:  could not execute EXPLAIN for cost estimation
>> DETAIL:  FATAL:  terminating connection due to administrator command
>> FATAL:  terminating connection due to administrator command
>>
>> There's no explanation what the "administrator" command was, and I
>> suspect this is really just a "I don't know what's happened here"
>> condition.  I don't think we should reach that point.
>
> That FATAL message is returned by remote backend's ProcessInterrupts()
> during some administrator commands, such as immediate shutdown or
> pg_terminate_backend().  If remote backend died of fast shutdown or
> SIGKILL, no error message is available (see the sample below).
>
> postgres=# select * From pgsql_branches ;
> ERROR:  could not execute EXPLAIN for cost estimation
> DETAIL:
> HINT:  SELECT bid, bbalance, filler FROM public.pgbench_branches
>
> I agree that the message is confusing.  How about showing message like
> "pgsql_fdw connection failure on <servername>" or something with remote
> error message for such cases?  It can be achieved by adding extra check
> for connection status right after PQexec()/PQexecParams().  Although
> some word polishing would be required :)
>
> postgres=# select * from pgsql_branches ;
> ERROR:  pgsql_fdw connection failure on subaru_pgbench
> DETAIL:  FATAL:  terminating connection due to administrator command
> FATAL:  terminating connection due to administrator command

Yes, that would be an improvement.

--
Thom