From: Tom Lane <tgl@sss.pgh.pa.us>
To: Joe Conway <mail@joeconway.com>
cc: pgsql-hackers@postgresql.org
Subject: Re: Getting rid of "unknown error" in dblink and postgres_fdw
In-reply-to: <02fa2d90-2efd-00bc-fefc-c23c00eb671e@joeconway.com>
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Comments: In-reply-to Joe Conway <mail@joeconway.com>
	message dated "Wed, 21 Dec 2016 16:12:34 -0800"
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Date: Wed, 21 Dec 2016 19:22:09 -0500
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Joe Conway <mail@joeconway.com> writes:
> I did notice that postgres_fdw has the following stanza that I don't see
> in dblink:

> 8<------------------
> /*
>  * If we don't get a message from the PGresult, try the PGconn.  This
>  * is needed because for connection-level failures, PQexec may just
>  * return NULL, not a PGresult at all.
>  */
> if (message_primary == NULL)
> 	message_primary = PQerrorMessage(conn);
> 8<------------------

> I wonder if the original issue on pgsql-bugs was a connection-level
> failure rather than OOM? Seems like dblink ought to do the same.

Oooh ... I had thought that code was in both, which was why I was having
a hard time explaining the OP's failure.  But I see you're right,
which provides a very straightforward explanation for the report.
I believe that if libpq is unable to malloc a PGresult, it will return
NULL but put an "out of memory" message into the PGconn's error buffer.
I had supposed that we'd capture and report the latter, but as the
dblink code stands, it won't.

In short, yes, please copy that bit into dblink.

			regards, tom lane


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