Re: ERROR: could not open relation

Sorry, I didn't have the evidence about the bgwriter before. It was
based on conjecture on IRC last night and newly gathered evidence
from this morning.

Here's a list of current postgres processes on the box.

postgres  1186  2.8  5.0 437812 417624 ?     S    Jul13  22:37
postgres: writer process
postgres  1187  0.3  0.0  5940 2688 ?        S    Jul13   2:54
postgres: stats buffer process
postgres  1188  3.1  0.1 13456 8856 ?        S    Jul13  25:16
postgres: stats collector process

My assumption is that it's typically the case that these three
processes generally get allocated sequential pids when postgres starts.

In the postgres log, we see these two types of errors, which were the
only ones that did not report an IP address:

2005-07-12 01:53:31 CDT 13390 :LOG:  statistics buffer is full
2005-07-13 17:44:51 CDT 13389 :ERROR:  could not open relation
1663/32019395/94144936: No such file or directory

So if we assume that pid 13390 referred to the stats collector from
yesterday, then presumably 13389 was the bgwriter.

Unfortunately, this is a system where the interloper is superuser
(and, yes, changing this has been a TODO). But even so, I need help
understanding how one backend could access the temp table of another.
Which is what brings me to vacuum or some other system process as a
culprit.

Recognizing that the application code will execute as superuser in
postgres, here is what is happening in a session:

Several temporary tables (some of which inherit from actual tables)
are constructed.
Data is loaded in.
If the data includes updates, in the same session, a VACUUM is
performed, else an ANALYZE is performed.

So we know these things:

1. This import process was running.
2. It had started the vacuum, which occurs in the same session as
temp tables that inherit from the table being vacuumed.
3. bgwriter reported an error about a missing relation file (I guess
this is a strong suspicion more than knowledge, but we strongly
suspect).

So could this be somehow related to the fact that VACUUM, as a result
of the inheritance relationship in the temp tables, is explicitly
attempting to access them?

--
Thomas F. O'Connell
Co-Founder, Information Architect
Sitening, LLC

Strategic Open Source: Open Your i™

http://www.sitening.com/
110 30th Avenue North, Suite 6
Nashville, TN 37203-6320
615-260-0005

On Jul 14, 2005, at 11:07 AM, Tom Lane wrote:

> Do you have some evidence that the bgwriter was what was reporting the
> error?  You didn't say that before.
>
> The bgwriter only works on dirty shared buffers, so the only way this
> could be happening is if a page of a temp table had gotten loaded into
> a shared buffer, which isn't supposed to happen really.  Is it
> possible
> that you had some backend deliberately trying to read a temp table
> created by another backend?  (You don't have to assume that the
> interloper tried to modify the table; a mere SELECT could have created
> the dirty-buffer condition due to hint-bit update.  You do have to
> assume that the interloper was superuser, though, else permissions
> would have stopped him from accessing someone else's temp table.)
>
>             regards, tom lane